I've written up a few blog posts on the journey!

https://tomasmcguinness.com/2026/08/18/my-heat-pump-installation-part-1/

https://tomasmcguinness.com/2026/08/28/my-heat-pump-installation-part-2/

https://tomasmcguinness.com/2026/09/04/my-heat-pump-installation-part-3/

Hoping someone here has hit this before, because I've run out of things to check against the manual.

Setup:

• Heat pump: Hitachi Yutaki M, RHUE-3AVHN, mono-valent (CONF = 1)
• System Controller: XEK23232A
• DHW via diverting valve (P1 = 1) — relay on the controller's DHW output closes an external motorized valve to isolate radiators during DHW loading
• All original Hitachi parts (heat pump, controller, room controller, RF unit, sensors), bought ~2 months ago as an unused/uninstalled unit. Self-installed (DIY).
• No timer input, no DHW electric heater fitted.

Current parameters: CONF 1, P1 1, P2 0, P3 5, P4 2.0, P5 15, P6 55, P7 1.0, P10 45, P11 5, P12 10, P21 20, P22 2, P23 20, P24 0, P25 0, P26 OFF, P27 4.0, P30 1, P31 3, P34 45

DHW works fine for a while (heats to 45°C, reheats correctly when it drops a few degrees), then randomly just... stops trying. TDHW will be sitting way below the P10‑P11 (40°C) reheat threshold — I've seen it stuck at 19°C — with SDHW = 0, relay de-energized, heat pump in standby, and FAUL = 0 the entire time. No fault, no error, it just doesn't register any DHW demand at all.

The only fix I've found is a full power-cycle of the System Controller — the moment it comes back up, it immediately starts heating the tank correctly (confirmed via power consumption monitoring). It's not slow to respond either — I measured a 19°C → 34°C rise in 21 minutes after a restart, so a full recovery clearly finishes well under an hour. That rules out the "Maximum DHW Loading Time" (P27) safety timeout as the cause, since there's no way a normal reheat cycle is running anywhere near that long.

What I've already ruled out / fixed along the way:

• P26 (Summer Switch-Off) was blocking all heating in summer — fixed, set to OFF.
• P30 (No-Load Function) was disabled, causing pointless cold-water circulation with zero demand — fixed, set to 1.
• Wiring/relay confirmed correct (on the DHW output, not the heating output).
• No parameter changes between a successful reheat cycle and the next failed one — it just silently stopped registering demand.

Question:

Has anyone seen the System Controller silently stop registering DHW demand (SDHW stuck at 0) despite TDHW being clearly below the reheat threshold, with no fault code, that only clears on a power-cycle? Is there some internal state/timer not covered in the SMGB0066 service manual that could cause this? Any pointers on how to monitor or reset it without a full restart?

Thanks in advance!

It's been a walk through treacle to get installers to come out and quote, but I finally have two to compare against. I also have a figure from a third (no report) and a stupidly high quote which we rejected. I also did my own heat loss survey using HeatPunk.

So, in order of size

Installer A   : 14.7KW - Tape measure and HeatPunk - rejected on grounds of price 

Installer B   : 13.7KW - Lidar and Spruce 

Myself         : 12.3KW - Tape measure and HeatPunk

Installer C   :  10.7KW - Lidar and Spruce

I've done a load of cross referencing (with much help from knowledgeable people on this forum, thank you once again) of my annual oil usage and wood stove : ~ 11.3 KW

Installer B is adamant that many of our rooms will be too cold unless we have a flow temperature of over 50 degrees and a 15KW Grant heat pump. He also says that our main bedroom is too cold and will need an additional radiator. That we currently run the UFH in that room, and most rooms, at about 45 and it is toasty in the winter makes no sense to us. He keeps saying that MCS requirements are very different to what we may find acceptable. This same installer is advocating ACH figures of between 0.7 and 1.7 (average 1.03), seems rather high? The house old, yes, but it not draughty. This is a third party doing the heat loss before passing it to a chosen installer to do the work - the installer will take overall responsibility from the point that the contract is signed, the heat loss surveyor walks away. Heat Pump offered is the 15KW Grant.

