How to calculate heat load…

[G and J]

Dear Buildhubbers,

 

I’ve been trying to find a formula to estimate our house design’s heat load and heat demand, but the combined intellect of google and me have failed.  
 

Please can someone enlighten me.  
 

Regards,

Confused of Suffolk.

[Mr Punter]

Have you looked at the JSH spreadsheet?

[JohnMo]

Start with the basics, so you understand where the calcs come from.

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

[G and J]

I’ve filled it in and I continue to play with it on a ‘what if’ basis. 
 

And yes, I’ll be very embarrassed if both figures are there already and I’ve failed to notice!

 

(But I’m ready.  I’ve my own conical ‘D’ hat and a stool waiting in the corner!)

[G and J]

17 minutes ago, JohnMo said:

Start with the basics, so you understand where the calcs come from.

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

Brilliant link.  Thank you. 

[G and J]

So, guess who has been up late googling.....

 

I found the following on an Australian website (I think it still works even though they are upside down over there):

"While the building heat load tells you how high the boiler output needs to be, the heat demand indicates the amount of energy that is required per square metre over the course of a year. The heat demand or building energy demand is therefore a criterion for describing the energy quality of a building."

 

Makes perfect sense.

 

So if i look at Jeremy's spreadsheet it calculates the heat required to maintain the internal temperature - default is 20c inside and -10c outside which gives me ~2.5kW.

 

But....

I'm not sure if -10c is excessive for Suffolk, 

There's no allowance for getting up to temperature, and

There's no allowance for hot water either.

 

And I'm really not sure how to derive the heat demand figure from his spreadsheet.

 

Help!

 

 

 

[Dave Jones]

easiest one ii found. 

 

Does the U values as well.

 

https://www.changeplan.co.uk/heat_loss_calculator.php

 

[G and J]

21 minutes ago, Dave Jones said:

easiest one ii found. 

 

Does the U values as well.

 

https://www.changeplan.co.uk/heat_loss_calculator.php

 

Ta for the link - won’t let me register at the mo but will keep trying.

[JohnMo]

1 hour ago, G and J said:

There's no allowance for getting up to temperature, and

There's no allowance for hot water either.

Heat pump you really should be running steady state, not on off like a boiler.

 

I take the following for DHW allowance.

2.5kW heat demand x 24 hrs is 60kWh. So now you need to take time out for DHW, so allow 2 hrs (2x heating sessions). So now you need to produce 60kWh in 22 hrs. 60 / 22 so 2.7kW.

 

So you are looking at a 4kW heat pump, which is about as small as you get. -10 is excessive for Suffolk, but in your case it's going to make much difference, you will still have an oversized heat pump. So you need a single zone to enable it to cope with limited cycling.

[marshian]

2 hours ago, G and J said:

So if i look at Jeremy's spreadsheet it calculates the heat required to maintain the internal temperature - default is 20c inside and -10c outside which gives me ~2.5kW.

 

But....

I'm not sure if -10c is excessive for Suffolk, 

There's no allowance for getting up to temperature, and

There's no allowance for hot water either.

 

And I'm really not sure how to derive the heat demand figure from his spreadsheet.

 

Help!

When I did my heat loss study it used -2.3 based on location (Norfolk) -10 is a little bit OTT for East Anglia

 

The Heat study came out at 14,500 kWh per year for CH and HW (HW was assessed at 3285 kWh of that overall figure) However they use a really high ACH rate due to the age of my house which I disagreed with (it has quite an impact)

 

Our overall gas usage per year is down to 7800 (CH and HW)

 

CH is 77.5% of our usage, HW is 21% of our usage and cooking is 1.5%

[ProDave]

-10 IS realistic as an outside temperature up here but not for many places in the UK.

 

Bet even at -10 my heat loss was about 2.5kW so a 5kW ashp, about the smallest you can buy, would only spend half it's time heating the house, so plenty of time to heat the DHW.  

 

[G and J]

40 minutes ago, JohnMo said:

Heat pump you really should be running steady state, not on off like a boiler.

 

I take the following for DHW allowance.

2.5kW heat demand x 24 hrs is 60kWh. So now you need to take time out for DHW, so allow 2 hrs (2x heating sessions). So now you need to produce 60kWh in 22 hrs. 60 / 22 so 2.7kW.

 

So you are looking at a 4kW heat pump, which is about as small as you get. -10 is excessive for Suffolk, but in your case it's going to make much difference, you will still have an oversized heat pump. So you need a single zone to enable it to cope with limited cycling.

Our MCS plumber dude told us straight of that the smallest heat pump we can have will be oversized, so I kinda took that as a given but corroborating opinions are useful.

