Fabric and ventilation heat loss calculator

[Great_scot_selfbuild]

3 hours ago, JohnMo said:

You need to allow room for DHW.

 

@JohnMo sorry - can you explain what you mean here? (Assuming DHW = Domestic Hot Water), but not sure what you mean about allowing room for it?

[MikeSharp01]

6 hours ago, Great_scot_selfbuild said:

but not sure what you mean about allowing room for it?

I guess he means making sure the heat pump (HP) can also cope with making domestic hot water (DHW). If your house heating needs 5kW then a 5kW heat pump won't be big enough, on the worst days, if the HP also has to make DHW because all its output will be needed for heating. The important thing is that this may only be a problem on the worst (coldest outside) days of the year when you could either accept a slight drop in heating while the HP is making the DHW or use the Immersion heater to make the DHW or get the next size up HP, or much bigger if you need loads of DHW see other threads here,  to let it cope with the DHW.

[Great_scot_selfbuild]

11 hours ago, SBMS said:

The ‘total heat loss’ cell (3090W) gives

your worst case heat loss when it’s -3 outside. 
 

I would question your house volume though that looks really small if you have 259sqm floor space multiplied by 2.4m that’s a minimum volume of 621m3. Are You sure?

@SBMS Thanks. Does that heat loss mean 3kW heat pump?! (not clear what duration it is over - I saw another post that referred to multiplying by 24, assuming it to imply it is an hourly heat loss figure, but basically I'm still unclear.

 

Volume - I hadn't clocked that was a manual entry and I hadn't changed it. You're correct, volume is 879m3; floor area is 131m2 but tall ceilings and upstairs within the roof space.

 

Thanks again

 

 

Edited Thursday at 06:00 by Great_scot_selfbuild
Saved/submitted too early

[jack]

2 hours ago, Great_scot_selfbuild said:

@SBMS Thanks. Does that heat loss mean 3kW heat pump?! (not clear what duration it is over - I saw another post that referred to multiplying by 24, assuming it to imply it is an hourly heat loss figure, but basically I'm still unclear.

 

'kW' is power, so it's a continuous number. There is no time component.

 

If you want to know how much energy the house uses, you multiply power by time. For example, 3 kW of power supplied for an hour is 3 kWh (the unit electricity is charged in).

[JohnMo]

9 hours ago, Great_scot_selfbuild said:

 

@JohnMo sorry - can you explain what you mean here? (Assuming DHW = Domestic Hot Water), but not sure what you mean about allowing room for it?

Pretty much as explained above.

 

So assume your heating power at design temperature is 3kW. Now to do hot water you need to allow 2 to 3 hrs for hot water production depending on cylinder size and usage. You now work on the basis that the heating would be needed for 24hrs at 3kW, so 3 x 24, so that is 72kWh. So in a 24hr period you need to produce 72kWh. However as you are spending say 3 hrs on DHW you now only have 21 hrs to produce 72kWh. So now we divide 72 by 21, which is 3.4kW. So the actual size of heat pump required is one that produces a minimum of 3.4kW at -3 degs.

 

However the biggest consideration is modulation. If you need 3kW of heat at -3, at the more average winter temp you only need help that so 1.5kW. So the ideal heat pump is one that can modulate down well. The better the modulation the more time the heat pump spends running and the less time on standby.

 

1 hour ago, Great_scot_selfbuild said:

Volume

You shouldn't need to adjust that figure as it's calculated from your inputs. 

[Marvin]

Hi @Great_scot_selfbuild 

 

My air source heat pump stops heating the radiators at the lower temperature and then heats my hot water tank at a higher temperature. Once the hot water tank is up to temperature it then reverts to the lower temperature. This is all achieved using a 3 port valve.

 

On a highly insulated property like we have, we have never noticed the building temperature drop whilst heating the 204 litre hot water tank. That said, we don't have any baths only showers.

 

[jack]

26 minutes ago, Marvin said:

On a highly insulated property like we have, we have never noticed the building temperature drop whilst heating the 204 litre hot water tank. That said, we don't have any baths only showers.

