Friends, I was peacefully preparing a tasty lentil curry the other evening while listening to the PM programme on Radio 4 when I heard an interview with a person talking nonsense about heat pumps.
Later, having recovered from my distress, I thought I might positively focus my frustration at what passes for intelligent comment on the radio these days by explaining why some people have difficulties with heat pump installation. It’s not about heat pumps themselves, it’s about hydronics. Hydronics?
What is Hydronics?
Hydronics is what engineers call plumbing (Wikipedia). It is the discipline that allows engineers to craft systems that move heat from one place to another via water flowing through a system of pipes and radiators.
Heat pumps themselves work fantastically. This is just a fact a life. They really do meet their specifications which typically means that 1 kWh of electrical energy will (on average through a typical winter) result in the production of between 3 kWh and 4 kWh of hot water for use in a central heating system. I’ve written about how heat pumps achieve this feat in these articles (1 and 2), both of which include videos.
And heat pumps will keep working even in extreme cold. That’s why they are popular in places that experience much colder winters than the UK (European Heat Pump Association). And they can heat any dwelling that can be heated by a boiler.
However, some people do experience problems with heat pumps. This is because the hydronic system connecting the heat pump to the house has not been properly configured.
The hydronic system is required to achieve two distinct, but linked, tasks.
- The first task is mainly mechanical: it is to take the heated water from the heat pump and move it around the radiator circuit in the dwelling, and return it to the heat pump. This circulation is powered by the water pump inside the heat pump. If the pipes in the hydronic system are too narrow then the pump may not be powerful enough to circulate water at the appropriate rate.
- The second task is mainly thermal: as the heated water flows through the radiators or under-floor-heating tubes, the circulating water transfers its heat to the dwelling and cools, returning to the heat pump approximately 5 °C cooler than when it started its circulation. If the radiators are too small, then not enough heat will be transferred to the dwelling.
In practice the two tasks are linked and a well-trained heat pump engineer will be able to make sure that the hydronic system is suitable for use with a heat pump. Let’s consider the mechanical and thermal requirements of the hydronic system separately.
Hydronic circuits for boilers and heat pumps: Mechanical Considerations
Let’s imagine that a home is currently being successfully heated with a gas boiler. Typically such a boiler will have a maximum heating power of 20 kW or more. The maximum heat loss in a typical UK home is around 7 kW, so such a boiler is easily able to heat most dwellings. In fact it typically cycles ‘on’ and ‘off’, heating the home episodically.
Click on image for a larger version. The graph shows typical ‘duty cycles’ for a gas boiler and a heat pump, both delivering 7 kW of heating to a dwelling. The gas boiler typically cycles on and off, but the heat pump typically operates continuously i.e. a ‘duty cycle’ of 100%.
When the hydronic system of pipes and radiators is set up, it is typically adjusted so that the water returning to the boiler has cooled by about 10 °C. The flow rate of hot water required to realise an average output of 7 kW of heat is approximately 10 litres per minute, or 600 litres per hour.
If a heat pump is to similarly deliver 7 kW of heating, the hydronic system of pipes and radiators is set up it is typically adjusted so that the water returning to the boiler has cooled by about 5 °C i.e. half the temperature drop for a boiler. The flow rate of hot water required to realise 7 kW of heat is now approximately 20 litres per minute i.e. twice the flow rate required when the boiler was used.
Click on image for a larger version. A schematic diagram of heat pump installation. Hot water is delivered to the home through a large diameter (22 mm or 28 mm) insulated pipe and returns to the heat pump after passing through radiators, approximately 5 °C colder.
Because the hydronic system attached to the heat pump has to circulate water at twice the rate of the hydronic system attached to the boiler, the water must necessarily flow faster through the pipes. Unfortunately, the resistance of the pipes to flow varies (roughly) as the square of the flow speed. So doubling the flow speed results in (roughly) four times the resistance to flow. This means that doubling the flow speed typically requires four times the pressure. This can be a challenge for pumps.