 

I was trying to calculate heat load/demand to get my head round how far off passive standard we are.

 

With the current set of assumptions I’m working to at -10c the heat loss divided by the internal floor area is around 17W/m2, so we are well over.

 

At the Norfolk level of -2.3c it’s 13W/m2 ignoring DHW - and just over 14W/m2 if I do the patent @JohnMo 24/22 DHW fudge factor - which doesn’t look so bad after all.

 

My confidence level in these numbers though is not high yet as I have the queasy feeling that I’m missing a basic, partly because they are so much better than I naturally expected (I still associate passivehaus with long straggly beards and 4’ thick cameldung and candyfloss walls, but then, I am an old reactionary at heart!).

Heat demand though still confuses the life out of me.

 

[G and J]

53 minutes ago, JohnMo said:

Heat pump you really should be running steady state, not on off like a boiler.

Agreed, but I didn’t want to assume that the heat load calc assumes that too.  

[ProDave]

12 minutes ago, G and J said:

My confidence level in these numbers though is not high yet as I have the queasy feeling that I’m missing a basic, partly because they are so much better than I naturally expected (I still associate passivehaus with long straggly beards and 4’ thick cameldung and candyfloss walls, but then, I am an old reactionary at heart!).

Heat demand though still confuses the life out of me.

When I was building my house, I too did not believe it would require so little heat.  So I did a test.

 

Once the building was complete, insulated and all windows in I put a simple electric convector heater on 24/7 in the middle of the downstairs, for a week, and took daily internal and external temperature measurements, and confirmed the difference was in line with what you would expect with that much heat input thus proving Jeremy's spreadsheet was bang on.  I then had the confidence to buy a heat pump.

 

 

[marshian]

2 hours ago, ProDave said:

When I was building my house, I too did not believe it would require so little heat.  So I did a test.

 

Once the building was complete, insulated and all windows in I put a simple electric convector heater on 24/7 in the middle of the downstairs, for a week, and took daily internal and external temperature measurements, and confirmed the difference was in line with what you would expect with that much heat input thus proving Jeremy's spreadsheet was bang on.  I then had the confidence to buy a heat pump.

 

I did a similar activity when validating my heat loss calcs - compared it with heat input once all the rooms were at target temp on the coldest day we during winter and it was pretty close. Enough for me to be confident that my replacement boiler didn't need to be 24 kW.

 

We traditionally heat only before we need it (mornings and afternoon/evenings) I wasn't confident enough to purchase an 11 kW boiler so settled for a 16 kW version (min output for 11, 16, 19, 25 or 32 kW all have the same 3.2 kW minimum)

[JohnMo]

I started with gas boiler and some heat meters came up cheap on here. So  installed and played with flow rates Andy temps to get house comfortable (it got down to -9) it also confirmed my heat loss calcs were pretty much as predicted.

[Sparrowhawk]

3 hours ago, marshian said:

CH is 77.5% of our usage, HW is 21% of our usage and cooking is 1.5%

How do you separate out the figures? I sat down to work out our kWh/m2 /yr and only have the 1 gas usage figure, so guestimated and deducted 4kWh/day for HW (cooking is electric here)

[marshian]

20 minutes ago, Sparrowhawk said:

How do you separate out the figures? I sat down to work out our kWh/m2 /yr and only have the 1 gas usage figure, so guestimated and deducted 4kWh/day for HW (cooking is electric here)

 

Do you want the full sad story or the precis? I'll stick with the precis I think but in a nutshell I've been a smidge OCD with meter readings (for smidge you can expect very OCD * See Note)

 

Summer water heating is easy to measure  no CH to worry about and our daily kWh usage averages out at 4.2 kWh

 

Winter water heating needs a bit more input energy wise - mains water is colder - loft tank is colder and boiler starts from a lower ambient temperature but a bit of math has that as a factor of 15% increase so 4.8 kWh as a daily average

 

Roughly

 

182 days a year at 4.2 = 766 kWh

182 days a year at 4.8 = 876 kWh

 

Annual Gas usage 7800 so HW (766 + 876) is 21% of the total used

 

Cooking whilst a small percentage is also easy to work out when you aren't using CH or HW

 

Only two people in the household typically 2 showers each daily and Mrs BC likes a long hot bath once a week - ~112 Litre Hot water tank heated to 50 every morning and normally lasts 18 hrs

 

*Note - Just to clarify how OCD - During the course of 2023 from Jan 1 to Dec 31 I have 1,658 meter readings tabulated in a spreadsheet (cross referenced against HDD and House/Water temps and boiler parameters)

[JohnMo]

17 minutes ago, marshian said:

Note - Just to clarify how OCD - During the course of 2023 from Jan 1 to Dec 31 I have 1,658 meter readings tabulated

I thought I was getting OCD doing once a day - so stopped now do it twice a month.

