Cold Weather Measurements of Heat Transfer Coefficient

Friends, it’s winter and the weather is reassuringly cold: average daily temperatures in Teddington are around 0 °C. And as I wrote the other week, that offers the opportunity to make measurements of the Heat Transfer Coefficient of a dwelling.

People with Gas Boilers

This is especially valuable for people with gas boilers who are thinking about getting a heat pump.

When the outside temperature is around 0 °C, the average heating power required to heat the majority of UK homes is typically in the range 5 kW to 10 kW.

Most gas boilers have a full power of 20 kW to 30 kW and so can heat a home easily. To keep the temperature just right, the boilers cycle on and off to reduce their average output to the required level. For most houses there is no possibility that a boiler will be undersized.

Heat pumps operate differently. They are typically less powerful than boilers and the maximum heat pump output must be chosen to match the maximum heat requirement of the house.

By measuring the daily use of gas (kWh) by a boiler on a cold day one can estimate the size of heat pump required to heat the dwelling to an equivalent temperature.

I wrote about this at great length here, but at its simplest one just takes the amount of gas used on a very cold day (say 150 kilowatt hours) and divides by 24 (hours) to give the required heat pump power in kilowatts (150/24 = 6.3 kW).

People with Heat Pumps

But the cold weather is not just for people with gas boilers: Heat pump custodians and people heating their house electrically can gain insights when it’s cold.

Starting on 1st December I looked up:

• the average daily temperature;
• the daily heat output from the heat pump (in kWh);
• the daily electricity consumption of the heat pump (in kWh);

The internal temperature was a pretty stable 20 °C throughout this period. So I first worked out the so-called temperature demand: that’s the difference between the desired internal temperature and the actual external temperature.

I then plotted the daily heat output from the heat pump (in kWh) versus the average daily temperature demand (°C). The data fell on a plausible straight line as one might expect. Why? Because the colder it is outside, the faster heat flows out through the fabric of the dwelling, and the greater the rate at which one must supply heat to keep the temperature constant.

In the graph below I have re-plotted this  but instead of using the average daily heat output from the heat pump (in kWh) I have divided this by 24 to give the average daily heat pump power in kilowatts.

Click on graph for a larger version. Graph of average heating power (in kW) versus temperature demand (°C) for the first 10 days of December 2022. Notice that the line of best fit does not go through the origin.

The maximum heat output from the 5 kW Vaillant Arotherm plus heat pump varies with the external temperature, but for flow temperatures of up to 45 °C, it exceeds 5.6 kW.

The maximum daily average power for the first 10 days of December is just over 3 kW, so I think the heat pump will cope well in even colder weather. Indeed I could probably have got away the next model down. But it does seem to be a general rule of heat pumps that one ends up with the model one size above the size one actually needs.

Click on image for a larger version. Specifications for the Vaillant Arotherm plus heat pump. For the 5 kW model at an external temperature of -5 °C and heating water for radiators to between 40 °C and 45 °C, the maximum output is between 5.6 kW and 6 kW.

The slope and intercept of the graph

The slope of the graph is approximately 0.166 kW/°C or 166 W/°C. This figure is known as the Heat Transfer Coefficient for a dwelling. It is the figure which characterises the so-called fabric efficiency of a dwelling.

However, as I noted many years ago when I looked at this problem using gas boiler measurements, the straight line does not go through the origin. The best-fit line suggests zero power output when the external temperature is 2.8 °C below the internal temperature.

This would imply that the heat flow through the fabric of the building was not proportional to the difference between the inside and outside temperature.

The reason for this is that there are other sources of heating in the house, and not all the heat pump output goes into the house. Specifically:

• People: each person heats the house with around 100 W, about 2.4 kWh/day.
• Electrical Items: All the electricity consumed by items in the house ends up as heat. For my home this amounts to around 10 kWh/day.
• Hot Water: Heat pump output that heats domestic hot water is mostly lost when the hot water is used. My guess for this house is that this amounts to around 3 kWh.day.

So to estimate the actual amount of heat dissipated in the house I should really take the heat pump output and:

• Add 2.4 kWh/day for each person in the house
• Add 10 kWh/day for all the electrical items
• Subtract 3 kWh/day for the hot water lost.

Together this amounts to adding 9.4 kWh/day to each heating estimate. Pleasingly, plotting the same graph with these corrections, the graph now intercepts within 0.5 °C of the origin. To me this indicates that I am now accounting for all the significant sources of heat within the house reasonably well.

Click on graph for a larger version. Graph of average heating power (in kW) versus temperature demand (°C) for the first 10 days of December 2022.

I haven’t included any solar gain in these estimates because at this time of year solar gain is generally very low unless a home has large south facing windows. Previously I have noted that solar gain seemed to be much more important in spring and autumn with longer and generally sunnier days.

COP

I also took the opportunity to evaluate how the daily averaged coefficient of performance (COP) varies with external temperature. This is based on readings from a heat meter and an electricity meter which monitors the heat pump.