 

Same. Our house temp changes about 1 °C per day with the heating off in the winter. Having no heating on for a couple of hours is imperceptible.

Compare that to old Victorian houses we've lived in, where the temperature would change several degrees over only a few hours.

[SBMS]

13 hours ago, JohnMo said:

You shouldn't need to adjust that figure as it's calculated from your inputs. 

Don’t think Jeremy’s spreadsheet calculates this it’s a manual entry 

[SBMS]

14 hours ago, Great_scot_selfbuild said:

Does that heat loss mean 3kW heat pump?!

Yes! Although you need a bit more Power to account for the demand on the heat pump

to heat your water as

@JohnMo says. 
 

When I look at the heat loss spreadsheet today it makes sense but it’s taken a lot of learning and understanding actually to make sense of it from a lay persons perspective. It’s just as others like @JohnMohave said probably much more eloquently than me but I’ll try and simplify it to my level!!

 

It’s not really immediately clear but the sheet provides you with two things. The first set of figures is all about heat loss sizing. So what the 3090W figure is telling you is, when it

is the worst case of -3 outside and you want say 20 inside how much heat is your house losing instantaneously. Actually it’s per second because W is already a measure of energy per second. 1W is 1 joule per second. So your house is losing 3090W - or 3090 joules energy to heat per second

 

So this sizes Your heat pump. To keep your house at the temperature you want your heat pump needs to produce 3090W of heat to offset the loss - every second. There’s also hot water production to bear in mind as mentioned above. Your heat pump might be producing hot water and offsetting heat loss so you need some additional power output. 
 

The second part to the sheet attempts to work out the total heat loss across a monthly/annual basis. This is where time comes in. If you have 1 hour at 3090W that’s 3kWh which is the unit of charge for electricity. So really the second part of the sheet attempts to calculate your

total consumption, factoring in less

consumption in summer to winter.  Don’t forget that the total consumption is NOT the total

electricity usage. Your 12kW heat pump

might only use 4kW of electricity to produce that 12kW. That’s its coefficient of performance and a unique feature to heat pumps over say gas boilers. So take your total consumption and divide it by the average CoP for a rough idea of electricity consumption - and therefore cost. 

 

sorry if this was all really obvious - i found

the sheet required a bit of basic knowledge which I didn’t have when i first started learning about this. Hope it’s useful for someone!

[SBMS]

14 hours ago, jack said:

kW' is power, so it's a continuous number. There is no time component.

I might have got this wrong but isnt wattage a measure of energy transfer per second??

[jack]

28 minutes ago, SBMS said:

I might have got this wrong but isnt wattage a measure of energy transfer per second??

 

Yes, power is the rate at which energy is delivered, so power = energy/time.

 

My comment about time was in response to the poster asking "what duration" the 3 kW heat loss was over:

 

15 hours ago, Great_scot_selfbuild said:

Does that heat loss mean 3kW heat pump?! (not clear what duration it is over - I saw another post that referred to multiplying by 24, assuming it to imply it is an hourly heat loss figure, but basically I'm still unclear.

 

[ProDave]

If the heat loss calculation gives an answer of 3kW then put simply that means you want a 3kW heater turned on all the time to maintain the temperature.

 

If you put less than 3kW into the house it will slowly cool down.  If the house is cold and you want to warm in up, you will need to put in more than 3kW

 

The point about water heating mentioned is as follows.  Most heat pumps will only heat the building or the hot water, usually done because then you can have different water temperatures for the 2 functions.  So in the perhaps 2 hours a day your heat pump is heating the water tank, it won't be putting any heat at all into the house.  So for the remainder of the day it would need to put more than 3kW of heat into the house just to get the average over the whole day up to 3kW.