Let’s look at some examples to see what a challenge this can present. I calculated the pressure required from the pump using this handy calculator.
Suppose the main pipe connecting a boiler to the radiator circuit is 10 m long and the return pipe is also 10 m long, a total length of 20 m. If the pipes were standard 22 mm diameter copper tube, the internal diameter of the tube is around 20.2 mm. Neglecting the restriction of the radiators themselves, to circulate 10 litres per minute through these pipes would require the water to flow at a speed of 0.52 metres per second and require a pressure differential (known as a pressure ‘head’) of 0.041 bar. [See note at the end of the article]
If we simply replace a boiler with a heat pump, then to deliver the same heating power as the boiler we need to circulate 20 litres per minute i.e. the water must flow at a speed of 1.04 metres per second, which would require a pressure differential of 0.137 bar i.e. the pressure head required is 3.3 times larger – not quite a factor of 4.
It may be that the heat pump cannot deliver such a large pressure head, in which case, the hydronic circuit would be unable to circulate water at the required rate.
There are three solutions to this problem.
- The first solution is to reduce the heating demand – by insulation – so that the required flow rate is reduced. This is generally a good idea, but is really a separate project in itself. And in some cases, it is not possible.
- The second solution, is to use larger diameter pipes in the heating circuit. If the 20 metres of pipe were replaced with pipe of an outer diameter of 28 mm (inner diameter 26.2 mm) then the required flow speed would fall to 0.62 metres per second and the pressure differential would be only 0.041 bar ( the same as for the boiler). Of course replacing all these pipes might require raising floorboards throughout a house which would be pretty disruptive.
- The third solution, is to install a so-called Low-Loss Header (LLH). I wrote about LLHs back in June 2021 (link). An LLH is a large diameter pipe connected between the input and the output of the heat pump. The recirculating pump generally has no problems flowing water through the LLH – but of course – none of it is going into the heating circuit! To heat the house an additional recirculating pump draws hot water from one end of the LLH, circulates it through the radiators, and then returns to the other end of the LLH. This arrangement generally allows the installation of a heat pump in properties in which there are long runs of pipe and still achieves adequate performance without the need to replace all the pipes in a house. However, it is very difficult to tune the flow rate of the additional recirculating pump to achieve optimal performance.
Click on image for a larger version. A schematic explanation of the role of a Low-Loss Header (LLH). An LLH is the equivalent of short circuit in an electrical circuit. A heat pump can easily circulate hot water with a very low pressure head through the low loss header. An additional pump taps off a fraction of this water to run through the radiator and returning the water to the LLH approximately 5 °C colder.
So there are solutions for replacing a boiler with a heat pump in so-called ‘difficult’ properties. However for properties with so-called micro-bore central heating, the pipe diameters (somewhere between 8 and 12 mm) are so small that the only option to allow heat pumps to work is to replace the pipework entirely.
In my own home, the external wall insulation reduced peak heating demand from around 7 kW to around 3.6 kW. The installers also installed a low-loss header. Over the last two winters the average coefficient of performance has been approximately 3.5 i.e. 1 kWh of electrical energy resulted in 3.5 kWh of thermal energy.
Hydronic circuits for boilers and heat pumps: Thermal Considerations
Let’s imagine once again that a boiler is currently delivering (on average) 7 kW of heating to a home and that the circulating water cools by 10 °C as it flows through the radiators. We know that this corresponds to circulating water at an average rate of 10 litres per minute (600 litres per hour).
The average heat output of a radiator (in kW) depends on four factors:
- Factor 1: The difference between the average temperature of the water flowing through it, and the temperature of the room: a larger temperature difference results in more heat output.
- Factor 2: The ‘duty cycle’ of the hot water flow: a duty cycle of 100% results in more heat output.
- Factor 3: The frontal area of the radiator: a larger area results in more heat output.
- Factor 4: The number of radiator fins and panels: more fins and panels result in more heat output.