Installer C is in agreement with us that if the rooms are remaining warm enough in the winter with reasonable flow temperatures, then we are probably fine. His ACH figures range between 0.5 and 0.9, with an average of 0.55. This is an installer who will be doing the work through his own company using dedicated subcontractors. He was a heating engineer,  is Heat Geek trained but doesn't operate through them anymore. Heat pump offered is the 12KW Vaillant mk 2. Flow temperature is designed for 45.

Fabric losses vary between B and C , 9,800 with B, 8,300 with C. 

Ventilation losses : B is 3850 and C is 2370.

So the combination of fabric and ventilation losses combine to give the 27% difference between each proposal.

My gut feeling suggests to me that Installer C is the more practical (although I wonder if the figures are trimmed to meet the Vaillant spec) and that installer B is protecting himself  and the installer that he passes the work on to.

I don't have the appetite to get another installer out, even if they agreed to come, it would be Christmas before we get to an installation point.

If you were betting on the right choice, assuming I have given enough information here, where would you put your money?

thanks

I’m taking a slow and cautious step-by-step journey along the path towards getting an ASHP. Below are my thoughts on replacement radiators, whether my current pipework is sufficient, and whether my hot water tank should be replaced. I’m a research scientist by profession, so like to understand and work things out for myself. However, I am also new to this area, so would much appreciate feedback and advice, including pointing out any faulty assumptions or missteps on my part.

Insulation

Back in 2023, we added as much retrofit insulation as was practical to our 1970’s 4-bed detached (146 sqm). That included toping up the loft insulation, adding underfloor insulation including above the integral garage, switching to high-performance double-glazed units, adding PIR insulation to the walls of the garage, and insulating the timber walls of the dormer. The house is now as well insulated as it reasonably can be.

Heat Loss

I have recently done heat loss calculations, both a theoretical room-by-room approach and a practical approach based on data from the heat energy going into the house and the temperature difference that created. See:

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/adventures-in-heat-loss-calculations/#post-66313

The heat loss worked out at about 6200W with at least 500W provided by other sources (electricity use, people, solar gain in the coldest months), so about 5700W needed from a heating source running for 24 hours per day.

Assuming hot water heating for two hours per day at 6000W, which is plenty for us, leaves 22 hours for heating, which comes out at about 6200W.

A 7kW heat pump would therefore appear to be sufficient. For example, a Vaillant Aerotherm 7kW, which provides its specified output or more at -5C or higher outdoor temperature and flow temperatures in the range 35-55C. It can also provide a flow temperature of 75C for heating hot water.

Radiators

The radiators are next on my to do list. The existing ones are type 10 and type 20 and have a combined output of about 12500W at DT50, at least they might have done when new. Most of them are the same age as the house, i.e. 55, and ready for retirement. Some are likely silted up, since a thermal camera shows the lower middle part of the radiator remains cold for a while on warm up. They often need bleeding and can be noisy – less so now that the gas boiler operates using weather compensation and flow temperatures are no more than 65C.

I have looked at the Stelrad radiator specs and chosen larger K2 radiators to match or slightly exceed the room-by-room heat loss at a flow temperature of 40C, i.e. a radiator temperature of 37.5C, assuming a 5C drop from flow to return. I calculated the radiator performance based on the published DT50 values, scaled according to (X/50)^1.3, where X is the difference between the radiator temperature and the room temperature. This gives a scaling factor of 0.29 for DT20 and 0.24 for DT17.5.

The new radiator for the hallway is under-powered by 15% as there isn’t space for a larger size, and fitting a K3 would be problematic due to the extra width. Taking a pragmatic view, if the hallway only reaches 17C on the coldest days of the year then that’s fine. In practice, I expect it will take some heat from the adjacent rooms, which have overcapacity. Aside from that I was able to match very well the room-by-room heat loss.

