Archive for the ‘My House’ Category


October 5, 2021

Friends, I gave a talk last week to the Richmond U3A – The University of the Third Age.

Disappointingly it was still a Zoom affair, but it appeared to be pass adequately.

After the talk I rashly thought I would run through it again and create a video presentation that I could share.

Having stared at my own face for several hours while trying every possible permutation of sound and video options in Windows™, I am no longer sure it was such a good idea.

But I’ve done it now. And as the saying goes, “I’ve suffered for my art, now it’s your turn…

You can download the Powerpoint file here.

The video of the presentation is in three parts, respectively 12, 16 and 24 minutes long.

Carbonaut: Part 1: 12 minutes

The first part is about the Climate Crisis and features a nice version of the Keeling curve – the curve that shows the increase in atmospheric carbon dioxide since 1959 – but expressed in tonnes of carbon dioxide in the atmosphere rather than concentration.

This makes it easier to understand how tonnes of emissions per person can lead to gigatons of emissions globally.

Carbonaut: Part 2: 16 minutes

The second part is about my ‘personal’ carbon dioxide emissions and explains how I have assessed the carbon dioxide emissions from house.

The key recommendation is to start reading your gas and electricity meter once a week.

Carbonaut: Part 3: 24 minutes

The final part is about what I have done to reduce carbon dioxide emissions from my house by 80%. It covers the installation of:

  • External Wall Insulation
  • Triple Glazing
  • Solar Panels
  • Battery
  • Air Source Heat Pump

It also covers what all this cost, the embodied carbon cost, and the likely carbon savings before the estimated date of my death. It’s quite a bit!

Cheap Electric Cars Are Coming!

October 3, 2021

Friends, back in March 2021 I wrote about the Wuling HongGuang – the most popular electric car in China – a car with a base model starting at $4,200.

Looking through the Fully-Charged channel’s coverage of last year’s Cheng Du motor show, many similar models are being manufactured in China – models which fill in all the gaps between the ultra-cheap and the normal price of an EV in the UK  – close to £30,000.

I’ve posted the video above, but I have also compiled a table of the cars showing a basic cost, battery size, and range.

Click for a larger version: Table showing basic statistics of cars discussed in the video above. All the numbers are uncertain and subject to change, but range of numbers is sufficient to show that EV’s can be made much cheaper than models currently on offer in the UK. Most prices are rounded up.

Summary: the future

From a Chinese perspective, I guess the UK looks like a small island in the Far West. But eventually their manufacturing capability will turn to exporting some of these models to us.

As I wrote previously, the mere existence of these models confirms that EV’s are actually simpler and cheaper to manufacture than ICE vehicles. And many of these models will be well-suited to retired people living in Teddington. And within their restricted budgets.

Of course, just replacing the overwhelming number of ICE vehicles with an equivalently overwhelming number of EV’s does not solve the UK’s problems with carbon emissions.

But the possibility that in a few years normal people will be able to buy EV’s with a range of properties and prices to match their needs does make clear that a realistic pathway is emerging for finally transitioning away from fossil-fueled vehicles.

And once that transition begins it will be hard to stop.

EV’s are cheaper to run and petrol stations run on low margins. As the amount of fuel purchased declines, petrol stations will go out of business, and in an ironic twist, ICE vehicle drivers will find themselves with “range anxiety”. They will have to carefully plan their journeys so as to be sure of finding an old-fashioned ‘filling station’.

Change is coming!

Legionella protection with heat pumps

October 1, 2021

Friends, I am loving our new Vaillant Arotherm+ heat pump.

Click image for larger version: Vaillant Arotherm+ Air Source Heat Pump has replaced our gas boiler. It provides up to 5 kW of heating but uses only (about) 1.5 kW electricity! It can either supply hot water – at up to 70 °C – to a cylinder, or circulate hot water through our radiators.

At the moment it is just heating our domestic hot water (DHW) but in the next couple of weeks I expect that it will begin to circulate water through our radiators in order to warm the house.

One aspect of using a heat pump for DHW at first sounds very alarming: it is a requirement to run an ‘Anti-Legionella’ cycle once a week or so.

‘Legionella’ is the name of an amoebic bacterium which lives naturally in water. It can breed in ‘warm’ water but is killed in ‘hot’ water. Very roughly it thrives in temperatures between 20 °C and 50 °C.

The bacteria are harmless to people when in the water – it’s fine to wash in the water. But if ultra-fine droplets are breathed directly into the lungs, they can cause a potentially lethal pneumonia called Legionnaire’s disease.