Boilers are easily able to recirculate water at temperatures up to 70 °C. Some modern heat pumps can also do this, but most heat pumps can still only output water at a maximum of 50 °C. However, all heat pumps become more efficient (i.e. are cheaper to run) if they re-circulate water at lower temperatures, ideally less than 40 °C.
A lower circulation temperature means that Factor 1 above is reduced. In order to compensate for this and still maintain the same heat output as a boiler, it is necessary to increase one or more of Factors 2, 3 or 4.
- Factor 2: A heat pump typically circulates water continuously whereas a boiler often switches on an off, so this automatically compensates to some extent.
- Factors 3 & 4: Increasing the size (frontal area) of radiators, and increasing the number of panels and fins will increase the heat output for a given flow temperature.
So there are many solutions to allow the use of a heat pump in a property with inadequate radiators. An Excel™ spreadsheet that quantifies how Factors 3 & 4 affect heat pump performance can be downloaded here and is illustrated below.
Click on image for a larger version. Extract from a spreadsheet enumerating factors 3 & 4 in the text. The maximum allowable flow temperature is set to be 40 °C. The graphic shows that it is not possible to output 7,000 W (7 kW) of heat even with 20 m^2 of the simplest type of radiator (Type 10). However, it would be possible with 9 m^2 of Type 22 radiator. Alternatively if one used 20 m^2 of Type 22 radiator, the flow temperature would fall below 30 °C, resulting increased efficiency and lower running cost.
In my own home, I changed none of the radiators, and the house is heated easily by the heat pump with a maximum flow temperature of just 43 °C. If I had elected to change some radiators, then this flow temperature might have been reduced further, with a concomitant increase in coefficient of performance i.e. the system would be cheaper to run.
Hydronic circuits for boilers and heat pumps: Overall
So let’s summarise: Heat pumps work as advertised and deliver multiple kilowatt hours of heat for each kilowatt hour of electricity used to operate them.
When installed in new homes or as part of large scale retrofit projects, there should be no problems in making them work with seasonally averaged coefficients of performance of 4 or greater.
However, when considered as a straight replacement for boiler, the hydronic system to which they are connected may need some modifications to ensure optimal performance.
Additional Note
When a boiler circulates episodically and outputs more than the average heating output for a short time, the flow rate is also greater i.e. the pressure head required is also larger. Thus the discussion in the text somewhat exaggerates the problem.
Protons for Breakfast 
August 2, 2023 at 2:56 pm |
A big problem for heat pumps/air conditioning in climates like the UK is the humidity in the air causing the external condenser fins to form frost and ice over. The heat pump then has to use power to defrost the fins at times like these the efficiency of the heat pump can be little better than a resistive element. Always good to site a heat pump on a south facing wall.
August 2, 2023 at 4:37 pm |
David, Good Evening.
Yes & No!
I agree that when heat pumps form frost on their condenser they do need to stop heating a home intermittently. They reverse their mode of operation and heat the condenser to melt the frost. This pauses heating to the home.
However, as the water vapour freezes on the condenser it releases extra latent heat which is used by the heat pump. So when it reverses its mode of operation and melts the frost, it is really returning the ‘extra’ heat it harvested previously.
I had not thought about the south facing wall option, but yes, on summer days in winter the heat pump may be able to benefit from some ‘solar gain’.
All the best
Michael
August 2, 2023 at 4:35 pm |
I heard PM that evening and the nonsense spouted by Lord Hockey. I had my annoyed email read out in response the following evening when they made up for it by responding to the deluge of people complaining that the positive side of heat pumps had been missed and the programme thankfully made up for that omission by questioning Lord Hockey’s comments the following evening and relating the comments of many heat pump owners. They also plugged the programme on BBC1 that evening about heat pumps which was interesting. Both PM and BBC1 yesterday were well worth listening to and watching and made up for the day before.
There is a problem with the take up of heat pumps and also media reporting. Part of the problem as with any technical subject is that your average journalist doesn’t feel qualified to call out misleading statements on the subject made on the fly in an interview.