The COP values below 3 are a little bit lower than I would like, but still acceptable. On subsequent cold days I will be seeing if there are adjustments I can make to the heat pump operation which can improve this.

Click on graph for a larger version. Graph of daily average COP versus daily average outside temperature (°C) for the first 10 days of December 2022. Extrapolating the trend suggests that the COP would reach unity at a temperature of – 12 °C.

Out of curiosity, I also evaluated the heat output using the Vaillant SensoApp. The figures were massively in error. For example, on 10th December the app suggested the total heat delivered to the house by the heat pump was 58 kWh. In fact the correct answer was 73.9 kWh.

Conclusion

Cold weather offers an opportunity to assess the so-called Fabric Efficiency of a dwelling by direct measurements of its Heat Transfer Coefficient.

The cold weather will be with us for a few more days so there’s still a chance to make measurements in your dwelling.

Weather Compensation: Experimental Tweaking

Friends, as I mentioned in my previous article, I have no real idea how to actually operate my 5 kW Vaillant Arotherm plus heat pump – or to check how well it is operating. That’s because there is no readable manual for the controller and the App does not do what it says it does.

But since I have an independent monitoring system, I have begun a series of experiments to tweak the heat pump weather compensation settings, and see what happens!

If ‘Reading the Manual‘ is like taking a course in theoretical heat pumps, then this is more like a course in experimental heat pumps.

This is quite a technical article, and it is nearly 1500 words long. So if you are not really interested in heat pump arcana I would recommend giving this one a miss. On the plus side, it does have some nice graphs :-).

Weather Compensation

Weather Compensation is the idea that when the weather is mild, one can heat water in radiators or under-floor heating to a low temperature – perhaps just 25 °C. But when the weather is colder, and the heating demand is greater, one can increase the temperature of the hot water to perhaps 40 °C or 50 °C to meet the heating demand.

Using weather compensation to match the output of a heat pump to the heating demand contrasts with using a thermostat for the same purpose.

Click on image for larger version. The heating supplied to a dwelling can be changed to match demand in two ways. In a traditional thermostat-based system, the radiator flow temperature is fixed and switches on and off to maintain a constant indoor temperature. In contrast, using weather compensation the radiator flow temperature is adjusted.

In a thermostat-based heating system, the flow temperature to which water is heated is pre-set: in boilers it is often as high as 70 °C, and for heat pumps it might be 50 °C. And then to match heating to demand, the thermostat switches the heating source on and off intermittently to maintain the desired temperature.

Weather compensation is particularly valuable when using heat pumps because the coefficient of performance (COP) of the heat pump varies with both flow temperature and environmental temperature. But it can be tricky to adjust the settings for any heat pump, but especially one with no decent manual!

Weather Compensation in action

The graph below shows data taken every two minutes during the week from 00:01 on 11th October 2022.

• The red curve shows the outside temperature
• The grey dots show the instantaneous flow temperature.
• The green curve shows the flow temperature averaged over 1 hour
• The orange curve shows the internal temperature

Click on image for larger version. Weather compensation in action. When the outside temperature falls, the flow temperature in the radiators increases to maintain the internal temperature.

Notice that when the outside temperature falls, the flow temperature in the radiators increases to maintain the internal temperature.

But on day 4 of the period shown in the graph above, I changed the setting of the Weather Compensation from the curve labelled ‘0.6’ to the curve labelled ‘0.5’ in an attempt to lower internal temperature of the house. I’ll explain more about these labels below.

The graph below shows that average for the 4 days before the change was 21.0 °C and the average for the 3 days after was 20.76 °C: so it does seem to have had a small (0.24 °C) effect, but I will need to continue experiments – see the end of the article for an update.

Click on image for larger version. Graph shows the internal temperature of the house detail averaged over a period of 1 hour. The weather compensation parameter was changed on Day 4 and it does seem to have slightly lowered internal temperature.

It is striking to me how stable the internal temperature is given that – as I understand it – it is based entirely on measuring the temperature OUTSIDE the house – not INSIDE it!

COP

To evaluate the COP, one needs to work out the ratio of the heat delivered to the electrical energy used, over some set time period.

The hourly averaged COP is shown in the graph below. The times when the COP is greater than 4 correspond to times when the difference between the flow temperature and the outside temperature is small, and so not very much heat is being delivered with these high COP values.

Click on image for larger version. Graph shows the hourly averaged COP. Considering only use for DHW the average COP was 3.1 and considering only use for space heating the average COP was 3.9. Overall, considering both DHW and space heating across  the entire period the average COP was 3.7. These averages are shown as dotted lines on the figure.

With a little spreadsheet untangling it is possible to extract the data corresponding to periods when the heat pump is heating DHW and periods when it is heating water for space heating. For DHW the average COP for heating water to 50 °C was 3.1 and for space heating the average COP was 3.9. Overall, considering both DHW and space heating across the entire period, the average COP was 3.7.