In total, the new radiators have an output of 26300W at DT50 and 6300W at DT17.5. By comparison, the lowest temperature that the existing radiators could possibly support is 50C, i.e. a flow temperature of 52.5C. Since, they are old and likely have quite a lot less output than new, that figure is no doubt optimistic.

For the new radiators, the total water content is 117 litres, with an estimated further 18 litres in the relevant copper pipes. Total water volume 135 litres, which works out at about 21.7 litres per kW. According to the info on the Heat Pump Water Volume calculator this is in the good range (>20).

https://renewableheatinghub.co.uk/does-your-heat-pump-have-enough-water-volume-to-run-efficiently/

At a design flow temperature of 40C a Vaillant Aerotherm 7kW is expected to give a SCOP of 4.13, which would be nice.

Pipes

Copper pipe is used throughout. This is 22mm for 10m on the flow and return from the gas boiler, splitting into two 15mm pipes (upstairs and downstairs) that are each about 40m long. The upstairs and downstairs 15mm pipes are not equal in terms of the heat energy they need to transport. The downstairs one needs to carry about 3700W and the upstairs one about 2500W.

The Vaillant Aerotherm 7kW operates at a maximum flow rate of 0.333 litres per second, with the difference between flow and return temperatures varying with output. At a 5C temperature difference, this corresponds to 7kW output.

A flow rate of 0.333l/s down a 22mm pipe corresponds to a flow speed of about 1.0m/s. A flow rate of 0.333l/s down two 15mm pipes in parallel, i.e. 0.167l/s each, corresponds to a flow speed of about 1.15m/s, which is in the standard range of 1.0 to 1.5m/s, and a little above the quiet range of 0.5 to 1.0m/s. The UK good practice limit is 1.2m/s for central heating circuits, so this would be met.

The pressure drop for 0.333l/s along one 10m section of 22mm pipe followed by two parallel 40m sections of 15mm pipe, then one 10m section of 22mm pipe is theoretically: 3.6 + 25.8 + 3.6 = 33 kPa = 0.33 bar, or 3.3m head. However, this value is for straight pipe, without junctions and corners that cause significant extra resistance to flow. Taking a pessimistic view that junctions and corners may add up to 50% gives a pressure drop of 0.5 bar or 5m head. This appears to be within the acceptable range for most central heating pumps, including my current one that delivers 6m head.

The 22mm hot water pipes separate off the main flow and return pipes after about 3.5m and run for a further 6.5m to the hot water tank. As we only ever do space heating or hot water heating, not both at the same time, this arrangement appears to be fine with flow rates and pressures (1.0 m/s and 0.07 bar or 0.7m head) for the hot water circuit below those for the heating circuit.

Hot water tank

Unlike the radiators, our hot water tank is not in need of replacement. Hence an economic argument would be needed to do so. Heating the hot water in the existing tank could be done using 75C flow from the heat pump with a COP of about 2.4. Whereas with a new heat pump specific tank, this could be done at lower temperatures, with a once-a-week anti-legionella cycle, with an average COP of about 3.2.

Our annual energy requirement for hot water is about 1600 kWh. The annual energy saving with a new hot water tank would therefore be about 167 kWh due to the increased COP. Assuming current cheap overnight electricity prices this amounts to a saving of about £15 per year. Assuming a Vaillant UniSTOR hot water tank and installation at £1500, which is perhaps optimistic, this equates to a payback period of 100 years, and so does not make economic sense.

Conclusions and questions

I aim to replace all the radiators next summer, but not the hot water tank, and then see how I get on trying out a 40C flow temperature over the following winter using my gas boiler as the heat source. It appears that my pipework can support a 7kW heat pump, but not a 10kW one, so I’ll need to make sure that any survey prior to fitting a heat pump concludes that a 7kW unit is the correct size.