The other more insidious problem is that for builders and suppliers of both heat pumps and gas boilers (Lord Hockey claims to sell both) is that they make more money from installing gas boilers because because there is no incentive for them to sell heat pumps and they can shift more gas boilers because they are quicker and easier to install and builders are well practiced in gas and may lack the training for a proper heat pump installation. The builder and suppliers don’t see the return on the investment – the savings go to the home owner.
I spoke to sometime in charge of social house building in the North West and he explained that despite new builds meeting Part L of the building regulations, so should well suit a heat pump, it’s cheaper to fit a gas boiler for them.
The incentive and investment reward only goes to the home owner at present. There’s no incentive for the builder and they only see potential problems down the line being called to fix issues with a badly installed heat pump or the education issue of customers complaining that their radiators don’t get stinking hot like they’ve been used to. So why take the risk?
Of course the Telegraph, Mail and Express love to talk down and politicise anything they can turn into a culture war.
This situation needed to change.
August 2, 2023 at 4:47 pm |
John, Good Evening.
And thank you for your intelligent comments. I had not really thought about the incentives from the perspective of builders and installers. My initial thought is that this is where regulation comes in.
Part of the problem arises from the fact that many people secretly think that it’s OK for homes to emit TONNES of CO2 each year. In my perspective it’s the equivalent of allowing homes to empty raw sewage into the street. Of course, installing drains and dealing with sewage costs money, and the only way to make people do that is to make it unacceptable to spill sewage in the street.Similarly, it just needs to be made illegal to dump TONNES of CO2 into atmosphere.
The installer problem is just one of many problems, but in my opinion it’s the job of government to create the environment where it makes financial sense for people to do what’s best for us all collectively.
Best wishes
Michael
August 3, 2023 at 10:27 am |
Michael,
A really excellent and well written piece. Thanks for spending the time composing it. I’ve followed the heat pump debate for many years and have read many articles and watched many videos on heat pumps. None of them has been quite as clear as your piece here.
Mark Brinkley
Author of the Housebuilder’s Bible
August 3, 2023 at 11:01 am |
Mark, Good Afternoon.
I’m blushing. Thank you for taking the time to let me know. As you no doubt know, writing something clearly takes ages – I am happy someone noticed!
Best wishes
Michael
August 4, 2023 at 7:54 am |
One minor factor you didn’t mention is the change in the viscosity of water with temperature. Reducing the circulating water temperature from 60˚C to 40˚C increases its viscosity by approximately 40% [1], increasing the resistance to flow in the same proportion. So the circulating pump will have to work harder, which should be taken into account when designing the hydronic system for a heat pump.
Max
[1] CRC Handbook, 97th Ed, page 6-1
August 4, 2023 at 10:49 am |
Max, Good Morning.
And thank you for raising the issue of viscosity. You are quite right, warmer water has significantly lower viscosity.
I am sure this effect will taken into account by heat pump designers, but I doubt people installing heat pumps account for it – even though it is significant.
Best wishes
Michael
Michael
August 4, 2023 at 11:05 am |
Nice analysis, as usual! I had two questions from my non-UK perspective. First of all I was wondering why a new build in the UK wouldn’t use forced-air from a heatpump rather than water (to a radiator on the wall or in the floor) — given that (in my vague memory of learning about Carnot cycles and heat engines almost 50 years ago!) the heat pump will work more efficiently when heating the interior air to the desired temperature *without* the thermal loss of maintaining an additional temperature gradient across an additional heat-transport fluid (e.g. water, in a traditional radiator). Well I guess it’ll all come down to how many metres of hydronics (or airflow) separate the heatpump from the space it is heating, and whether the heatloss from the transport system is all truly “lost” or is heating up a neighbour’s apartment or some bit of flooring in your own house. Anyway I’d *guess* there’s some design space for the only sort of heatpump I see in NZ homes — the forced-air variety, with a wall-mounted unit that’s blowing air past a (small) radiator that’s being heated (or cooled) by a refrigerant (such as R134a) that’s being cooled (or heated) from ambient air outside the house.