Electrical and Thermal Power

Calculation of COP requires evaluation of both electrical power consumed and thermal energy delivered. The graphs below show both these quantities measured every 2 minutes throughout the week or so under consideration.

Click on either image for larger version. Graphs show hourly averages of electrical and thermal power. The DHW cycle runs once a night using cheap rate electricity. The separation of the two uses of the heat pump is not quite perfect: sorry.

Tweaks

So far I have just showed a week or so of data. Now I will explain what I hope to achieve with some ‘tweaks’ First let me explain, about how Vaillant implement Weather Compensation.

Their scheme is illustrated in the figure below. The flow temperature of water in the radiators is set depending the temperature outside. The sensitivity of the weather compensation is set by picking a curve labelled by a number from 0.1 to 4. For example, when the outside temperature is 5 °C,

• the curve labelled 0.6 would result in a flow temperature of about 34 °C but
• the curve labelled 0.5 would result in a flow temperature of about 32 °C

Click on image for larger version. The flow temperature of water in the radiators is set depending the temperature outside. The sensitivity of the weather compensation is set by picking a curve labelled by a number from 0.1 to 4. When the outside temperature is 5 °C, the curve labelled 0.6 would result in a flow temperature of about 34 °C but using the curve labelled 0.5 would result in a flow temperature of about 32 °C.

On Day 4 I adjusted the weather compensation from 0.6 to 0.5. To see if this tweak is working we can look at the second figure in this article in this article which I have reproduced below.

Click on image for larger version. On Day 4 the weather compensation setting was changed from 0.6 to 0.5. If we look at cold spells before and after the change it does look as though as the flow temperature is perhaps a degree or two than one might otherwise have expected.

If we look at cold spells before and after the change it does look as though as the flow temperature is perhaps a degree or two cooler than one might otherwise have expected. And since this article has taken a day or two to prepare, I now have a couple more days data on the internal temperature with WC curve 0.5. It does indeed seem to have maintained an internal temperature about 0.23 °C cooler than using WC curve 0.6.

Click on image for larger version. Updated version of the second graph in this article with 3 extra days data. The graph shows the internal temperature of the house in detail averaged over a period of 1 hour. The weather compensation parameter was changed on Day 4 and it does seem to have slightly lowered internal temperature.

Conclusion

My main conclusion is that the weather compensation adjustment does seem to be sort-of working. I will continue experiments and let you know how they go.

My second conclusion, is that observing these effects is really hard and it takes hours of analysis to unearth this kind of insight!

My third conclusion – which you may have already spotted – is that my 5 kW heat pump is just too big. It only needs to output 1,500 W to maintain a temperature of just over 20 °C in my home when the outside temperature is 5 °C i.e. with 15 °C of demand. This seems to indicated that a 3 kW heat pump would have been adequate to heat the home down to (say) – 5°C.

This oversizing is probably responsible (at least in part) for the rapid cycling on and off of the heat pump – exactly what weather compensation was supposed to avoid!

 

Vaillant Arotherm Plus Heat Pump: The good, the bad and the ugly.

Friends, it’s been just over a year now since we had our 5 kW Vaillant Arotherm Plus heat pump installed.

The Good

In short, I love the heat pump: it is super quiet; uses low GWP propane as a working fluid; and can even heat water to 70 °C if I should ever desire.

And it is has worked well with a seasonal average COP (sCOP) of 3.6 in its first heating season during which the internal temperature of the house has been steady at ~21.5 °C 24/7. And we have lots of pleasantly hot hot water.

But, not everything is good. And this article is about the things that are seriously bad and the things that are downright ugly.

The Bad

There is no User Manual! There is lots of excellent engineering documentation and installation instructions, but I personally would appreciate a relatively short document that explained how to adjust various aspects of the heat pump.

Click for a larger version. Engineering documentation for the Vaillant Heat Pump. But no user manual.

For example, if one uses a thermostat, it is relatively easy to programme a setback period overnight where the temperature is lower. But it would nonetheless be nice to have instructions.

But if you switch to using weather compensation instead of a thermostat? There is no explanation of how the weather compensation interacts – if it all – with the thermostat setting. And the settings are four or five layers down in a menu system that is labrythine in its obscurity. A manual would be helpful.

The SensoComfort controller looks great: sleek and black. But using it is a nightmare. Each time one attempts to achieve a particular task one has to decide whether it should be looked for under a variety of confusing menu headings:

• Installation
• Basic System Diagram Configuration
• HP control module configuration
• Heat Pump 1
• HP control module
• Circuit 1
• House
• Domestic Hot Water

There is no logic to this and it’s just guesswork every time because there is no manual!

The Ugly

The Vaillant SensoAPP does just about have some basic functionality.

For example, it allows one to set a period of absence or trigger a boost to the domestic hot water. However, it frequently fails to do even these basic tasks, commonly reporting a variety of errors.

