Knowing kwh used and ashp sizing

[Nevik]

Hi, I’m wondering how to calculate the size of a ashp. I know how many kwh of heat a year I use including hot water. I know the sq ft of my house etc. I’ve found calculators that start from zero, asking about walls, windows, insulation etc. as I know how many kwh of heat I use I would think it’s more accurate to go from there. I’m on a shared biomass at the moment, incoming water temp is 50c, it proving to be unreliable when you need it the most. So thinking about a ashp in the future when the biomass smokes it last polluting pellet.

[JohnMo]

You really need to complete a proper heat loss calculation. Start from the basics and work from there.

 

You need to spend a couple of hours heating hot water, so then that leaves 22 hrs to supply the house with heat on your coldest design day. So for example a heat loss of 4kW is 4x24hrs, so 96kWh, then divide by 22 hrs, which is 4.3kW. You then look at the heat pumps datasheet and if your design temperature is -3 you choose a heat pump that can output at least 4.3kW.

[RobLe]

With a few caveats, you can estimate the heatpump size from your yearly heat use:

 

Heating requirement (in kW) = Yearly heat use (in kWh) / 2900

 

There is some maths behind the above, and it relies on:

Home heating dominates over hot water

South England based HDD (Heating Degree Days) data

South England min temps

21C home temps all the time with both heating sources

 

For example, if you use 20000kWh / year of heating, then the maximum heatload is 7kW.  Most people would go up a size to a 11kW heatpump because:

You do need a little more heat to keep a home at 21C all the time instead of only twice a day (which is common in the UK, and only possible with oversize heating sources).   

Air source Heatpump nameplate power does not take into account cold weather defrost cycles, so again that needs headroom.

 

maths:

https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/

 

 

[JohnMo]

10 minutes ago, RobLe said:

With a few caveats, you can estimate the heatpump size from your yearly heat use:

 

Heating requirement (in kW) = Yearly heat use (in kWh) / 2900

 

There is some maths behind the above, and it relies on:

Home heating dominates over hot water

South England based HDD (Heating Degree Days) data

South England min temps

21C home temps all the time with both heating sources

 

For example, if you use 20000kWh / year of heating, then the maximum heatload is 7kW.  Most people would go up a size to a 11kW heatpump because:

You do need a little more heat to keep a home at 21C all the time instead of only twice a day (which is common in the UK, and only possible with oversize heating sources).   

Air source Heatpump nameplate power does not take into account cold weather defrost cycles, so again that needs headroom.

 

maths:

https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/

 

 

But to be meaningful you need to operate your heating system in a similar way to how you operate a heat pump. Otherwise a good chance of rubbish in-out. Rules of thumb can lead to big over or undersized heat pump, neither are good. 

[Nevik]

I was kind of hoping as I know my heating kwh pretty accurately and floor/ cubic area of the house, that would give me heat loss calculations and size needed if the correct maths are applied. 3000sq ft and 45,000 kwh pa (old house solid brick walls) . The current flow temp in is 50c  which I think is ok for a heat pump. So I’m thinking if with those numbers it’s all good isn’t that enough  real world info to get the sizing correct?

[MrPotts]

You need a proper heat loss survey to be sure of getting the correct size heat pump. In my case the "protons for breakfast" website states my home has 4kw heat loss based on my annual gas usage but using the Heatpunk website I have calculated the heat loss to be a little over 7.5kw, a big difference!

[JamesPa]

@Nevik

This is an area of some disagreement, here is my take and experience, you will need to decide for yourself.

 

"Proper heat loss surveys" can be utter garbage if the surveyor (for example) ignores fabric upgrades they cant see (which some do as a matter of principle) or puts in an unrealistic value for air changes, or miscellaneous other sources of error.  The spreadsheet/software looks sophisticated, but as ever GIGO applies.  Personally I wouldn't rely on a 'proper heat loss survey' alone unless you validate both the inputs and the calculation and also have some independent handle on air change rate.  I had two done, one of which I paid for, and they were total nonsense because the surveyors ignored invisible fabric upgrades ('if we cant see them how do we know they are there' was their argument, an argument which I have heard from others since) and double counted room to room losses.  

 

To be tolerably certain I would

• have a survey done, then check (a) the u-values used (b) how they have treated room to room losses (they should be counted for for radiator sizing but not for whole house sizing, unless room to room gains are also counted) and (c) what value for air changes has been used
• if you can collect smart meter data from a season where you operate your boiler fairly constantly and are heating your house a good proportion of the time, then do various plots eg daily consumption vs degree days, average power over 3, 6, 12, 24 hrs and utilise what this tells you
• Do the rule of thumb calculation, although a divisor of 2200 or even 2000 is also suggested by some and there are arguments for various values in between, probably depending on the assumptions you make about boiler efficiency and location.
•  

For the second and third of these, if you can, make some intelligent assumptions about your boiler.  If its very old then it probably has an efficiency ~75%.  If its a condensing boiler but you have plumes of vapour coming out of the flue (ie the norm in the UK), then its not actually condensing, so efficiency might be ~90%.  If its a modern condensing boiler, run at a FT of 55 or less, and there are no plumes of vapour, then it is condensing and might be running at an efficiency of 110%.  All figures rough and ready.  If you cant work out what assumptions to make, at least you can account for the uncertainty.

 

Once you have all of that (if you can be bothered to go to this much trouble) then see what this tells you.  If they are all tolerably consistent then you are good to go.  If there are major discrepancies then I would suggest you might try to work out the explanation.  In my case the discrepancy was a factor of two, the difference between 8kW (measured) and 16kW (calculated), a difference which has major knock-on consequences.   If your house is low loss (eg ~3kW) it matters less because you are anyway going to be limited by available models and the need to have sufficient capacity to heat DHW, so you are probably going to end up with 5-6kW anyway.  

 

All that said a tolerably oversized heat pump is probably not a disaster unless it pushes you into consequential actions such as upgrading pipework, requiring planning permission or something else, all of which are more likely for a high loss house than a low loss house.  An oversized pump will cycle more than a right-sized pump, possibly even at very low temperature.  This does reduce efficiency, but nobody really seems to know by how much with modern inverter driven pumps (perhaps a topic for another thread sometime).   Heat loss surveys will tend to overestimate because that's 'safe' for the installer.  Whether its 'safe' for the customer depends on the circumstances as exemplified above.  It sounds like your house fits into the high loss category in which case a sizing error might make a massive difference to the scale of works needed.

Edited October 12 by JamesPa