I also recalled — not very fondly — the steam radiators in Duluth MN. The weather is seriously cold there in winter. 7kW would be enough to heat a well-insulated room in winter but as soon as you open the door, well, the air (sometimes as low as -30 degrees C) would blow in… so we had essentially airlocks at all entrances (a front door, a small anteroom, and another door that’d open into the house) and boilers which would heat the water hot enough that the little steam vents would hiss pleasantly when the heat was on… thereby raising the inside humidity to a level where it was safe to kiss your partner *if* you always remembered to touch your hand to some less-tender part of their body before you touched lips. Touching a metal door handle was always a bit of a gamble! Ah such memories. Anyway: since it takes only a 7kW (24000 BTU) boiler to heat an average (poorly insulated?) UK home in your region, then the design space for interior heating is quite quite different than in Duluth MN, or for that matter in Auckland NZ (where I — like many — don’t mess around with heating a bedroom or even a kitchen, but do like a warm living room & dining area… and 2.5kW of heating capacity from a small forced air wall-mounted heatpump is plenty for that. Anyway I suppose that’s all irrelevant to you in your part of the UK!
August 4, 2023 at 1:03 pm |
Hi. Good Afternoon.
You are quite right. Modern homes *should* need very little heating and Air-to-Air seems or forced air systems would be ideal. One would still need to deal with the hot water, but there are self-contained that pump cylinders.
But unfortunately, the building regulators have effectively been captured by the building industry and our building regulations as regards energy efficiency are poor.
The focus on air-to-water systems is because they are (to some extent) a direct replacement for gas boilers when retro-fitted into a property. They provide space heating and hot water.
Unfortunately, heat pumps have got caught up in a culture war with right-wingers stating they absolutely don’t work.
It can be depressing at times.
Anyway, best wishes: I hope you enjoy the end of your mild winter.
M
August 5, 2023 at 12:26 am |
BAU + CH4 -> FUD. A strongly exothermic reaction in the UK’s current political climate, it would seem! No photons emitted, merely a surplus of low-grade “waste” heat which won’t spin any turbines. The FUD is fuelled, in part, by the poor COP of many of the heat-pump systems fitted in UK homes by early adopters. Reading between the lines… it would seem that many of the poor-performers were in homes whose owners would take long hot showers without realising how many additional litres of not-as-hot water from their home’s retrofitted boiler they were consuming? In the NZ context — and I have *no idea* whether it’s applicable in your part of the UK — it’s a no-brainer to put a small PV array on the rooftop of a home with a bog-standard old-fashioned electric HWC that’s fitted with a “solar smart” controller. Even a few hundred Watts of “spare” PV power will (slowly) heat the water in the HWC; and to my way of thinking this is a *much* better outcome than “dumping” the household excess PV power onto its local line during mid-day… i.e. pushing down the middle of the “duck’s back” (which is increasingly problematic in some parts of the world, notably California, Hawaii, the Far North of NZ…).
There’s a nice graphic of the yearly-average duck’s back in California over the past decade at https://www.eia.gov/todayinenergy/detail.php?id=56880.
Again… I have no idea whether your part of the UK has local lines that are suffering from a “duck’s back” syndrome but … even if you don’t, I do find myself idly wondering whether it’d be appropriate, given whatever solar irradiance you have in your part of the UK, to oversize a PV array so that it can reliably keep your heatpump HWC hot enough for hot showers without sucking power from the grid, or whether you could save a lot of capex (and quite possibly also reduce carbon footprint) with a smaller PV array, a resistive HWC, and a solar-smart controller on the HWC. I do realise that any heat-pump HWC will have a significant COP advantage over a resistive HWC but is it economic for either $ or CO2e? Oh… ah… yes… in a retrofit of a boiler on a house which had been supplying water for both radiators as well as for use in showers and taps, there’ll be significant costs in separating the plumbing for these two uses. The devil is always in the details when you’re trying to shift off BAU!