Question 1: Rather than match the calculated heat loss for each room, should I up-size the new radiators further, e.g. 70cm high instead of 60cm in some rooms? The idea being that they could then be balanced down to the right output. The extra cost would be relatively low, extra water volume would be added to the system, and there would be more scope to turn them up if needed to get the correct overall balance. Or is this a waste of money?

Question 2: Feedback would be most welcome on pipework considerations. For those with lots of experience in this area, would the normal expectation be that my 22mm/15mm pipework is likely fine for use with a 7kW heat pump? And are the upstairs and downstairs 15mm pipes capable of carrying 2500W and 3700W respectively?

Question 3: As an alternative to what I propose doing, is there an argument for getting a heat pump first and then changing the radiators later? Are there any advantages in doing the radiator changes at the same time as a heat pump is installed (aside from draining and filling the system one less time)?

Why do insulation first?

If I had not upgraded the insulation first, then the heat loss would be about 9400W, 52% more than it currently is. Hence the need for a 10kW heat pump instead of 7kW, at an additional cost of around £1000 and possibly higher installations costs too. Instead of 40C flow temperature with the new radiators, it would be around 47C, reducing the SCOP. The maximum flow rate for 10kW is 0.574l/s. This flow rate down a pair of 15mm pipes equates to 1.97m/s, which is well over the practical limit of 1.5m/s, likely to cause excessive noise and erosion of the lining of the pipes. Hence, the main 15mm pipework both upstairs and downstairs would need replacing, at significant cost and disruption. The pressure drop for the existing pipework would be 9.9 + 70.7 + 9.9 = 90.5 kPa, 0.9bar, or 9m head. This is again an underestimate due to junctions and corners. Even an optimistic uplift of only 10% gives a pressure drop of over 1 bar or 10m head, which is too high.

As expected, there are strong arguments for a "fabric first" approach, insulating as much as possible first. This reduces the energy required for heating due to the lower heat loss, and more so with a heat pump by enabling lower flow temperatures and hence higher efficiency. Insulating first potentially avoids the need for a more costly higher output heat pump, that may require expensive and disruptive changes to pipework that would not otherwise be required.

The electricity supply is from the front of the house facing East close to parking but somewhat exposed, with the meter and consumer units (CU) on an inside wall.

The CU was installed as part of a house refurb a couple of years ago and has a separate unit with a 40A circuit to the garage via an (iirc) 80A cable, (don’t ask, the pain is still fresh) and another 40A sub main that will eventually connect to another house CU (again, don’t ask, the house was previously split into a granny annex and main house, so we have two supplies and meters, one of which I hope to remove someday).

The garage has a CU (older, plastic, full) which feeds the garage/workshop/utility and is about 15m from the incoming supply. It’s complicated, a diagram would help if I get the time.

We don’t currently have Solar/battery but may in the future, so it would be useful to be able to integrate if that transpires. I realise this may be the wrong order to go about this, but that’s where we are.

The obvious solution is to fix to the front wall on the other side of the meter/CU cupboard.

As it’s potentially a bit of an eyesore and a trip hazard, as cables would need to pass over an entrance path, then I was considering an untethered unit, however a tethered unit would not be a deal breaker, though probably need greater than 5m cable to be safe.

The alternative would be to put it in/on the garage. The main issue with this would I expect be routing the cable for the current transformer for load balancing control and WiFi.

As we are out in the weeds, down a dip non line of sight to any mast, the 4G service at any location is poor, which leads me to rule out anything that relies exclusively on 4G, along with what appears to be poor customer service from companies supplying such devices.

That leaves units that connect via WiFi (or possibly fixed Ethernet). WiFi at the outside of the wall is currently non-existent though, as the walls of the house are stone, 600mm thick, and in places insulated with metal foil lined boards. I am currently investigating moving the main router and adding a mesh unit, which does significantly help.

Some initial questions:

The specifications of some units state only IP54 ingress protection (and one rather oddly lists in the user manual that it is actually IP45, but IP54 on the data sheet). One claims IP65 but then says that the backplate is only IP22 and should be protected under cover. As a former engineer that has put equipment through rain and dust testing, this gives me cause for concern. Given that this is to be installed in the South West, where it rains, a lot, are standard units not suitable for exposed wall mounting and what IP rating is adequate, or am I reading the wrong data or overthinking this?