But the one thing I would like the App to do would be tell me the Coefficient of Performance (COP) of the heat pump. The COP tells the owner or an engineer, how well the heat pump is working.

The COP is the ratio of the amount of heat delivered to the house, to the amount of electrical energy used to operate the pump. Typically COP lies in the range 2 (poor) to 5 (outstanding) and this provides the most significant measure of a heat pump’s performance.

Ideally, the App would report the COP for hot water and for space heating separately. But actually there is just nothing!

In my opinion it is scandalous that the App does not report the COP.

There are signs that the App should be able to show the COP, but then using the available data it gives erroneous answers. In short, when it comes to monitoring heat pump performance, it is literaly useless.

Let me explain.

The App offers no direct readout of COP– which is disappointing – but the ‘Information’ screen on the SensoAPP appears to offer the opportunity to see the electrical consumption and the thermal output (called the ‘environmental yield’) for both space heating and domestic hot water.

Click image for a larger version. The information page on the Vaillant sensoApp looks like it should have all the information one needs to calculate the COP.

Using these data it should be possible to evaluate the COP. Sadly this is not the case.

In an attempt to do this I downloaded the weekly data for the electrical consumption and checked it against the completely independent MMSP monitoring system I have installed.

The weekly consumption information screen for DHW looks like the figure below. It is highly suspicious to me that the data appear to be exactly whole numbers of kWh every day – but the screen tells me that I used 8 kWh of electricity that week for domestic hot water, and that is the figure I recorded.

Click on the figure for a larger version. The electrical consumption for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily consumptions are all exact numbers of kWh.

My weekly MMSP data runs Saturday to Friday while the Vaillant data runs Sunday to Saturday, so we might not expect the data to be identical, but the data (below) are similar. I was hopeful when I saw this correspondence.

Click on the figure for a larger version. The blue curve shows the weekly electrical consumption (kWh/day) as self-reported by the Vaillant App. The red curve shows the same quantity as measured by an independent monitoring system.

Over the 61 weeks since installation the Vaillant reported consumption of 2,147 kWh – 4.3% less than the MMSP system. Not great agreement.

However, if one looks at the thermal data – the environmental yield – the data are both dodgy and missing.

How can they be both dodgy AND missing? As the screen grab below shows, the graph suggest the environmental yield is an exact whole number of kWh every day – something which is very unlikely. This makes the data seem dodgy to me.

But in this case we can also add up the daily yield very easily – it comes to 8 kWh that week. However the App does not do that summation for me – it simply states that the total heating over the entire installation time is 721 kWh. The weekly data are just missing!

Click on the figure for a larger version. The environmental yield for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

While I can add up the data in the bar chart above quite easily, this not possible for other screens such as that below – which again simply states the total yield over the entire installation period.

Click on the figure for a larger version. The environmental yield for space heating in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

This means that it is impossible to work out the COP.

I did try working out the overall COP since installation, but the results were not believable. The Vaillant self-reported average COP is 2.0 whereas the MMSP monitoring system indicates an answer closer to 3.51 .

The good, the bad and the ugly

Summarising, the heat pump is fantastic, and works well.

But I only know that because I have an independent monitoring system.

If I didn’t have independent monitoring I would literally have no idea how well the heat pump was working.

And there has been no improvement or new software updates in the last year

Overall, this is shockingly bad.

First Winter with a Heat Pump

Friends, our first winter with a heat pump is over.

Last week:

• I switched off the space heating, and…
• I changed the heating cycle for domestic hot water (DHW) from night-time (using cheap-rate electricity) to day-time (using free solar electricity).

From now until the end of July, I am hopeful that we will be substantially off-grid.

Let me explain…

No Space Heating 

The figure below shows the temperatures relevant to our heating system for the week commencing Saturday 9th April.

The week started cold, with overnight temperatures close to 0 °C and daytime temperatures peaking at 12 °C.

But the week ended with much warmer temperatures, and even in the absence of any heating flow, the household temperature rose above 21 °C. At this point I decided to switch off the space heating. You can see this on the monitoring data below.

Up to the 15th April, the heat pump would operate each evening – you can see this because radiator temperatures oscillated overnight as the heating circuit struggled to deliver a very low heating power.

From the 16th April – with the space-heating off – you can see the radiator temperatures simply fell after the DHW water heating cycle.

Click image for a larger version. Graph showing four temperatures during the week beginning 9th April 2022. The upper graph shows the temperature of radiator flow and the domestic hot water (DHW). The lower graph shows the internal and external temperatures. In the colder weather at the start of the week, the radiator flow temperatures cycled on and off. In the warmer temperatures at the end of the week, heating stopped automatically. On 16th April I switched the space heating circuit off.

Heating DHW during the day 

The next graph shows the same data for the following week. Now there is no space-heating in the house, but the insulation is good enough that household temperature does not fall very much overnight.

On the 20th April I switched from heating the domestic hot water at night (using cheap rate electricity) to heating during the afternoon (using electricity generated using solar PV).