Even the droplets from a shower are unlikely to be capable of causing infection – they are too large.

But the disease presents an interesting risk profile.

  • IF the disease is caught (which is unlikely) it is potentially fatal to the elderly,
  • BUT prevention is trivial – just heat stored DHW to 60 °C or above.

What’s this got to do with heat pumps?

Most gas boilers are ‘Combi’ boilers which instantly heat the water as required, and don’t store hot water at all – so they offer no risk of harbouring Legionella bacteria.

So-called ‘system’ gas boilers which store water in a DHW tank typically heat the water to 70 °C – and so kill any legionella bacteria present.

But heat pumps typically struggle to heat water above 50 °C, and typically store water at 50 °C – which is easily hot enough for normal domestic purposes – but just opens the window to potentially allow Legionella to thrive in a DHW tank.

For that reason, modern heat pumps typically run an ‘Anti-Legionella’ cycle once a week which heats the water to above 60 °C. For older heat pumps this can involve the use of a direct electric immersion heater, but more modern heat pumps (such as mine ;-)) can heat water to 60 °C or even 70 °C with no problem.

No problem? So why am I writing this?

When we first started using the heat pump, I noticed that occasionally the water from the taps was extremely hot. It felt dangerously hot.

So I began to measure the temperature of the tap water at three outlets in the house: the kitchen, and two of the bathrooms. The temperatures were typically within ±0.5 °C of each other.

I found that on the morning after the anti-legionella cycle, the tap temperatures were just a bit less than 70 °C. It felt to me like it would be just a matter of time before someone was hurt by this.

Looking on-line (1, 2) I saw that the time to ‘scald’ was less than one second at these temperatures.

Click image for larger version: Graph showing the immersion time before ‘scalding’ for water at various temperatures. For flowing water, these times would be reduced. Notice that the vertical scale is logarithmic.

Safety Action Step#1

After about 1 month of use, I realised why the water was getting so hot. The anti-legionella cycle had been timed to run exactly after the daily heating cycle. It was this ‘double-heating’ which was producing the high water temperatures.

So cancelling the daily DHW heating cycle for that day (Wednesday) meant that the water coming out of the taps was now only about 60 °C – still ‘too hot’ in my opinion.

Safety Action Step#2

A little research on line showed that thermostatic devices existed which could prevent excessive temperatures reaching the taps. They were called ‘blending valves’ or ‘anti-scald’ valves.

These devices act like the valves in thermostatic showers and blend cold water with the hot water to maintain a set temperature – set-able between 45 °C and 65 °C.

The wonderful Twickenham Green Plumbers installed such a device on the top of the DHW cylinder and now my concern about scalding  is a thing of the past. The hot water temperature at the taps is now a consistent 47 ± 1 °C independent of the day of the week.

Click image for larger version: Graph the temperatures of the water emerging from 3 outlets in the house versus time. The anti-legionella cycle takes place early on Wednesday mornings. The installation of a blending valve on the cylinder means that now the tap temperatures does not vary from day-to-day, and does not reach potentially harmful temperatures.

Coefficient of Performance

The practical miracle of heat pumps is that they extract heat from the environment in order to warm our houses, and so provide more heating energy than the electrical energy used to operate them.

The ratio of the heating effect of a heat pump to its electrical energy consumption is called the coefficient of performance or COP.

The graph below shows the COP for the DHW heating cycles (in which the water was warmed to 56 °C) and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C (in both cases these temperatures were exceeded by about 5 °C).

Click image for larger version: The graph shows the COP for the DHW heating cycles in which the water was warmed to a nominal 50 °C and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C. In all cases actual temperatures were exceeded nominal temperatures by about 5 °C.

For the normal DHW cycle, the average COP is 3.3; for the very high temperature combined DHW and anti-legionella cycle, the COP fell to 2.5; but for a normal anti-legionella cycle the average COP is 2.9.


The idea that preparing domestic hot water could potentially create a life-threating hazard is at first alarming.

But in fact the anti-legionella heating cycle – when programmed correctly! – is very simple and reduces COP by only a small amount.

Adding a blending valve to the DHW cylinder output maintains a safe temperature at the taps and has one additional benefit: it allows the DHW cylinder to store extra thermal energy.

Assuming the water is heated from 15 °C, a tank of water at 60 °C contains 28% more thermal energy than a similar tank at 50 °C. If hot water demand were high – e.g. visitors! – the tank could supply 30% more water at a safe discharge temperature of 47 °C.