On the 4G type units with control buttons, if 4G is lost, the units appear to be still useable to charge at full rate subject to some considerations. Does this include load monitoring and charger throttling or is it full power or nothing?

WiFi units appear to generally have no local control and don’t appear to be able to be commissioned without WiFi, so will they still work as a basic charger if WiFi is lost?

Would it be better to use a local independent installer rather than the supplier install route with less control over the installation?

Sorry for the long post, but I don’t want to commit to a particular model/location/installation route until I’m fairly satisfied I know what I’m getting, due to the cost and the fact that the wife will be mostly using this. I will of course ask detailed questions of any potential suppliers.

TIA

My house has run on a hybrid system since it was built in 2013: a Viessmann Vitocal 200-S air source heat pump working alongside a Viessmann Vitodens 200 gas boiler, installed together on Viessmann controls. The heat pump is type AWB-AC 201.B13, a 13 kW split, with the boiler covering the peak and doing all of the hot water as a design decision.

The heat pump has now reached end of life, with Viessmann confirming that spare parts are no longer available. Even though I had the fan motors and control PCBs replaced three years ago, I am worried that something else will break. Rather than waiting for this to happen, I would like to replace the unit and take the opportunity to remove the boiler at the same time to come off gas completely. I need to find a unit that will carry the entire load on its own and also do the hot water.

That makes the design load figure the whole question and it is where I have run into trouble. I have two answers that differ by a factor of 1.7, and I would like someone to tell me whether I have got this wrong before I commit to equipment.

The house

It is a developer-built detached house completed in 2013 in Surrey, on four storeys including a basement and rooms in the roof, with around 550 m2 of floor area excluding the garage. There is underfloor heating on three floors, radiators on the second and MVHR throughout.

The as-built U-values are 0.28 for the external walls, 0.16 for the pitched roof, 0.21 for the flat roof, 0.22 for the ground floor and 1.8 for the windows. Air permeability is 8.2, according to the EPC.

The data

The hybrid arrangement has given me two distinct measurement regimes, which is the reason this post might be useful to anyone else working a design load out from consumption figures.

The heat pump was switched off entirely from 2014 until February 2021 and the boiler carried the whole load, which gives a clean gas-only record; my meter readings cover 2015 to 2018 within that period. Since 2024 I have had daily gas consumption metered separately for hot water, along with heat pump output logged into InfluxDB, which gives a period where both heat sources are running and can be separated from each other.

Three methods

Firstly, I regressed twenty gas meter intervals from 2015 to 2018 against heating degree days computed hourly from ERA5 data at my coordinates. The hot water baseline was taken from each year's measured summer consumption rather than assumed, and the fit was forced through the origin so that only one parameter remained free. That gives 815 W/K at a fitted base of 13 C, with an R2 of 0.96.

Boiler seasonal efficiency is taken as 90 per cent gross, which is reasonable for a Vitodens 200 condensing on underfloor return temperatures. It is worth noting that gas meter readings convert on gross calorific value, so the 98 per cent net figure manufacturers quote is not the number to use here. Varying it between 85 and 92 per cent moves the result by about 1 kW.

Secondly, I took the 2024-25 season and combined heat pump output with boiler heat to give total delivered heat. That gives 814 W/K.

Thirdly, and this is the one I trust most, the heat pump saturates at around 7 kW below about 5 C and its output barely rises as the weather gets colder, so the boiler supplies almost all of the additional load. That makes the slope of boiler output against outside temperature a measurement taken entirely from the gas meter, with no heat pump instrumentation involved at all. The boiler contributes 538 W/K and the heat pump 276 W/K over the same weeks, giving 814 W/K combined. This is only possible because it is a hybrid, and it is the strongest evidence I have.