[Great_scot_selfbuild]

@JohnMo @SBMS @MikeSharp01 @jack @Marvin Thanks for all your input - most helpful. After challenging/questioning the suppliers' quotes they have revised their proposed solution and Cool Energy currently is looking most favourable (MCS umbrella etc.) and has reduced the ASHP proposed down from 14kW(!) to the following:

 

Proposed Solution:

We recommend the following setup based on your requirements:

 

• Heat Pump Output: We recommend a heat pump with an output of 4.354kW.
• Design Temperature: -3°C ambient temperature with a 35°C water flow temperature, suitable for underfloor heating (UFH). 
• Heat Pump Model: We recommend the Cool Energy CE-ET10, which can modulate between 3.89 kW to 7.48 kW at the above design temperature.
• Hot Water Cylinder Size: 250L, calculated at 50L per person plus an additional 50L.

 

Design Temperature Explanation:
The design temperature of -3°C ambient and 35°C water flow is chosen to ensure your heat pump system provides reliable heating even on colder winter days while keeping the water temperature low enough for underfloor heating efficiency. This balance helps maximise energy efficiency and comfort throughout your home.

 

My interpretation of this is that whilst the say 'recommend an output of 4.354kW' which sounds ok to me based on my estimated heat losses of just over 3kW plus an allowance for the DHW - I'm not sure how to calculate/estimate this? and haven't searched around yet; however their model can modulate between 3.89 - 7.48kW; am I correct in assuming this would be referred to as a '7.5kW ASHP'?

 

Just wanted to share an update and see if anyone had an opinion on this proposed ASHP solution?

[SBMS]

1 hour ago, Great_scot_selfbuild said:

@JohnMo @SBMS @MikeSharp01 @jack @Marvin Thanks for all your input - most helpful. After challenging/questioning the suppliers' quotes they have revised their proposed solution and Cool Energy currently is looking most favourable (MCS umbrella etc.) and has reduced the ASHP proposed down from 14kW(!) to the following:

 

 

Proposed Solution:

We recommend the following setup based on your requirements:

 

• Heat Pump Output: We recommend a heat pump with an output of 4.354kW.
• Design Temperature: -3°C ambient temperature with a 35°C water flow temperature, suitable for underfloor heating (UFH). 
• Heat Pump Model: We recommend the Cool Energy CE-ET10, which can modulate between 3.89 kW to 7.48 kW at the above design temperature.
• Hot Water Cylinder Size: 250L, calculated at 50L per person plus an additional 50L.

 

Design Temperature Explanation:
The design temperature of -3°C ambient and 35°C water flow is chosen to ensure your heat pump system provides reliable heating even on colder winter days while keeping the water temperature low enough for underfloor heating efficiency. This balance helps maximise energy efficiency and comfort throughout your home.

 

My interpretation of this is that whilst the say 'recommend an output of 4.354kW' which sounds ok to me based on my estimated heat losses of just over 3kW plus an allowance for the DHW - I'm not sure how to calculate/estimate this? and haven't searched around yet; however their model can modulate between 3.89 - 7.48kW; am I correct in assuming this would be referred to as a '7.5kW ASHP'?

 

Just wanted to share an update and see if anyone had an opinion on this proposed ASHP solution?

What have they calculated your design heat loss as? 7.5kW or 4.35?

[Great_scot_selfbuild]

1 hour ago, SBMS said:

What have they calculated your design heat loss as? 7.5kW or 4.35?

They used a statement in my SAP assessor’s report that stated heating requirement as 16.7627w/m2; 260m2. (Screenshot attached)

 

 

[JohnMo]

23 minutes ago, Great_scot_selfbuild said:

They used a statement in my SAP assessor’s report that stated heating requirement as 16.7627w/m2; 260m2. (Screenshot attached)

 

 

So a big heat pump for house load.  The smaller 6kW one isn't big enough at -3.  So not the best fit for your heating system.

[Great_scot_selfbuild]

27 minutes ago, JohnMo said:

So a big heat pump for house load.  The smaller 6kW one isn't big enough at -3.  So not the best fit for your heating system.

@JohnMo sorry, not sure I follow - you’re saying the 7.5kw isn’t the best fit? They’ve used this SAP figure to give a 4.35kw requirement, and then (presumably) scaled it up to a 7.5kw HP to cover the DHW.

