@grahamf 

I had a similar journey, had 2 people do heat loss calculations. The first was very professional and looked at solar gain for each south facing room, as my house is EW facing.

The second survey was done after I’d had the extension built, usually they decline to do it at that stage as there is no insulation or ceilings in the extension so heat loss will be massive, fair enough, but the calculation came out 1/3 higher than the thorough one.

I didn't want the large 8kW HP as it would block the rear access to the garden. The next option from Octopus was 6kW Cosy6 which I wanted as it used R290 gas and is more efficient.

Unfortunately my Heat Loss calcs said this wasn't enough, and Octopus told me (they actually used very harsh words) that if I insisted on having the smaller HP, then I wouldn't get the £7500 grant.

That was a shock: I had 2 options, suck it up or negotiate.

I showed them the professional Heat Loss calcs I’d had done, and asked to see their calcs, which they were very cagey with. After half hour of poring over the data, we finally concluded their calculation of my Heat Loss was 300W higher than the Cosy6 output, apparently the Cosy6 is not 6kW, its 5.6kW.

Light Bulb Moment! Thats why my professional Heat Loss calcs of 5.9kW wont work with the Cosy6 aka Cosy5.6!

We tweaked a few assumptions and a note was added that the survey was only valid when the finished insulation is installed.

My Cosy5.6kW has worked well all year, it had a hard January as the insulation was not correct and was clearly unable to cope with the unfinished insulated rooms. I opted to insulate higher than required, and am insulating the last outside wall with insulated plasterboard

I had already opted to fit PV on each side of the roof, maxing out the PV to 5kWh, adding a 5kW battery which charges on cheap rate electricity, gave me an average 12.3p kWh this past year

The comment about only heating 4 out of 5 bedrooms Ive read about and would suggest is not good. I have my unused bedrooms turned down to 18C

When you do your Heat Calcs, their are 3 types of wall:

Inside, Party, Outside

an Inside wall has NO cavity insulation so is determined to be a high heat transfer wall. Having an unheated room next to a heated room sucks heat from the heated room. The unheated room also causes thermal draughts.

The last piece of my puzzle for controlling heat is to box in the stair well and fit a door. This will keep the heat downstairs from escaping to the upstairs landing. I’m sure you've all noticed it get hotter as you climb the stairs.

When I had Gas CH with radiators downstairs, the upstairs landing was a massive heat trap, and a cold draught was always coming back down. Since having UFH with the Heat Pump, the heat is introduced in the floor and the heat doesn't convect up the stairs like happened with radiators; fitted under windows and covered with curtains so the heat warms the windows first, worst design flaw of a heating system

Posted by: @grahamf

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@jamespa unfortunately, I don't think heat loss calculations can ever be terribly accurate.

It's not necessary to be precise, it's only necessary to be precise enough, which I personally define as sufficiently good  to be reasonably certain about the major design forks, and to ensure that an avoidable error hasn't been made.  How precise that is depends somewhat on the house in relation to the granularity of heat pump capacity and pipe sizes, and where are house sits in relation to that granularity.

I went through quite a few steps to get to my 7kW figure.   Had I relied blindly on two  professional surveys, each taking 3hrs and both coming out at 16kW, I would have ended up having to upgrade pipework and with a heat pump over twice the required capacity.  Simply comparing the survey results with my 20MWh annual gas consumption triggered the alarm that the survey results were highly suspect.  By combining all the information available I was able to convince myself that the loss was actually between 6.5 and 8kW, which was good enough to tie down the major design decisions. 

If the industry were just to take the simple step of comparing surveyed loss with measured gas consumption and investigating major inconsistency I think it's highly likely that a good proportion of major sizing errors would be eliminated.  Certainly that applies to a high proportion of the cases on this forum where gross oversizing appears to be a contributing factor.

@jamespa yes, I agree it doesn’t need to be perfectly accurate - only enough to ensure you size the system to a reasonable level.  Do you now have firm evidence that the heat loss really is 7kW, or is it more or less than that?

Our gas usage for 2024 was 25MWh - only 25% more than yours.  That would put us at maybe 9kW.  I had a feeling that it might be that low.  However, it might be reasonable to add another 25% due to the gas heating having been off at night, upstairs being on for only a few hours each evening  and 4 bedrooms being turned off.  Maybe that gets us to 12kW, which compares to the heat loss calculation of 15.1kW.