Where the calculator disagrees

The de Podesta HTC estimator gives 474 W/K and a 10.2 kW heat pump from the same consumption figures.

The entire difference comes from one assumption. The estimator places the balance point 3.5 K below internal temperature, which for me gives a base of 18.5 C and roughly 2,550 degree days a year. Fitting the base to the data instead gives 13 C and about 1,350 degree days. Dividing the same annual consumption by half as many degree days doubles the coefficient, so neither of us has made an arithmetic error, we have simply used different denominators.

The tiebreaker

In the week ending 13 January 2025, at a mean outdoor temperature of 0.8 C, the house consumed 12.75 kW continuously for seven days. Of that, 5.01 kW came from the boiler, which is a gas meter reading with no modelling in it whatsoever. The calculator's 474 W/K predicts 8.4 kW at that temperature, so the house measurably used half as much again as the model says it can.

Where I think I am weakest

A base of 13 C against a 21 C setpoint implies around 6.5 kW of free gains, which is considerably more than a normal house. Some of that is real, since there is a lot of south-west glazing and a secondary hot water loop was dumping about 900 W into the fabric around the clock until I put it on demand. But some of it is almost certainly that not every room sits at the thermostat setting, so 21 C describes the hall rather than the basement or the roof rooms.

Another thing worth knowing is that Viessmann's energy balance is calculated from temperature sensors rather than metered, and Viessmann themselves state it can deviate by up to 20 per cent. The bias is not random: calculated heat output tends to read high while calculated electricity reads low, so the resulting COP flatters and several owners have logged discrepancies of 30 per cent against calibrated meters. That is why the third method matters. If my counter overstates output by 20 per cent, the answer falls only to 760 W/K and 18.3 kW, because the heat pump contributes just a third of the fitted slope.

Where I have landed

The heat transfer coefficient is somewhere between 760 and 815 W/K, which gives a design load of 18 to 19.6 kW at -3 C on the MCS basis that ignores gains, or 14 to 16 kW if you credit the gains the house actually has.

I am specifying delivered output of 15 to 16 kW at A-3 / W37, to be evidenced from the manufacturer's capacity table rather than the A7 / W35 headline rating. Hot water is a comparatively small addition on top: around 10 kWh a day into a 200 litre cylinder at 48 C, which I intend to schedule on Agile Octopus cheaper rates rather than reheat on demand as the boiler currently does.

There is one piece of good news in all this. My weather compensation curve delivers 35.5 C flow at design conditions, which is low for a house this size. In 2018, running on the boiler alone, the curve was set to 54 C to hold the house one degree warmer than it is now, but that was using room controllers to regulate the temperature and I am not doing that any more.

Questions

I know there is quite a lot to take in here! But does anything above look wrong to you?

I don't think there are any monoblocs on the UK market that deliver 15 to 16 kW at A-3, so this looks like two units in parallel sharing the load. I should be clear that I mean parallel rather than a two-stage refrigerant cascade, which is what usually comes up when you search the term. Has anyone here actually run one domestically, and would you do it again? I have read the Grant Aerona 2x10 thread, which is largely about when the second unit should cut in, and that is precisely the part I would want designed rather than guessed at.

I am also open to being told that keeping the hybrid is the sensible answer at this size, but I would rather be rid of the gas and the standing charge that goes with it, and the boiler is thirteen years old anyway.

Separately, is anyone running active cooling into an MVHR duct cooler? I have that now and want to keep it,.

Full disclosure: I did the regressions and the degree day work with Claude, which pulled the hourly ERA5 data, ran the fits across a range of base temperatures and did the sensitivity analysis. The meter readings, the heat pump logs and the U-values are all mine and measured, and the arithmetic is all checkable. I mention it because the conclusion here contradicts a widely used spreadsheet and I would rather people know how it was produced and pick holes in it than take it on trust. I am happy to post the raw data, the workings or the code if anyone would like to.