Using Radiators with Heat Pumps

September 8, 2021

Click for a larger image. A typical dual-panel radiator transfers heat from the hot water flowing through it to the room in two quite distinct ways. By direct heating of the air in contact with the metal surfaces, and by radiation from the outer metal surface.

Switching to heating a house with a heat pump rather than a gas boiler is not entirely straightforward. But it is much easier if one can keep using one’s existing radiators.

But heat pumps operate most efficiently when circulating water at lower temperatures – ideally 40 °C or so. However radiators don’t work so well at these lower temperatures, so in the worst case it might be possible that not enough heat will be transferred to the house to keep it (and you!) warm.

In this article I thought I would explain how radiators work and how one can estimate how well they will work when the water flowing through them is at lower temperatures.

This article is a little bit technical and involves tables of data and mathematical formula: sorry.

The key to understanding radiators is that radiators transfer heat to the room using two quite distinct physical mechanisms:

  • radiation
  • convection.

And in fact, convection is generally more important that radiation. Let’s look at each mechanism in turn.

How radiators work: Radiation

The heat transferred by radiation occurs mainly from the outer panel facing the room and the amount of heat transferred (in watts) is given by a fancy formula.

Click for a larger version.

The power radiated into to the room depends on:

  • The Stefan-Boltzmann constant 5.67 x 10^-8 W/m^2/K^4
  • The front surface area of radiator in m^2 i.e. height (m) x width (m)
  • The physical property of the surface known as emissivity – typically 0.9 for many painted surfaces.
  • The difference between the temperature of the radiator surface and room temperature. But the it is not just the simple difference between the temperatures. It depends on the difference between the 4th power of absolute values of the temperatures.
  • To find the absolute temperature one adds 273.15 K to the temperature in degrees Celsius. So a room temperature of 20 °C corresponds to 293.15 K (kelvin) and a flow temperature of 50 °C corresponds to 323.15 K

The graph below shows the amount of power radiated from the front surface of a radiator at various temperatures

Click for larger version. The heat radiated by a typical radiator with a surface area just over one square metre. Warming the temperature of the surface from 30 °C to 40 °C results in 57 W of additional heat transfer to the room. Further warming the temperature of the surface from 40 °C to 50 °C results in 63 W of additional heat transfer to the room.

The emissivity of the radiator has a maximum value of one – and so can’t be increased very much from it’s typical value of 0.9.

So to radiate more heat from a radiator one must either increase its area, or its flow temperature.

How radiators work: Convection

The heat transferred by convection occurs at the all the vertical heated surfaces of the radiator.

Click for a larger version. For a radiator with 2 heated panels, convection is induced on 4 vertical surfaces.

Heat is transferred by direct contact between the air and the painted surface. Since the heated air has lower density, it become buoyant and a self-sustaining upward air flow is developed.

It is difficult to develop an exact formula that describes the heat transfer process, but most simple analyses assume that heat transfer is proportional to the temperature difference between the radiator and the room.

However, at higher temperature differences, the moving air speed increases and this further improves heat transfer to the air. This leads to a slight non-linear dependence on the radiator temperature.

Convective and radiative heat transfer can be calculated using complex mathematics at this web site.

The graph below shows the amount of power transferred by convection from the front surface of a radiator at various temperatures.

Click for larger version. The heat transferred by convection from just the front surface of a radiator is compared with the heat radiated the front surface of the same radiator as in the previous figure. Warming the temperature of the surface from 30 °C to 40 °C results in 36 W of additional convective heat transfer to the room. Further warming the temperature of the surface from 40 °C to 50 °C results in 39 W of additional convective heat transfer to the room.

However even a single-panel radiator can transfer heat convectively from two surfaces (front and back). And a double-panel radiator can transfer heat from 4 surfaces (the front and back of each panel).

And we can increase the convective heat transfer further from a radiator by more adding vertical surfaces for air to flow past. For example, for example, the figure below shows the design of several Stelrad Radiators. There are several additional ‘corrugated’ fins with a length which exceeds the basic width of the radiator.

Click for a larger view. These are cross-sections of radiators showing different numbers of panels and fins. All the radiators have roughly the same radiated output 317 W: this is proportional to the frontal area. But the overall power outputs are 1568 W for the K1 model, 2155 W for the P+model, 2770 W for the K2 model. This extra power is achieved by additional convective heat transfer from the panels and the fins which can have a much larger surface area than the panels. All figures assume 70 °C water flow.


For a single panel radiator, with no fins, radiation and convection contribute roughly equally to heat transfer.