My plan was that by 2:00 p.m., the battery would be substantially re-charged, and heating the hot water at that time would:

1. Minimise exports to the grid and maximise self-use of solar-generated electricity.
2. Heat the domestic hot water using air that was ~ 10 °C hotter than it would be at night – improving the efficiency of the heat pump.

Click image for a larger version. Graph showing four temperatures during the week beginning 16th April 2022. The upper graph shows the temperature of radiator flow and the domestic hot water (DHW). The lower graph shows the internal and external temperatures. The radiator flow was switched off. On 20th April I switched from heating the domestic hot water at night to heating during the day.

One can see that household temperature has fallen a little during the week, but only to around 19 °C, which feels quite ‘spring-like’ in the sunshine.

The big picture 

The graph below shows:

1. The amount of electricity used by the household
2. The amount of electricity drawn from the grid

It covers the whole of 2021 and the start of 2022 up to today (almost) the end of April. The graphs show running averages over ± 2 weeks.

Click image for a larger version. Graph showing the amount of electricity used by the household each day (kWh/day) and the amount of electricity drawn from the grid each day (kWh/day). Over the 8 months of the winter heating season, 27% was supplied by solar generated electricity.

The 4 kWp solar PV system was installed in November 2020 and was just beginning to make a noticeable difference to our electricity consumption in the spring of 2021.

In March 2021 we installed the Powerwall and immediately dropped off the grid for just over 2 months! In mid-summer we had a run of very poor solar days and we began to draw from the grid again.

In July 2021 we installed a heat pump and this extra load (for DHW) coupled with the decline in solar generation caused us to need to draw a few kWh from the grid each day.

Over the 8 month heating season from the start of August to the end of April, the household used 4,226 kWh of electricity for all the normal activities (~ 2,200 kWh) plus heating using the heat pump (~2,000 kWh). Over this period the heat pump delivered just over 7,000 kWh of heat for a seasonally averaged COP of around 3.5.

However, even in this winter season, only 3,067 kWh were drawn from the grid – mostly at low cost. The balance (27%) was solar generated.

Summer and Winter Settings

The optimal strategy for the Powerwall is now becoming clear.

In the Winter season, daily consumption can reach 25 kWh/day and solar generation is only ~ 2 kWh day. So in this season:

• We operate the household from the grid during the off-peak hours.
• We time heavy loads (dishwashing, tumble drying and DHW heating) to take place in the off peak hours.
• We buy electricity from the grid to fill the battery (13.5 kWh) with cheap rate electricity – and then run the household from the battery for as long as possible. Typically we would need to draw full price electricity from the grid only late in the day.

Click image for a larger version. Images showing the time of day that we have drawn power from the grid (kW) in half-hour periods through the day. Each image shows the average for one month. The graph was assembled using data from the fabulous Powershaper software (link).

In the ‘summer’ season, daily household consumption is ~11 kWh and average solar generation is typically 15 kWh/day. So given that the battery has 13.5 kWh of storage, we can still stay ‘off-grid’ even during a periods of two or three dull days.

So during this period

• We switch the battery from ‘time-based’ mode to ‘self-powered’ mode.
• We time heavy loads (dishwashing, tumble drying and DHW heating) to take place in the afternoon.

This year and last year 

Last year (2021), as soon as we installed the Tesla Powerwall battery, we dropped off-grid within days.

But this year (2022) we have an additional daily electrical load. Now we are heating DHW electrically with a heat pump which requires ~ 1.5 kWh/day.

Nonetheless, I hope it will be possible to remain substantially ‘off-grid’ for the next few months. Time will tell.

Heat Pump: First Space-Heating Results

Friends, the unseasonably warm autumn has meant that I have had to wait until 23rd October for our 5 kW Vaillant Arotherm plus system to switch itself on.

I only have a couple of day’s data, but the results are interesting and promising. First I will show the data for each day and then discuss what it means at the end.

23rd October 2021

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 23rd October 2021.

From midnight until 6 a.m., there is no space heating, but the heat pump operates for 1 hour to heat the domestic hot water (DHW) tank. Performance for hot water heating is described in these articles (1, 2)

From 6 a.m. until midnight the system is responding to the thermostat set at 19.5 °C and heated water is circulated around the radiators.

The heat pump operates 12 times up until 6 p.m. after which no additional heating was required.

As shown on the graph, 12.1 kWh of heat was delivered using only 2.9 kWh of electrical power. which over 12 hours amounts to an average heating power of around 1 kW using only 250 W of electrical power.

The ratio of thermal to electrical power is known as the coefficient of performance (COP) and this is summarised in the graph below

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 23rd October 2021.

The COP seems to increase slowly through the day peaking for a few minutes in each cycle at values as high as 6, but the space heating average is 4.2.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 23rd October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

The initial flow temperature is ~34 °C which falls through the day to ~28 °C. This reduction in flow temperature is in response to the increase in external temperature from ~12 °C to ~15 °C.