Over the last few days, I’ve done some heat loss calculation for my house, a 1970's 4-bed detached (146 sqm), which has as much insulation as we could reasonably add without it turning into a full retrofit. As a stubborn and pragmatic scientist, I decided I’d work out the heat loss as far as possible myself from the basics. I used 24 degrees as the design temperature difference (i.e. -3C outside and 21C inside).

I calculated the heat loss in two ways and also tried out a simple estimate:

1. Theoretical: based on the attributes of the house (room sizes, construction materials and their U-values etc.).
2. Practical: based on the energy going into the house that kept it significantly warmer than outside during December 2024 and January 2025. (I used those months because our only heating system then was a gas boiler).
3. Quick estimate: using the Michael De Podesta Formula

1. Heat loss (theoretical):

1a. Thermal conduction: I worked out the areas of the various building components (floors, ceilings, walls, windows etc) on a room-by-room basis and either looked up their U-values from the rdSAP 10 manual or worked them out from the U-values for the individual materials used and their thicknesses. (The open university web pages were useful in that respect, see https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-3.2.4 and the next few pages). From the various U-values, areas, and the temperature difference, I worked out the heat loss.

The overall thermal conduction losses came to about 3720 W.

1b. Thermal Bridging: This takes account of the joins between walls and roof, walls and floor, windows and walls etc. which tend to be areas of high heat loss. rdSAP 10 accounts for this by increasing the U-values of everything in 1a above by a fixed amount depending on the age of the property, e.g.by 0.15 for my 1970's house.

The overall thermal bridging losses came to about 1180 W

1c. Ventilation Losses: These are given by a simple formula: 0.33 x temp difference x house internal volume x ACH. Where ACH is the number of air changes per hour. Appropriate values for ACH are an area of some debate as they can vary widely for two ostensibly similar houses. As I’ve taken care that my house is well sealed, I used a value of 0.5 for ACH, however, I currently have no hard evidence for that value.

The overall ventilation losses came to about 1340 W

Total theoretically determined heat loss 6240 W

Thermal conduction was 60% of the total, thermal bridging 19% and ventilation losses 21%.

2. Heat loss (practical):

I have a semi-professional weather station in my back garden. The outdoor and indoor units record temperature every 30 minutes, hence I could easily get average indoor and outdoor temperatures for Dec 2024 and Jan 2025 and the average difference in temperature over each of those months.

I then worked out the total heat energy going into the house from the following:

• Gas consumption figures, assuming 0.9 boiler efficiency (condensing except when heating hot water).
• Subtracted gas consumption for hot water heating, since the water is still hot when it drains out of the house.
• Electricity consumption figures, since all use of electricity within the house eventually turns into heat.
• Two people in the house at 100W each for an average of 16 hours per day.
• Solar gains from 10 kWh per sqm incident energy on east and west facing windows over each whole month. (Double glazed unit g-value of 0.49, glass to frame ratio of 0.7, overall window area measured).

Correcting from the actual difference in average indoor and outdoor temperatures gave an input energy of 147.8 kWh per day to achieve a 24-degree temperature difference.

Total practically determined heat loss 6160W

3. Michael De Podesta Formula

The Michael De Podesta Formula is simply annual gas usage divided by 57.3 multiplied by the 24-degree temperature difference.

Total heat loss based on Michael De Podesta Formula 6031W

Thoughts:

The first thing that struck me was that the figures from the two completely different heat loss calculations are surprisingly close. As they are worked out completely differently, this gives me some confidence that they are about right. Further, the simple Michael De Podesta Formula also gives a value that is within 5%.

The ACH value has the most uncertainty and varying that has a big effect. I don't know if there is anything I can do to reduce that uncertainty short of paying for an air tightness test.

There is quite some heat that goes into the house from sources other than the heating system. This amounts to about 570W from non-heating electricity use, people burning calories, and solar gain through the windows even at the dullest time of year.

If I just want the maximum demand on the central heating system, then it looks like this is about 5500 to 5700W, not including hot water heating.

Comments welcome!