But for more complex radiators with additional fins and panels, convection is much more important for heat transfer. For the K2 radiator in the figure above, convective heat transfer is 8 times larger than radiative heat transfer.

The physical models of heat transfer are too complicated to calculate for every variety of radiator. So there is a standard curve adopted for calculating overall (radiative and convective) heat transfer for water flow at lower temperatures.

This standard curve is shown as a dotted line in the figure below. It matches the physical models reasonably well, but predicts a slightly lower heat output.

Click for larger version. The heat transferred by convection from the four vertical surfaces of double panel radiator, and the heat radiated the front surface of the same radiator. Their sum is shown in black and the standard de-rating curve is shown as a dotted line. Operating the radiator at 70 °C (dangerously hot) results in a total heat output of 1072 W. Cooling the temperature of the surface to in (roughly) almost a 50% reduction in heat transfer to the room. Cooling further to 40 °C the de-rating is close to 70%. And using a flow temperature of 30 °C will result in an 85% reduction in heat out put compared with the nominal radiator specification.

But the summary is simple. The nominal heat output of a radiator is specified assuming that the room is at 20 °C and the water flowing through the radiator is at an average temperature of 70 °C.

  • The estimated heat output with a flow temperature of 50 °C is reduced to ~50% of the standard output.
  • The estimated heat output with a flow temperature of 40 °C is reduced to ~30% of the standard output.
  • The estimated heat output with a flow temperature of 30 °C is reduced to ~15% of the standard output.

The standard de-rating factor F is given within 1% by this formula:

where both temperatures are expressed in degrees Celsius.

So what temperature should I set my hot water flow? 

This is difficult to work out. But I think the procedure work like this.

  • First work out how much heat is required to heat a home on a cold winter day. In the south of England where I live this typically corresponds to an outside temperature of about -2 °C. Based on my weekly readings of the gas meter on the coldest week last winter (average temperature 0.2 °C) the peak heating required for the house was around 72 kWh/day – or around 3000 W.
  • Next one considers all the radiators and measures their height and width. Analysing the Stelrad data for about 40 different radiator sizes I saw that:
    • K1 type radiators are rated at about 1600 watts per square metre,
    • K2 type are rated at about 2800 watts per square metre.
    • I then guessed that my old single-panel no-fin radiators will give roughly 700 watts per square metre.
  • Collating all the data I arrived at a table like that below.

Click for a larger version. Analysis of all the radiators in the house estimating first their ‘standard output’ and then their output with a flow temperature of 40 °C.

  • This table suggests that a flow temperature of 40 °C, the radiators should output 3214 watts of heating – which just about matches the 3000 watts required in the coldest weather.

So I am hopeful that my existing radiators will work fine with the new heat hump at the reasonably low flow temperature of 40 °C.

According to the specifications of my 5 kW Vaillant Arotherm plus (excerpt below) with a flow temperature of 40 °C through the radiators, the seasonal coefficient of performance should be over 4.

If most of the electricity is purchased at night using the Octopus Go rate of 5p/kWh, this means that the cost per kWh of heating will be around 1.25 p/kWh i.e. around 30% of the cost of heating with gas.

Click for a larger version. Excerpt from the operating specification of the Vaillant Arotherm Plus heat pumps. The 5 kW model is highlighted in blue. At 40 °C flow it claims to be able to deliver 6 kW of heat when the external temperature is – 5 °C, with a seasonal coefficient of performance of 4.13.

One final issue is whether the heating is in the right places in the house. The bedrooms are often very warm, and our kitchen is the coldest room, having only a single old single-panel radiator and this may need to be upgraded.


Assessment of Heat Pump heating water to 50 °C and 70 °C

September 7, 2021

Friends, our Air Source Heat Pump (ASHP) (a 5 kW Vaillant Arotherm Plus) has been installed for over a month now and I am beginning to get a feel for how it is working.

At this time of year (early September) we have no space heating requirements so the work load for the heat pump is low.

Most of the day it sits in the garden admiring itself, and consuming 12 W of electrical power (0.29 kWh/day)

Our heat pump idling away the late summer days in the back garden. It only works for an hour a day!

Each night it wakes itself at 3:00 a.m. and if the hot water tank requires a top up, it operates for about an hour, heating the tank to roughly 50 °C.

  • Typically it uses ~1 kWh of electricity and delivers ~3 kWh of heat.

On Wednesday mornings it additionally heats the water in the tank to 70 °C in a so-called Anti-Legionella cycle.

  • This typically uses ~3 kWh of electricity and delivers ~7 kWh of heat.