This so-called ‘weather compensation’ allows the use of lower flow temperatures, which enables the system  to operate with the highest possible COP.

I was surprised that even with my unaltered radiators, flow temperatures of 35 °C were sufficient to warm the house.

24th October 2021

Here is the equivalent data to that for the 23rd – the graphs are similar and I show them only to show that the system seems to be behaving reproducibly.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 24th October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

Conclusions

My first conclusion is that the system works. This is quite a relief!

My second conclusion is that the system works slightly better than I had been hoping for.

Being able to heat the house with these low radiator temperatures means that over winter the average COP could be higher than my prior estimate of 3. This means I will use less energy, emit less CO2, and spend less than I had estimated.

My third conclusion is that average heating power was about 1 kW during both days. But the system has plenty of margin to deliver more power either by changing the duty cycle – the pump could stay on for longer – or by increasing the flow temperature.

As soon as the weather settles down to being reliably cold I will begin to carry out experiments to optimise the weather compensation.

My fourth conclusion will be the basis of another article – but I can see already that as far as heating is concerned – I am going to have a very cheap winter.

Keep warm.

Heat Pumps#1

Installation of an air source heat pump (ASHP) in my own house is sadly on hold while the installers await delivery of a part. So I thought I would take this opportunity to update you with one or two things I have learned about how real-world heat pumps operate.

What is a heat pump?

I am preparing an article about how heat pumps work internally, but considering only their operational behaviour, they work like the device illustrated below.

Click for a larger version.

• Powered by electricity, they extract heat from the air.
• Cold water enters the the heat pump.
• Warm water flows out.

The engineered ‘miracle’ of a heat pump is that 1 kWh of electrical energy can extract between 2 kWh and 4 kWh of heat energy from the air.

It might seem that nothing could be simpler or more wonderful? But the engineering reality behind the ‘miracle’ requires that the heat pump be operated carefully.

The problem

The key problem is that heat pumps require a high flow of water through them in order to enable efficient operation of the heat exchangers which extract heat from the air. Typical flows are in the range 20 to 40 litres per minute of water.

For my 5 kW heat pump, this can warm such a flow of water by only 2 or 3 °C. So how can such a device heat water to 55 °C?

Domestic Hot Water

When the heat pump is configured to heat domestic hot water – for sinks and bathrooms – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats domestic hot water. See text for further details.

In DHW-mode, the water in the heat pump circuit is passed through a steel tube wound inside an insulated water storage cylinder. This acts as a heat-exchanger between the water in the heat pump circuit, and the water in the cylinder.

But remember, the ‘hot’ water in the heat pump circuit is just a degree or two warmer than the returning ‘cold water. So how can this ever heat the domestic water to 55 °C.

The trick is having a smart heat-pump controller and low losses in the connecting pipework.

The heat pump controller first sets the heat pump operating parameters to warm the water returning from the DHW by a few degrees.

As the DHW tank warms, the returning water also warms, and the controller slowly adjusts the operation of the heat pump to increase the temperature of the water it supplies to the DHW tank. Eventually the controller detects when the water in the DHW tank has reached its set temperature.

So for example, if the outside temperature is 10 °C, and the water returning from the DHW tank is initially at 20 °C, then:

• Initially the controller configures the heat pump to heat the flowing water to (say) 22 °C. Pumping heat from air at 10 °C to water at 22 °C can be done much more efficiently than pumping heat from 10 °C to 55 °C.
• At first the temperature of the water returning from the DHW tank will be only slightly above 20 °C. But as heat is transferred to the DHW tank the temperature of the water returning from the DHW tank increases.
• In response to this increase in the temperature of the returning water, the controller re-configures the heat pump to an incrementally higher temperature.

By adopting this clever strategy:

• The first part of the heating can be done with higher efficiency – perhaps resulting in 4 units of heating for each unit of electrical work.
• The later part of the heating is less efficient and might only results in 3 units of heating for each unit of electrical work.
• So overall – depending on the maximum temperature required – the so-called coefficient of performance (COP) is usually somewhere between 3 and 4.

Space Heating 

When the heat pump is configured for room heating – so called ‘space heating’ in the lingo – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats radiators. See text for further details.

I was surprised to find that in this mode of operation the water from the heat pump is not passed directly through the system of radiators.

Instead, most of the water passes through a short section of tubing called a ‘low loss header’ and goes straight back to the heat pump. This allows the heat pump to operate at high flow rates.

The water used in the radiators is drawn from the top of the ‘low loss header’ and returns – cooler – to the top of the ‘low loss header’.

However there is almost no pressure difference between the top and bottom of the ‘low loss header’ – and so very little water would naturally flow through the radiators. So a hydraulic pump is used to push water through the radiators.

The cooled water from the radiators now mixes with the main flow at the bottom of the ‘low loss header’ and returns to the heat pump.

Click for a larger version. Schematic illustration of a ‘low loss header’ See text for further details.