 

But saying the above, now have a heat pump monitor system coming with segregated CH and DHW. Every statistic ever needed - I think 

Edited August 22 by JohnMo

[marshian]

13 minutes ago, JohnMo said:

I thought I was getting OCD doing once a day - so stopped now do it twice a month.

 

But saying the above, now have a heat pump monitor system coming with segregated CH and DHW. Every statistic ever needed - I think 

 

Trying to optimise an unsuitable boiler to be as efficient as possible on Ch as well as HW - I went down a rabbit hole looking for Alice and was doing DHWP manually

 

With new boiler I'm back to once per day - once I have DHWP and Weather comp working I'll run with once per day as the start of the heating period and then happy with the results/efficiency I'll go back to monthly (I hope)

[Dave Jones]

4 hours ago, G and J said:

Our MCS plumber dude told us straight of that the smallest heat pump we can have will be oversized, so I kinda took that as a given but corroborating opinions are useful.

 

I was trying to calculate heat load/demand to get my head round how far off passive standard we are.

 

With the current set of assumptions I’m working to at -10c the heat loss divided by the internal floor area is around 17W/m2, so we are well over.

 

At the Norfolk level of -2.3c it’s 13W/m2 ignoring DHW - and just over 14W/m2 if I do the patent @JohnMo 24/22 DHW fudge factor - which doesn’t look so bad after all.

 

My confidence level in these numbers though is not high yet as I have the queasy feeling that I’m missing a basic, partly because they are so much better than I naturally expected (I still associate passivehaus with long straggly beards and 4’ thick cameldung and candyfloss walls, but then, I am an old reactionary at heart!).

Heat demand though still confuses the life out of me.

 

 

same boat as you, did my calcs and the smallest heatpump was sufficient. In march we had no loft insulation and downlight holes all open to roof. It was zero outside at night and with flow temp of 35 it was too hot inside. 22/23.

 

The cop was as high as 8.

[Sparrowhawk]

3 hours ago, marshian said:

Do you want the full sad story or the precis? I'll stick with the precis I think but in a nutshell I've been a smidge OCD with meter readings (for smidge you can expect very OCD * See Note)

I'm all for other people's OCD, it makes mine feel less

 

Thanks for that, I'll apply similar logic to mine. We had a Viessmann fitted in June (and a previously undetected 4.8kWh/day internal gas leak sorted) so next year should be significantly better than this year.

[marshian]

4 minutes ago, Sparrowhawk said:

I'm all for other people's OCD, it makes mine feel less

 

Thanks for that, I'll apply similar logic to mine. We had a Viessmann fitted in June (and a previously undetected 4.8kWh/day internal gas leak sorted) so next year should be significantly better than this year.

Happy to take one for the team so to speak 😉

 

OT - Which Viessmann did you go for??

[SteamyTea]

10 hours ago, G and J said:

not sure if -10c is excessive for Suffolk, 

There's no allowance for getting up to temperature, and

There's no allowance for hot water either

Quick answer is yes, no and you need to add to it.

 

The longer answer is you need to know the total power per degree kelvin.

Then you need to know your weather profile i.e. how often, in hours, it is -10, -9, -8 ......8, 9, 10....up to your heating off temperature (mine is a daily mean OAT of 9°C).

Then you look that the middle 99% of that profile and accept at the negative temperature tail, you may need to turn on a fan heater.

If you know your DHW energy demand, say 5 kWh/day, and your heating system is saying 5 kW, then you are heating water for 1 hour, so you have to make sure your heating system can, in the coldest days, supply enough energy in 23 hours.

 

If you have ever lived in a place with a properly designed and executed E7 system, you will we used to this. You get all your energy stored in 7 hours (or less) then spread it out for the remaining 17 hours.

 

In practice, you will find that your combined maximum heat load is not a nice round number that the heating system comes as i.e. 8 kw. So you have to round up to the next available size.

Gas combination boilers are generally sized to deliver instantaneous hot water, so are often massive oversized i.e. 24 kW, but the modulate down to possibly a couple of kW power output for space heating, then start short cycling (what buffer tanks are used for).

 

[Sparrowhawk]

2 hours ago, marshian said:

OT - Which Viessmann did you go for??

Vitodens 100-W B1HF System 11kW GB. Noisier than expected, but we now reliably have hot water and the promise of gas burned heating the radiators not just going out the flue this winter, so all good.