Before the heat pump installation, I reckoned it might be as low as 9kW and that it was probably in the range 12-18kW.

The early signs are that it is nearer to the lower end.  The calculation had an impact on our design, but it has not been serious.  I really wanted a 16kW Viessmann Vitocal, but the installer recommended the slightly more powerful 17kW Grant Aerona 3 R32.  We compromised on a Grant Aerona 290 15.5kW, which claims a higher heat output than the Viessmann at -3C.

We probably upgraded more radiators than we needed to.  The result is that our flow temperature may not go above 37C at -2C, when the design said 45C.  That made the capital outlay higher, but I am pleased that we have an efficient system.  If the temperature ever drops to -10C, which it did even in south Manchester on 11 January 2025, I am optimistic that we still have enough heating capacity to cover it.

Posted by: @grahamf

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  Do you now have firm evidence that the heat loss really is 7kW, or is it more or less than that

Doing the plot from heat pump output gives 6.5-7kW also.

Posted by: @grahamf

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 However, it might be reasonable to add another 25% due to the gas heating having been off at night, upstairs being on for only a few hours each evening  and 4 bedrooms being turned off.  

Depends on how cold house gets.  Heat loss is dependent on house temperature Vs outside not whether or not the heating is on or not, and for sure house loss is equal to heat delivered.   As you have a physics degree you know this of course!

Posted by: @grahamf

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 I am pleased that we have an efficient system.

That's good!

Posted by: @grahamf

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Our gas heating bills gave an indication of heat loss, but we used to keep the downstairs temperature at 18-19C and turned on the upstairs zone for only a few hours in the evening.  We did not heat 4 out of 5 bedrooms.  Then, we turned the whole system down to 15C at night.  Now that we have the heat pump, we are heating the whole house as one zone and setting the thermostat in the hall to 20C for 24 hours per day.  They just aren't equivalent.

I am not making any judgement but it  made me smile. 

It seems a core part of the design of most ASHP systems, to warm the house at a constant temperature all the time. Unless people works shifts.. For most people it is better to have a duvet than trying to sleep with a light cover, generally healthier. Our house has quite low heat loss but we still enjoy it a few degrees lower at night..

With heat loss calculation reports, a friend of ours had one done recently. They have a back extension that is essentially a conservatory with the usual insulation (even the roof is just double glazed). 

They had a heat loss report and apparently did not get warned they would need a couple more rads in the room (or do something sensible like apply some insulating layer to the ceiling..). Their living room is freezing, the rest of the house is always at the same temperature. And this could seem to be because of their new heat pump! And, I am sure their pump is very efficient... 

So if these reports are not used to address these topics, what are they really for, I wonder..

@batpred exactly why i knocked my conservatory diwn and built a brick wall, super insulated roofed extension in place of it to house the machine room and 4th bedroom

Posted by: @batpred

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This made me smile, it seems a design fault of most ASHP systems, warming the house at a constant temperature all the time.

I'm not sure where you are getting your design information from, but its incorrect.  The standard design temperatures, as specified in MIS3005(D), are 21C in living areas and 18C in bedrooms, not the same all over at all.  If you want it colder still in the bedroom(s), just turn down the lsv, or if you wish fit a TRV (the former is preferable).

If your ashp supports it natively (not all do) and your house cools sufficiently quickly, there is absolutely nothing to stop you dropping the temperature throughout a couple of degrees at night if you wish.  If your ashp doesn't support this function natively, then homely or havenwise do.  However if you do this you should be aware that it may not save any money.  If you are on a night time cheap tariff, it is almost certainly cheaper to raise the temp a couple of degrees at night!  You can still have the bedrooms, where presumably you sleep, cooler if that suits you, provided of course your house fabric is such that you can maintain the temperature difference.

Posted by: @batpred

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They had a heat loss report and apparently did not get warned they would need a couple more rads in the room (or do something sensible like apply some insulating layer to the ceiling..). Their living room is freezing, the rest of the house is always at the same temperature....

So if these reports are not used to address these topics, what are they really for, I wonder..

They should be used for this and if they weren't warned the installer is at fault, and should offer to return and fit the rads for the cost of the rads plus the time to fit them, but should not charge for the additional drain and refill operation that is necessary as a result of not doing it at the same time as the rest of the work.