So for example, if the heat pump is supplying 20 litres per minute of water at 55 °C to the ‘low loss header’:

• The hydraulic pump draws perhaps 4 litres per minute of water at 55 °C leaving 16 litres per minute to flow straight through the header.
• The return water from the radiators is cooled to (say) 45 °C.
  • From this one can calculate that the radiators have provided heating of 2.8 kW.
• So at the bottom of the ‘low loss header’ there is a mixture of:
  • 16 litres per minute of water at 55 °C
  • 4 litres per minute of water at 45°C
  • When mixed together this makes 20 litres per minute of water at approximately 53 °C which is returned to the heat pump.

At first I was puzzled by this arrangement, but then I realised it was clever trick.

• It allows the heat pump to operate at high flow rates and yet heat water only over small temperature differences.
• And it allows the radiators to operate with lower flows and bigger temperature drops.

For those with experience of electronics, it is analogous to the ‘impedance matching’ effect of a transformer.

It’s complicated…  

Things are more complicated than these diagrams would suggest.

Firstly, the heat pump can only operate in one mode at a time.

So the heat pump controller changes modes by operating a valve to direct the water from the heat pump either to the DHW storage tank or the radiators.

Secondly, there are numerous features incorporated for reasons of safety or maintainability.

Some of these guard against the effects of thermal expansion of the water, some guard against the (low risk) of Legionella infection, and some are filters or energy monitoring components.

But I hope the explanations above come close to getting to the gist of heat pump operation.

I have lots more to say about heat pumps: so stay tuned!

Air Conditioning versus Air Source Heat Pump

Click for a larger version. Similarities and differences in how an air source heat pump (ASHP) or an air conditioning (AC) system warms a home. All the components inside the dotted green line are contained in the external units shown. A key design difference is whether or not the working fluid is completely contained in the external unit. See text for more details.

Regular readers will probably be aware that – having reduced the heating demand in my house – my plan is to switch away from gas heating and install an electrically-powered air source heat pump to heat the house and provide domestic hot water.

But next week I am also installing air conditioning, something which is traditionally not thought of as very ‘green’. What’s going on?

Why Air Conditioning?

I have two reasons.

My first reason is that, as you may have heard, the whole world is warming up! Last year it reached 38 °C in Teddington and was unbearably hot for a week. I never want to experience that again.

During the summer the air conditioning will provide cooling. But assuming the heating comes with good weather, the air conditioning will be totally solar powered, and so it will not give rise to any CO2 emissions to make matters worse!

My second reason is that in the right circumstances, air conditioning is a very efficient way to heat a house. That’s what this article is about.

Heat Pumps

Air Conditioners (AC) and Air Source Heat Pumps (ASHP) are both types of heat pumps.

In scientific parlance, a heat pump is any machine that moves heat from colder temperatures to higher temperatures at the expense of mechanical work.

Note: to distinguish between the general scientific idea of a heat pump, and the practical implementation in an air source heat pump, I will use abbreviation ASHP when talking about the practical device.

The general idea of a heat pump is illustrated in the conceptual schematic below.

As shown, the pump uses 1 unit of mechanical energy to extract two units of heat energy from air at (say) 5 °C and expel all 3 units of energy (1 mechanical and 2 thermal) as heat into hot water at (say) 55 °C.

Click for a larger version. Traditional representation of the operation of heat pump.

Heat pumps can seem miraculous, but like all good miracles, they are really just applied science and engineering.

A heat pump is characterised using two parameters: COP and ΔT.

• A heat pump which delivers 3 units of heat for 1 unit of work is said to have a coefficient of performance (COP) of 3.
• The temperature difference between the hot and cold ends of the heat pump is usually called ‘Delta T’ or ΔT.

Obviously engineers would like to build heat pumps with high COPs, and big ΔTs and they have used all kinds of ingenious techniques to achieve this.

But it turns out that heat pumps only operate with high COPs when the ΔT is small and when the heating power is low. There are two reasons.

• Firstly, the laws of thermodynamic set some absolute limits on the COP achievable for a given ΔT.
  • Most practical heat pumps don’t come close to this thermodynamic limit for a variety of mundane reasons.
  • The maximum COP for moving heat from 5 °C to 55 °C is 6.6.
  • The maximum COP for moving heat from 5 °C to 20 °C is 19.5.
• Secondly, in order to heat a room to (say) 20 °C, the hot end of the heat pump needs to be hotter than 20 °C.
  • Typically the hot end of the heat pump must be 5 °C to 10 °C warmer than the room in order that heat will flow out of the heat pump.
  • Additionally the cold end of the heat pump must be 5 °C to 10 °C colder than the external air in order that heat will flow into the heat pump.
  • The interfaces between the ends of the pump and the environment are called heat exchangers and designing ‘good’ heat exchangers is tricky.
  • A ‘good’ heat exchanger is one that allows high heat flows for small temperature differences.