Later in the year I expect that the heat pump will begin to be required to heat the house, and I’ll write about that in a little while.

But for now let me just describe how the system is working at present.

A Normal Cycle

Two typical water-heating cycles from the 5th and 6th September are shown below. The external air temperature in each case was about 15 °C.

Click for a larger version. Typical performance of the heat pump when heating domestic hot water. The two upper panels show data from 5th September and two lower panels show data from 6th September. In each case the left-hand panel shows electrical power consumed (watts), the heat delivered to the cylinder (watts) and the water temperature (°C). The right-hand panel shows instantaneous COP and dotted lines show two estimates of the average COP. 

The key measure of how well a heat pump works is its coefficient of performance (COP) which measures the ratio of thermal energy delivered, to electrical energy consumed.

The graphs on the right above show how the COP varies from minute to minute through the heating cycle.

Also shown as dotted lines are two estimates of the average COP.

  • The blue estimate includes all the electrical energy which the heat pump uses during the 23 hours when it is not ‘working’.
  • The purple estimate includes only the electrical energy which the heat pump uses during the heating cycle’.

Depending on which measure one uses, the COP is between 2.5 and 3 i.e. the heat pump delivers between 2.5 and 3 times as much as heat as the electrical energy it uses

An Anti-Legionella Cycle

Legionella bacteria, which can cause Legionnaires Disease, are capable of lurking in hot water systems at temperatures below 60 °C.

To counteract this, every Wednesday morning the heat pump system additionally executes an Anti-Legionella cycle which heats the water to 70 °C. It should be noted that it is very unusual for heat pumps to operate at all at such high temperatures.

Click for a larger version. Typical performance of the heat pump during an anti-legionella heating cycle on 1st September. The left-hand panel shows electrical power consumed (watts), the heat delivered to the cylinder (watts) and the water temperature (°C). The right-hand panel shows instantaneous COP and dotted lines show two estimates of the average COP. 

From the graphs above one can see that heating to higher temperatures is hard work for the heat pump and the average COP falls from the range 2.5 to 3.0 when heating to 50 °C, to just around 2.1 when heating to 70 °C.

Hot Water Temperatures

Click for a larger version. The measured temperature of hot water at three hand-basins in the house over a period of 20 days. After the anti-legionella cycle in the early hours of Wednesday morning, the flow temperature of water at the taps can reach almost 70 °C, a potential scalding hazard. At other times, the hot water is delivered at just under 50 °C

One unanticipated feature of the Anti-Legionella cycle is that on Wednesday mornings, the temperature of water delivered from the hot water taps is very high – almost 70 °C.

With our level of water use, the system typically skips the Thursday heating cycle because the water is still hot from Wednesday’s ‘super’ heating. Indeed, the water does not return to ‘normal’ temperatures until Saturday!

Delivering water at almost 70 °C is a significant hazard and so I will shortly have anti-scalding valves fitted to the outlets which will limit the maximum temperature of hot water to about 45 °C.

Once I have finished with my tests, I will also reduce the normal hot water temperature by a few degrees.


Overall the system is doing well.

Click for a larger version. COP performance of the heat pump during normal heating cycles and during anti-legionella heating cycles. Heating the water to 70 °C degrades the performance of the heat pump.

Looking at the performance during normal heating cycles, the heat pump heats water from around 15 °C to 50 °C with a COP of typically 3.4

Looking at the performance during anti-legionella heating cycles it heats water from around 15 °C to 70 °C with a COP of typically 2.4

These COPs do not include the electrical energy consumed during the 23 hours when the heat pump is on ‘stand by’. This better indicates the operating performance of the pump, but of course this ‘stand by’ energy still has to be paid for.

Overall (including the ‘stand by’ consumption) the heat pump is delivering on average 4.5 kWh/day of hot water heating at the expense of about 1.77 kWh of electricity/day.

At this time of year, all this electricity comes from solar energy stored in the battery and so costs nothing.

But as the winter season draws in, we will eventually operate this using mains electricity on the Octopus Go tariff. This provides electricity at 5p per kWh between 00:30 and 4:30 a.m. each day.

So the cost of 4.5 kWh of hot water in winter will be about 1.77 kWh x 5 p/kWh = 8.85 p per day.

This is equivalent to just under 2p/kWh (thermal) – which is about 40% cheaper than gas heating which costs about 3.3p /kWh (thermal)

Things will be a little harder in winter as the average external temperature falls, but I am very curious to see how the Vaillant ASHP performs.



Spreading the word

September 1, 2021

Click for a larger version. I have put a sign outside my house!