Not that I disagree, just trying to understand - which bit of this shows that 6kW isn’t big enough at -3? And is 7.5kw big enough at -3, plus covering DHW?

[JohnMo]

37 minutes ago, Great_scot_selfbuild said:

@JohnMo sorry, not sure I follow - you’re saying the 7.5kw isn’t the best fit? They’ve used this SAP figure to give a 4.35kw requirement, and then (presumably) scaled it up to a 7.5kw HP to cover the DHW.

Not that I disagree, just trying to understand - which bit of this shows that 6kW isn’t big enough at -3? And is 7.5kw big enough at -3, plus covering DHW?

You need a couple of hrs for DHW heating. So for 4.35x24, so 105kWh per day.  To do that in 22 hrs, 105/22, is 4.77kW not 7.5kW.

 

According to the Cool Energy datasheet for the 6kW unit, at -3 outside it has max output of 3.8kW at 35 Deg flow temp.  So isn't big enough for your calculated heat load of 4.35 or 4.77kW. So the only option they seem to have for you is a 10kW unit.  The Cool Energy unit offered has a minimum modulation of 3.89kW, which is fine.  But at 7 degs outside (based on 4.35kW) your heat demand will be around 2kW, so would cycle on off about 50% of the time. Cycling like that is OK, for off time hits CoP, a unit that tick away most the time has great CoP and almost no standby costs.

 

A Panasonic 5kW puts out 5kW at -3 and will modulated lower at 7 degs

Just been reading up on Hisense 4.4 ASHP, made by Hitachi, will do 5kW at -7 and modulate down to 1.85kW at 7 degs. And only £2k incl VAT (Heat Pump Warehouse).

 

 

[Great_scot_selfbuild]

51 minutes ago, JohnMo said:

You need a couple of hrs for DHW heating. So for 4.35x24, so 105kWh per day.  To do that in 22 hrs, 105/22, is 4.77kW not 7.5kW.

 

According to the Cool Energy datasheet for the 6kW unit, at -3 outside it has max output of 3.8kW at 35 Deg flow temp.  So isn't big enough for your calculated heat load of 4.35 or 4.77kW. So the only option they seem to have for you is a 10kW unit.  The Cool Energy unit offered has a minimum modulation of 3.89kW, which is fine.  But at 7 degs outside (based on 4.35kW) your heat demand will be around 2kW, so would cycle on off about 50% of the time. Cycling like that is OK, for off time hits CoP, a unit that tick away most the time has great CoP and almost no standby costs.

 

A Panasonic 5kW puts out 5kW at -3 and will modulated lower at 7 degs

Just been reading up on Hisense 4.4 ASHP, made by Hitachi, will do 5kW at -7 and modulate down to 1.85kW at 7 degs. And only £2k incl VAT (Heat Pump Warehouse).

 

 

@JohnMo amazingly helpful and clear - thank you!

[ADLIan]

It used to be, and I'm not aware of any change, that SAP was specifically not to be used for heating system design.

[JohnMo]

12 hours ago, Great_scot_selfbuild said:

They used a statement in my SAP assessor’s report that stated heating requirement as 16.7627w/m2; 260m2. (Screenshot attached)

 

 

Just taken another look at the figure used and it's not the heat load in kW, it's the number of kWh per year expressed as kWh per m² per year. So cannot be used directly to size the heat pump.

 

To make an estimate based on predicted energy input.

 

So 16.7 (from EPC) x 250 (m²), say 4200kWh, if you then divide by degree days, let's use 2900 (will vary by location and house design), that's 1.4kW. Which is pretty close to what you calculated I believe?

 

Your heat demand at -3 is around 1.4kW. So ideally you are looking at a 4 to 5 kW heat and certainly one with great modulation. You would have one that size purely for DHW heating.

 

But with a demand that low, do you really need a heat pump? If it's not being installed for free, I wouldn't bother. A couple of storage heaters? Simple use a time of use tariff. If you already have UFH installed - Willis heaters?