So now we have seen how heat pumps are characterised, let’s see how heat pumps are used domestically.

Air Source Heat Pump (ASHP) versus Air Conditioner (AC)

The schematic diagrams  below show how a house is heated by an ASHP and an AC system. Both systems operate using a working fluid such as butane, which is ingeniously compressed and expanded. The details of this process are not the topic of this article so here I am glossing over the fascinating details of the device’s operation. Sorry.

Click for a larger version. How an air source heat pump (ASHP) warms a home. All the components inside the dotted green line are contained in the external unit shown. A key design feature is that the working fluid is completely contained in the external unit and heat is transferred to the central heating water by a heat exchanger.

Click for a larger version. How an air conditioner (AC) warms a home. All the components inside the dotted green lines are contained in either the external unit or the fan coil unit shown. A key feature is that the working fluid itself flows into the fan coil unit and heats the air directly.

We can compare the operation of the two systems in the table below.

Air Conditioner	Air Source Heat Pump
Air at (say) 5 °C is blown over a heat exchanger and evaporates the working fluid.	The same.
The working fluid is then compressed – that’s the bit where the work is done – and liquefies, releasing the captured heat.	The same.
The hot working fluid – now at ~30 °C then flows through a pipe to an indoor heat exchanger (fan coil unit) where air is blown over the pipe and heated to 20 °C.	The hot working fluid – now at ~60 °C then flows through a heat exchanger and transfers the heat to water in my central heating system at ~55 °C
No corresponding step	The 55 °C water then flows through a radiator in my room, heating the room by radiation and by convective heat transfer to air at ~20 °C.

Looking closely at the figures and table above, one can see that the operation of the ASHP and the AC system are broadly similar.

However the ASHP has to operate with a bigger ΔT (~55 °C versus ~25 °C) than the AC system, and also has to transfer heat through an extra heat exchanger.

Both these factors degrade the achievable COP and so for my application, the specified COP for an ASHP is just over 3, but for the AC system, it is just over 5.

In my well-insulated house, when the external temperature is 5 °C, I require typically 36 kWh per day of heating, equivalent to 1.7 kW continuous heating. I can achieve this in several ways:

• Using gas I must burn ~40 kWh of gas at 90% efficiency costing 40 x 3p (£1.20) and emitting 40 x 200 g = 8 kgCO2
• Using an ASHP with a COP of 3, I must use ~36 kWh/3 = 12 kWh of electricity costing 12 x 25p (£3.00) and emitting 12 x 200 g = 2.4 kgCO2
• Using an AC system with a COP of 5, I must use ~36 kWh/5 = 7.2 kWh of electricity costing 7.2 x 25p (£1.80) and emitting 7.2 x 200 g = 1.4 kgCO2
• Using a domestic battery and buying the electricity at night for 8p/kWh, I can reduce the cost of using an ASHP or AC system by a factor of 3 to £1.00/day or £0.60/day respectively.

[Note: In these calculations I have assumed that the carbon dioxide emissions per kWh are same for both gas and UK electricity (200 gCO2/kWh) which is roughly correct for 2021]

So using an AC system I should be able to achieve domestic heating with lower carbon dioxide emissions than an ASHP.

My plan

In my case I need to heat water for my home to 55 °C for use in showers and basins. So I need an ASHP for that. And since I already have radiators in every room, hooking up the ASHP to the radiator circuits is smart double use.

The AC system I am having installed will have 1 external unit and 2 internal ‘fan coil units’. One unit will be in my bedroom (a sheer indulgence) and the other will be high up on the stairs, allowing air to be either blown down to the ground floor where I hope it will circulate, or blown towards the bedrooms.

My hope is that, when used together, the AC system (COP~5) will reduce the heating output required from the radiators so that I can reduce the flow temperature of the water from 55 °C to perhaps 40 °C. This reduces their heat output, but increase the COP of the ASHP from 3 to perhaps 4.

The main difficulty that I foresee is the extent to which the AC heating will actually permeate through the house and so reduce the amount of heating required by the ASHP.

So I am not sure how much heating will be required by the ASHP acting through the radiators, and whether the radiators will work at low flow temperatures. It is possible I might need to replace a few radiators with ones which work better at low temperatures.

It is not at all obvious that this plan will actually work at all – but I think it is worth a try.

Kit

The air conditioning I am having installed is a Daikin 2MXM40 multi-split outdoor unit with two FTXM25 indoor air units. (Brochure)

The model of heat pump I will have installed is a Vaillant Arotherm plus 5 kW. It can supply up 5 kW of heating at 55 °C with a COP of 3  – i.e. it will use just 1.6 kW of electrical power to do that – and heat water to 55 °C. Water storage will be a 200 litre Unistor cylinder. A brochure with technical details can be found here, and a dramatic video showing the kit is linked at the end of this article.

When I have come to terms with how much money I am spending on this, I will share that information. But at the moment it hurts to think about it!

Anyway: the adventure begins next week!