Posted by: @agentgeorge

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@batpred exactly why i knocked my conservatory diwn and built a brick wall, super insulated roofed extension in place of it to house the machine room and 4th bedroom

Great, you did the right thing! I am not clear if you replaced the conservatory before the first before heat loss calculation? 

What I cannot understand is how my friends could have heat loss calculations done and nobody raised the obvious risk they had (and they did have after the ASHP installation) where their living room was open to the conservatory and it had a couple of small capacity rads that were nowhere near enough. 

It seems that instead of oversizing things a bit, people are being pressed to have exactly what they need for the whole house. Does it really cost much more to run a slightly larger system? I suspect if we all follow the advice and request several quotes, the cost of manpower to produce various heat loss assessments easily trumps the extra cost for larger kit?  

Posted by: @batpred

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It seems that instead of oversizing things a bit, people are being pressed to have exactly what they need for the whole house.

In 3 years on this forum I have seen very, very few cases indeed of under sizing of the type you are describing.  In contrast there have been dozens of cases of gross oversizing of the outdoor unit, which can have dramatic knock on effects.  Thus I very much doubt that 'people are being pressed' in the way you suggest, this is an unusual case not reflective of any norm reported here.  If you would care to post the heat loss calculation that might help understand what happened.  

In any case up sizing a radiator is a quick and relatively cheap operation so little or nothing is lost by doing 'after' the main install.  Why do your friends not simply do this if it matters to them?  See my comments above about what should be deducted from the cost and what they could reasonably be expected to pay imho.  If this were to happen and  based on what you have said, they would be in exactly the same position as if the radiators in question had been done with the main install.  So the question to my mind is why is it not happening if it matters?

@batpred

I had the first heat loss calculation done before the Conservatory was removed, the surveyor marked the room as unheated.

I correctly kept the french doors between the dinning room and the conservatory when it was built. It was a hopeless room, 40+C in the summer, 6C winter. You could only comfortably use it in Apr-May and Sept-Oct. I was considering a solid roof replacement and was quoted 6k; cheaper to demolish and replace with a super insulated room with warm deck roof.

Those calculations were based on a dining room with french window doors leading to the outside. As a bonus the surveyor also calculated the heat loss of the dining without french window doors leading through a closable door into a well insulated extension.

@jamespa I am really impressed that your heat loss estimate was so accurate.  May I ask what kind of house you live in?

In winter, when we had the gas heating, we used to go upstairs to a cold bedroom and overnight the temperature could get down to 15-16C.  It was still cold when we went down to make our early morning cup of tea.  The 4 other bedrooms could get really cold.  As you point out, the heat loss depends on how hot the house is.  30-40% of the house was cold all the time and all of the house was pretty cold at night.  Hence my caution about estimating heat loss from the gas bills.

@batpred

Posted by: @batpred

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I am not making any judgement but it  made me smile. 

It seems a core part of the design of most ASHP systems, to warm the house at a constant temperature all the time. Unless people works shifts.. For most people it is better to have a duvet than trying to sleep with a light cover, generally healthier. Our house has quite low heat loss but we still enjoy it a few degrees lower at night..

I suspect there is probably a heat pump owner maturity curve.  People start by trying to get the last 5% efficiency out of the system.  Then, they remember that there are more important things in life and just let it run!  I am frustrated that I think I have the system running really sweetly, but the Grant Aerona 290 provides no way of tracking the COP and SCOP.  Very frustrating.  I am not too keen on spending £800 on the excellent heat pump monitor, because I cannot see what decisions I would make to increase the efficiency of the system.

Given our stage in life (68/70), we have decided that we have had enough of being cold, so we are no longer going to lower the thermostat to shave money off the bills.  We are just setting it to 20C.  I agree that a cooler temperature at night is preferable, but that is where my penny pinching is still in effect.  We have the 7p per kWh Intelligent Octopus Go EV tariff. Therefore, if the system turns off or down at night, we will be reheating at 28p per kWh.

It may also depend on the type of bed you have.  Our Tempur mattress feels cold when we go to bed, even though it must be at 20-21C.  It takes a long time to warm up, but by morning, it is definitely too hot.  

As I write this, a team of people is ripping our roof off ready to reroof and install solar panels and batteries.  After that, I anticipate that our usage pattern will change again.