One of the aims of my mission to reduce carbon dioxide emissions from my house was to make sure that, in the end, the house looked normal.

And with annual carbon dioxide emissions reduced by an estimated 80%, I feel I have succeeded: the house still looks very ordinary.

I felt that if the house looked futuristic or weird, it might deter people from doing something similar.

But one flaw in that strategy is that as people walk past – they don’t notice the house at all!

So I have put up an A4-sized notice board in the front garden to tell people how amazing the house is.

A notice board? 

I am aware that the 21st Century offers opportunities for communication other than noticeboards.

I have heard that entirely visual apps such as Tickety Tok and Instantgram are very popular with the under fifties.

But there are also a lot of people filling those channels of communication with a tsunami of… stuff.

I am targeting the pensioners and families of Teddington, many of whom – but by no means all – are in a position to do something similar to their own homes.

Frankly, I am not optimistic – but I thought I would give it a go.

I’ll let you know how it goes.


Articles about my house

August 31, 2021

Friends, I have just added a static page to this blog called “My House”.

It contains links to the all the articles I have written over the last couple of years on my efforts to reduce carbon dioxide emissions from my house.

If the link is not obvious to you – you can find the page here:



Heat Pumps: Power, Noise and Condensation

August 30, 2021

Friends, I had a visit the other day from a couple who were considering installing a heat pump in their home, but were concerned about the noise.

To get the heat pump to operate, I ran the hot water for 10 minutes and then requested a hot water ‘boost’ using the app on my phone.

We then stood around the heat pump chatting until the visitors started to get cold. The reason? The heat pump had started up and was blowing cold air over their legs. But they had not heard a thing!

I told them to wait – and slowly the heat pump speeded up and became audible. But it was not what I would call ‘noisy’. In the garden, 5 metres away – you would not be aware of it as a separate sound against the (quiet) suburban background.

In fact, the need for heat pumps to be quiet constrains their design significantly and actually determines their physical size! It would be possible to make heat pumps differently – but they would be either noisier or drippier!

Let me explain…

Click for a larger version. How a heat pump works. A fan rotates and blows air out of the heat pump cabinet. This draws in air which flows over a so-called heat exchanger. This consists of many small diameter pipes containing coolant. The coolant absorbs heat from the air which is later delivered to the house.

Thermal power and air volume

When designing a heat pump, the first thing one needs to know is the thermal power the heat pump must deliver: Let’s say its 6 kW.

If it operates with a coefficient of performance (COP) of 3, then 2 kW out of those 6 kW will be from the electrical motor, and 4 kW will be extracted from the air.

Heat pumps obtain this energy by cooling outside air by roughly 3 °C using a so-called heat exchanger. The heat capacity of air is (more or less) fixed, ~ 1 kJ/kg/°C (source) and 1 kg of air occupies a volume about 0.83 cubic metres.

So, if the heat pump extracts heat from 0.83 cubic metres of air per second, cooling it by 3 °C, then it will extract 3 x 1 kJ = 3 kJ of heat per second i.e. 3 kW.

So to achieve its target of extracting 4 kW of heat, it must pass 33% more air over its heat exchanger i.e. about 1.1 cubic metres of air.

Air speed and noise

Heat pump noise arises from air flow over and around surfaces, and the noise increases with the speed of air flow.

A heat pump can draw a given quantity of air over its heat exchanger in (broadly) two ways.

  • By increasing the speed of air flow over a given area of heat exchanger
  • Or by increasing the area of heat exchanger and keeping the air speed low.

In practice, the faster the air flows, the noisier the heat pump becomes.

So when more heating power is required, manufacturers can speed up a fan a little to increase air speed, but  generally they increase the area of the heat exchanger.

Click for a larger version. Heat pumps made by Vaillant. In order to extract more heat while keeping the air speed low, heat pumps need to be physically larger to accommodate larger area heat exchangers.

Heating Power and Condensation

The heating power of a heat pump is linked directly to the volume of air it passes across its heat exchanger, and the amount by which the air is cooled.

So one other option for increasing the heating power extracted from the air while maintaining low air speeds (i.e. low noise) is to cool the air more.

However when air is cooled, then depending on…

  • the air temperature,
  • the initial humidity, and
  • the temperature drop,

…water may or may not condense. The larger the temperature drop, the more likely water is to condense.

Water condensation is not especially harmful, but at low temperatures, condensation can freeze around the heat exchanger and stop the heat exchanger working.

Heat pumps can detect this and intermittently melt any ice on the heat exchanger – but this makes the operation of the heat pump less efficient.