August 5, 2023 at 2:15 pm |
Good Afternoon. Yes! For people with no battery, a solar diverter (as they are called in the UK) makes excellent sense if a household has any DHW storage at all.
And yes, there are a lot of options. In general, I would recommend to put up as much solar as they can conceive of as fast as possible. My experience is that nobody laments that they have too much!
In my case, in summer the solar PV powers the heat pump directly which heats water to 55°C with a COP of between 3 and 4. So the 20% efficiency of the panels and the 300% efficiency of the heat pump combine to heat water with around 60% efficiency. If I had solar water heating directly, I would also have around 60% efficiency, but I would be able to heat water hotter and store more energy.
In winter, we heat water at night to take advantage of cheap electricity and the COP is nearer to 3.
There are many options and it can be bewildering for people trying to do the right thing without the ability to analyse teh different options.
Are you enjoying ‘spring’ yet? Or is t still ‘winter’? M
August 7, 2023 at 2:18 pm |
Another fantastic article. Very clearly explained, thank you.
August 7, 2023 at 4:37 pm |
Paul, Thank you: I am glad you enjoyed it 🙂
M
November 1, 2023 at 2:05 pm |
Hi Michael,
I missed this one back in the summer!
Can you point me towards the equations for calculating the temperature drop (and thus heating power) across a radiator of a given surface area?
I’m struggling to get my head around *exactly* why the DT is typically around 5° on a heat pump and 10° on a gas boiler. On my combi, the difference in outlet and return temperature is usually around 12°, despite the outlet temperature being ~43°C. My radiators are single-panel/single fin! My low desired temperature – 19° – probably helps.
November 1, 2023 at 2:33 pm |
Dan, Good Afternoon.
Why is DT 5 °C for a heat pump and 10 °C for a boiler? I think it must be connected to the efficiency of heat transfer to the flowing water within the boiler/heat pump. The much higher temperatures of the combusted gases allows good heat transfer to water even with a simple heat exchanger. For a heat pump, the working fluid might be at (say) 60 °C and so to transfer heat to flowing water at (say) 30 °C requires a complex heat exchanger with lots of surface area (=expensive) for heat exchange. So I think the nominal 5 °C is a compromise that optimises heat transfer within the heat pump.
For a radiator one needs to solve two equations.
The Room-Radiator heat transfer equation is that the heat output is proportional to the average temperature of the radiator surfaces minus the average room temperature. This arises from radiative heat transfer from the front surface of a radiator and convective heat transfer from each vertical surface. I wrote about the physics of radiators here
This gives the heat output for a given average surface temperature.
The Fluid-Radiator heat transfer equation is that the heat transfer is proportional to difference between the inlet and outlet temperatures and the flow rate and the heat capacity.
Heat Transfer (Power) = Flow rate x Heat Capacity x DT
Notice that this second equation involves DT not T_room or T_radiator_ or T_flow. You need to solve the two equations together to find out the temperature at which the two equations are satisfied.
It’s simple physics, but it does my head in each time I think about it!
Enjoy
M
January 13, 2024 at 4:13 am |
Hi Michael,
Big fan of your blogs, they are very very helpful.
I’m in Melbourne, Australia and it’s quite an immature market for Hydronic heating and even more so with Heat pumps.
We are ripping up our subfloor (suspended timber) in our old Victorian double brick as it needs to be replaced and are planning on installing hydronic heating (and hopefully cooling) via radiator panels (sensible cooling) at the same time.
I’m finding it hard to understand the required heating kW’s for the whole house when I don’t have a current gas system to calculate from (using your other spreadsheet). If I base it off ~1.5kW per room (as per other online calculators) then I’d be requiring 2 x large type 33 units in each room to achieve the required square meterage your spreadsheet indicates, this would be a fairly big inconvenience finding the space. Could I be over-estimating the kW’s required or is it that using radiator panels won’t be the best option and I should look at fan coils (more expensive but should only require 1 reasonable size unit per room instead of 2 bulky radiators)?
Being in Melbourne, our coldest winter days would be 10deg maximum for the day (it can get down to 2-3deg at nights but maybe only 3-4 times a year).
