@scalextrix I think what you are describing is exactly why I am so sceptical of room by room assessments and more supportive of previous use estimates. 

In my case I knew my previous bungalow was fine with a 5kW heat pump and estimated that the new one would be too with some radiator changes. I also did some experiments with running my gas boiler at a lower temperature. 

Guess what. I was right. I don't think it's rocket science to get a ballpark figure, run some tests and if unsure add on a small amount for safety. 

On several occasions I have looked at a house and estimated it's rough heat loss in minutes with a much greater degree of accuracy than many of the experts with their heat loss assessments who frequently over egg it. 

Worst case scenario is add some more insulation if you are widely wrong. 

I await a torrent of abuse over my comments 😁

• Mine was rubbish. Very poor EPC assessment driving massive over assumption on heat loss and a resulting system much bigger than I need (cascade).  Ours is a large old but very well insulated, double glazed property with UFH throughout downstairs. Room by room is a bit pointless as so many of the rooms touch each other - resulting in a higher spec. I even told them of our actual heating usage in each of the last 5 years  - which for cop of 3 would mean a 10kw system  (not 17kw which is what was installed) 

so far this year since feb1 ( when the installation wrinkles were sorted) the whole sytem has been OFF for 25% of the time, and very rarely do both pumps operate.(I’ve seen it happen once)

@bontwoody thanks, do you have any tips for a real world data approach.

It does feel a little risky basing my estimate on a single day, unfortunately I had to travel quite a bit over winter so don't have a lot of good data to run with.

@carpenterstation sounds like what I'm afraid of, hopefully you can get it sorted.

Thanks for your feedback.

My heat loss survey came back at 7.4kW

We keep our house a little cooler than design temp, at around 19-20C, and 18C overnight. On the coldest day last winter, we generated 95kWh of heat, divided by 24h gives ~4kW, substantially below the heat loss survey calculation. I attribute the difference to the high air changes per hour value MCS used at the time, and that we do not heat to 21C.

Our situation was further exacerbated by the fact that the installers used Samsung who offer 8kW and 12 kW models, and the 8kW model cannot deliver 7.4kW output at design temp (-2C), so they insisted on installing the 12kW model. The upshot is we ran it at bare minimum all winter, and in the shoulder months of autumn and spring, we just run for a few hours and turn off like a conventional boiler as it's putting out too much heat for the property (we certainly turn off for 8-10h overnight)

That said, it has still performed really well and our COP figures are in line with Samsung's published data but continuous running is not possible for much of the year.

The usual problems are not including known insulation improvements or the defaults for air leakage.

If you have an actual air loss measurement it can be used by a potential installer (3 used it, one didn’t) instead of defaults. Cost ~£300 might be saved on the heat pump unit alone if a smaller one proves suitable.

20% or even 50% over-sized is not a crucial issue. Problems occur when units are 2-3x oversized 

@scalextrix When do you intend taking the plunge? Do you have next winter to work with to get some good data?

If you have the house area and a feel for the insulation level you can get a rough estimate using the heat geek basic guide. 

Https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

Its very rare for a heat pump to be undersized compared to the opposite situation so I would start on the assumption that 7kW is a reasonable starting point.

Also try and find a house similar to yours using heatpumpmonitor.org or visitaheatpump. You can then ask how their system is performing. 

@judith where it does start to make a differnce is installation cost and noise.  Thanks for your reply.

Posted by: @old_scientist

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My heat loss survey came back at 7.4kW

We keep our house a little cooler than design temp, at around 19-20C, and 18C overnight. On the coldest day last winter, we generated 95kWh of heat, divided by 24h gives ~4kW, substantially below the heat loss survey calculation. I attribute the difference to the high air changes per hour value MCS used at the time, and that we do not heat to 21C.

Our situation was further exacerbated by the fact that the installers used Samsung who offer 8kW and 12 kW models, and the 8kW model cannot deliver 7.4kW output at design temp (-2C), so they insisted on installing the 12kW model. The upshot is we ran it at bare minimum all winter, and in the shoulder months of autumn and spring, we just run for a few hours and turn off like a conventional boiler as it's putting out too much heat for the property (we certainly turn off for 8-10h overnight)

That said, it has still performed really well and our COP figures are in line with Samsung's published data but continuous running is not possible for much of the year.

 

Thanks, so yeah yours is quite oversized, we are the opposite, we need to keep the huse warmer than the standard.

For me: Two full surveys (each taking 3hrs, one of which cost me £300) got to 16kW.  Correcting for obvious errors in the surveys got to 10.5kW.  Measured loss based on gas consumption 7kW.  Found Two installer prepared to take this into consideration.

Installed '7kW' Vaillant (actually specced at 8.5 kW at design conditions). More or less perfect, perhaps 0.5-1kW over.  Really pleased with the installation.

@scalextrix for our house 1950s detached the air leakage proportion was30% of the supposed total, using the default values. In fact with the measurements it was 2%, so that alone would have given an over estimate.

Noise and insulation go together on heat loss on windows because seals and droughts are often poor on single glazing. Double glazing solves both issues together on windows but not elsewhere. Single glazing transmits external noise inside.

Your bills are the best estimate of what heat pump size you will need. Do you know your annual consumption. This rule of thumb works well for most people https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/  

Different surveyors have different hang-ups. We had a range from 8 to 12kW and the reason for the upper value for that surveyor was we couldn’t prove the old retrofit cavity wall insulation still worked. Doh! It kept us warm didn’t count for that company. Our usage said 7-8kW according to the method used.

If you want the house hotter say so, they can go up but not down.