To cope with condensation all heat pumps are equipped with a drain which allows condensed water to simply drip out the bottom of the casing. This is why it is important to mount heat pumps level – so the designed draining port is actually at the lowest point.

But where does the water go after it drains away?

Allowing water to just drip on the ground – and potentially freeze is not a great idea.

Plumbing the drain into an existing drainpipe may seem adequate but it is not. In winter, when the heat pump is operating below zero, this will freeze and may cause icy spillages, and blockages.

So best practice is dig a ‘soak-away’. For my heat pump we used a ground auger to drill a 15 cm diameter hole a full 1 metre deep. We then filled this with small stones.

The drain hose from the heat pump has a 30 cm long internal heater that prevents icing until the condensate is about 15 cm below ground level. Hopefully the temperature there will be above 0 °C!

Click for a larger version. Arrangement for removing condensation from a heat pump. The casing must be level and water is drained away from the lowest point in the cabinet into a soak-away. The drain is heated along its length to prevent it freezing up at low temperatures.

How much condensation is there?

The amount of condensation depends on many factors but because I knew you would ask, I wrote a spreadsheet to calculate it. (Excel .xlsx file: Calculation of Condensate Volume)

A typical output is shown below. The graph shows the number of litres per day of condensation for a heat pump which delivers 6 kW of heating when the external temperature is 0 °C.

This calculation assumes the relative humidity of the air is 85% and that the temperature drop across the heat pump heat exchanger is either 3.5 °C or 7.0 °C – potentially extracting double the heating power.

In this case the larger temperature drop causes a roughly 10-fold increase in the rate of condensation

The reason for the shape of the curves is that:

  • At low external temperatures, the heat pump must run at high power and so extract heat from a larger volume of air.
  • At low external temperatures, the amount of water in the air is much less than at high temperatures.

Together these two factors combine to produce maximum condensation at temperatures between 5 °C and 10 °C.

Click for a larger version. The graph shows the amount of condensation (litres per day) expected when a heat pump is operating at the external temperature shown so as to maintain an internal temperature of 19 °C. The thermal power at 0 °C is 6 kW and heat pump is assumed to cool the air by ΔT = 3.5 °C  or by ΔT = 7.0 °C. The relative humidity of the air is assumed to be 85%. Notice that cooling the air more drastically increases the amount of condensation.

Non-combatants may wish to stop reading here.

But for those interested, I will explain the calculation below.

Click for a larger image. Spreadsheet for calculating the amount of water which condenses from a heat pump. The text below explains each column in the calculation. The actual spreadsheet is downloadable from a link in the text.

The basic inputs are the shown in red text with a yellow background.

  • The desired internal temperature (19 °C)
  • The thermal power required to maintain 19 °C when the external temperature is 0 °C. (6000 W = 6 kW)
  • The Coefficient of performance of the heat pump (3) which is assumed to be constant.
  • The humidity of the air (85%)
  • The amount (ΔT) by which the heat pump cools the air (3.5 °C)

Column 1: shows the external temperature.

Column 2: shows the temperature demand, the difference between the internal and external temperatures

Column 3: shows the thermal power required to heat the dwelling, assuming it is proportional to temperature demand.

Column 4: shows how much thermal power must be extracted from the air based on the COP.

Column 5: shows the volume of air per second that must be cooled by ΔT in order to extract the required heating power. More air flow is required at low temperatures as the heating demand increases

Next we work on the humidity

Column 6: shows the specific humidity of saturated air with the numbers entered from a data table. This expresses the maximum density (in grams per cubic metre) of water that air can hold without condensing.

Column 7: shows the the same quantity as column 6 but derived from a formula designed to closely match the actual data. This allows me to interpolate between the points in the data table.

Column 8: shows the specific humidity of the air under consideration i.e. with relative humidity less than 100%.

Column 9: shows the specific humidity of saturated air which is ΔT colder than the external temperature.

Column 10. If the specific humidity of the actual air (Column 8) exceeds the specific humidity of saturated air at its new lower temperature, then condensation will occur.

Column 11. If condensation occurs, then the excess water (the difference between columns 8 and 9) will become liquid.

Column 12. Expresses the condensation per cubic metre in terms of condensation per second.

Columns 13, 14, 15 and 16. Expresses the condensation rate in terms of litres per second, per minute, per hour and per day respectively.

Heat Pump – First Operational Data

August 14, 2021

Click for a larger version. The heat meter estimates the heat delivered by the heat pump by measuring the flow of hot water [in kilograms per second] and the difference between the temperature of the water delivered by the pump (T1), and the temperature of the water returning to the pump (T2).