Any assistance would be greatly appreciated.
Kind regards,
Ben
January 13, 2024 at 5:43 pm |
Ben, Good Evening,
Well Melbourne is a long way away but I seem to recall being told it has a mild climate with wet winters. I have just looked up Melbourne on https://www.degreedays.net and the climate looks to have about half the heating demand of an equivalent home in the UK.
So my first thought is that such large radiators are rarely needed in the UK so they are likely to be more than is required in Melbourne. But it really is hard to offer advice from this distance.
I have lots of questions to ask before I can really offer sensible advice.
1. How was the house heated previously? Electricity? Can work out how much extra you consumed in winter?
2. Do you know the construction of your home? Can you find out the U-values (UK terminology) or R-values (US) of the room/home and estimate heating demand.
3. The output of the radiators depends up on the temperature of the water flowing in them. I wrote about this in the article below.
What flow temperature are you designing for?
4. In the UK a typical old home has a heating requirement of 100 to 150 kWh per year per square metre. These are very ball park figures – but in Melbourne a typical old home would probably be half of that. You might use that to make a guesstimate of your annual heating requirements.
5. Remember that if you cool radiator panels below about 10 °C, they will collect condensation. If summer cooling is a big part of your requirements, then fan-coil units which can have a drain fitted might be a better option.
6. If you are taking up the floor, are you considering insulation the floor, or even using underfloor heating?
Enough questions!
Best wishes
M
January 14, 2024 at 3:11 am
Hi Michael, Thank you for your prompt response. You are right, Melbourne is a long way away!
The property this is regarding is one we’ve just purchased but settlement isn’t until April so I’m up to my neck in planning what needs to be done to the house before we can move in and can start straight away with a plan in place. (it needs a bit of work)
As a result, we have no consumption data to go off unfortunately.
1 – I don’t believe there is any heating or cooling currently in the property, it’s about 150m2 of floor space with 3.9m ceilings. (There are 5 fireplaces though…)
2 – I’ve found some information regarding U-values just based off the external walls which is 1.96. So with roughly 200m2 of external wall, the heating requirement for 10degree external difference would be 3.9kW/h and with a 15 deg difference it would be 5.9kW/h. That doesn’t including ceiling, flooring, windows though. (https://anewhouse.com.au/2013/11/insulation-basics-double-brick-walls/)
3 – Your article regarding the radiators and heat pumps was very helpful, mainly the points about heating requirement per day and the conversion from kW/h to watts. I think this was a key calculation I was missing, however it’s really hard to judge as I don’t have any data to go off. I tried using our current situation but the house is poorly insulated (lots of drafts and lots of single pane glass) and we use an inefficient gas ducted heating with our consumption averaging 110kw/h per day during winter but thats not with the heating running 24 hours a day, normally between 7hrs (work day) to 14hrs (weekend), and that figure includes consumption by our gas instant hot water system, so it gets complicated….
I’d be looking at a flow temp of 45deg unless I’m advised otherwise. It seems they are scoping systems using 50deg here in Australia from speaking with a couple of retailers.
4 – I was thinking I’d try and guesstimate based on best effort and research of similar property builds, insulation types and take in climate factors. Can you tell me what the kW/h per year calculation would mean? Do you divide by 365 to get daily requirement? so 150kW/h a year for 150m2 would equal 61kw/h per day, so 2,541 watts?
5 – The cooling would be more of an experiment to begin with, to see if any additional cooling would be required or the slight affect of “cool” radiators would be enough (maybe add some ceiling fans). I understand I’d need a humidity sensor to be able to calculate the lowest temperature we could run the flow at before condensation would occur. If 10deg is about that number then I suspect it would be a pretty good outcome on a dry 40deg Mebourne day!
6 – I had a brief look at underfloor heating but we want to use solid timber floor boards ontop of red-tongue in some areas (hallway/Living) and carpet ontop of red-tongue in the bedrooms. Costs and effectiveness don’t look like they add up in this case. I’ll be insulating the subfloor with R2.5 batts.