Friends, please let me tell you about the first data I have on the operation of the new heat pump – a 5 kW Vaillant Arotherm Plus.

[Edit 6/9/2021: Initially I stated this a 7 kW version because I had forgotten that in the end I opted for the lower power version]


As part of installation, I paid for a “Metering and Monitoring Service Package” (MMSP) which monitors: the heat delivered by the heat pump; the electrical energy it consumes; alongside the local internal and external temperatures.

All this data is measured every 2 minutes (!) and then whizzed into The Cloud where I can view and download it.

Just as importantly, the data is aggregated by Ofgem (Office of Gas and Electricity Markets) who can then assess real world performance of heat pumps ‘in the field’. And Ofgem will – I hope – eventually pay me for the data!

Aside from electrical power measurements, the key element of the monitoring system is a heat meter, whose operation is illustrated at the top of this article.

This clever device integrates several measurements:

  • The temperature of the water delivered by the heat pump
  • The temperature of the water returning to the heat pump
  • The flow rate of the water.

…to estimate the heat delivered by the heat pump.

Click for a larger version. The Sontex Superstatic 449 heat meter installed near the hot water cylinder. The meter indicates that since installation, the heat pump has delivered 78.533 kWh of useful heat.


At this time of year, we don’t need any space heating so the only way to assess the performance of heat pump is for heating domestic hot water (DHW).

The MMSP can detect whether electrical power is applied to the 3-way valve (see picture at the top) and so can tell if the hot water is being delivered to the radiators or the DHW tank.

The heat pump is set to heat the DHW tank between 3 a.m. and 4 a.m. each day. The data below is from the early hours of 13th August 2021.

The graph below shows electrical power drawn by the heat pump (watts), the thermal power delivered (watts), and the temperature of water (°C) versus time. The water temperature should be read against the right-hand axis.

Technical Note I have smoothed the power data by averaging it over 10 minutes to make it easier to see what’s happening.

Click for a larger version. Graph showing the operation of DHW heating cycle. The electrical power drawn by the heat pump (watts) and thermal power delivered (watts) are shown against the left-hand axis, and the temperature of the water (°C) is shown against the right-hand axis.

The first thing to notice is that the thermal power delivered by the heat pump is larger than the electrical power consumed. This is the ‘magic’ of heat pumps. The extra energy is drawn from the outside air which is cooled by about 3 °C in the process.

The second thing to notice is that initially the thermal power delivered is high (peaking at nearly 3.8 kW) and the electrical power is low (just under 1.0 kW). But as the water temperature increases from 30 °C to above 50 °C, the heat pump has to work harder (electrical power increases) to deliver slightly less heat.

This is the nature of heat pumps – they work best when heating lots of water through small temperature differences rather than heating small amounts of water through large temperature differences.

The ratio of the heat energy delivered to the electrical energy used is called the Coefficient of Performance or COP. This is shown on the graph below.

Click for a larger version. Graph showing the operation of DHW heating cycle. The Coefficient of Performance (COP) is shown against the left-hand axis, and the temperature of the water (°C) is shown against the right-hand axis.

For most of the heating cycle the COP is above 3 and almost reaches 4 when the water temperature is about 40 °C. The spike in COP at the end of the heating cycle is probably an anomaly caused by the smoothing of the data, and the fact that heat is delivered from pre-warmed pipes after the electrical energy was reduced.

Averaging the electrical power drawn by the pump over the whole day – the standby power is 12 W – the effective COP is around 2.8 i.e. the heat pump provided me with 2.8 times more thermal energy than the electrical energy I used to power it.

For those of you unaware of other household developments, this electrical energy came from a battery which stored solar power generated earlier in the day. So our hot water is 100% carbon dioxide free in the summer.

Future Improvement 

I am pretty happy with this performance. But I think it can still be improved.

Firstly, there is a four metre section of pipes between the heat pump and the DHW cylinder which are still not insulated. After insulation, more heat should be delivered and the overall COP should increase.

Secondly, I need to see how the temperature of the hot water in the taps and showers is affected by lowering the hot water storage temperature. I think somewhere between 45 °C and  50°C might be acceptable and that should improve the COP still further.

Finally, when used for space heating I am hoping to keep the temperature of the water circulating through the radiators as low as possible – perhaps 45 °C will be possible – which should again improve the overall COP. To achieve this I may need to change one or two of the older radiators. But that is a problem for the autumn.

For now I am enjoying the wonder of thermodynamics in action.

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