I’ve been looking at 10kw to 12kW ASHP’s which might be a little overkill for now but we would like to undertake an extension to the rear of property in a few years time which will increase the heating requirements by a little (150m2 floorspace increasing to ~190m2), using 25mm pex piping setup in a home-run configuration and type 22 radiators. I’ll be DIY’ing all flooring, piping and radiator installs. Everything but the ASHP install really.
Is 12kW massive overkill? Do you think a 7kW to 9kW would be sufficient?
January 14, 2024 at 2:27 pm
Benjamin, Good Afternoon.
It’s clear that there are many uncertainties in your project, most notably that you have no real idea about the maximum heating requirement. I’ve read through each point you have made, but I got only more confused the more I read! So here are my suggestions.
1. Starting with the Heat Transfer Coefficient (HTC). My UK home (162 m^2 over 3 floors with one-shared wall with a neighbour) used to have a (HTC) of around 300 W/°C. That means that to heat the home 1 °C above the outside temperature required 300 W of power. So, to heat the home 20 °C above the outside temperature of 0 °C requires around 20 × 300 = 6,000 W or 6 kW.
2. The HTC depends just on the construction type, level of leakiness etc. If this home (before I insulated it) were in Melbourne then – given your generally milder winters 6 kW would seem like a good first guess.
3. When installing a heat pump from scratch, remember to use large diameter pipes (typically 28 mm OD in the UK) from the heat pump to the central heating cylinder and heating circuit.
4. I would suggest you consider dealing with your heating and cooling needs separately. This is what I chose to do in my home. There was such confusion about the heat pump installation (it was during COVID) that although the heat pump can also cool, I opted not to use that function. Instead, I installed a small mini-split air conditioner (2 indoor units) which cools our bedroom and the main hallway. This simplified things conceptually – and has proved important in summers providing better cooling that the heat pump could have. I had also envisaged that if my heat pump calculations were very wrong, the AC could be used for supplementary heating in winter.
5. So space heating and hot water would be covered by a hydronic system, and cooling would be dealt with by a modest AC installation – which presumably you would run off solar power and a battery in the summer?
Finally, Melbourne is big enough that there is a probably a nearby expert in thermal assessments. Members of Greenpeace or Friends of the Earth are often know about such folks. In the UK it costs about £250 to have someone survey a house and make an estimate of the heat loss from a home and the heating requirement in each room. Might be worth doing before your start. There are also calculators online for doing the same e.g.
https://heatpunk.co.uk/home
Here you can draw your home and tell the system information about the walls and floors and roof. It’s tedious – but that’s why you can just pay someone to do it for you!
Also in the UK we have site which monitors heat pumps remotely and shows how they perform. My heat pump only has a SCOP of 3.6 – keeping my home at 20 °C all winter – which I used to think was good, but the heat pump monitor has data on installations in cold places in the UK with a SCOP of 5! This leads to low running costs.
https://heatpumpmonitor.org
Enough! I have another enquiry from Slovenia to deal with now!
Best wishes with your endeavours! M
January 15, 2024 at 4:07 am |
Hi Michael,
Firstly let me thank you for your generosity for giving me your time in writing such detailed responses.
They have been immensely helpful!
The heatpunk website is of great use, I’m coming up with around 6,500 W of heat loss for the 150m2 at 5 deg which is about what I was thinking which is great.
I’ve done a lot of reading around flow rates and pressure from your blogs and have noted the pipe diameter requirements, amongst many other things!
The heat pump monitor site is very interesting, I suspect the SCOP 5.0 configs would be underfloor installations..?
Anyway, thanks again. I’ll let you know how things pan out in a few months time!
Re: Cooling, I’m going to look into ducted fan coils such as Carrier Idrofan systems which are usually for commerical setups but I don’t see why it can’t be used in residential. Plan on setting up it’s own zone and then turn off radiator zone when cooling is in operation. Need to do some more research though.
All the best.
Regards,
Ben