Heat Loss - Seems very low!

[RomyD]

Hi,

So we have a 3 bed TF det bungalow in Suffolk undergoing renovation. It was built in 1990 to quite a high (Swedish) spec for the time. Anyway I spent a few hours (days actually) building a spreadsheet to work out heat loss and therefore give a basis for what kind of heating system we would need. Then I found Jeremys spreadsheet (thank you!) and pleasingly the numbers were within a few watts of each other.

 

The problem I have is that in my head it just seems so low. Fabric heat loss comes out at just over 2kW. The re-design u-values are walls 0.14, floor 0.21, ceiling 0.10 and windows and doors at 1.2. Does the heat loss sound sensible?

 

We don't know airtightness so went with an estimate based on data I found on the web;

https://eprints.leedsbeckett.ac.uk/id/eprint/2109/3/Johnston %26 Stafford accepted Dec 2015.pdf

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjL_aSKiPrzAhVQT8AKHaakA9UQFnoECA0QAQ&url=http%3A%2F%2Frandd.defra.gov.uk%2FDocument.aspx%3FDocument%3DEE01005_6849_FRP.doc&usg=AOvVaw0u9awmIx7t003FO38u924B

 

I know the maths don't lie but I am having trouble believing all we will need is around 2 -3 kW space heating!

 

Can anyone give examples of their build; calculated / actual heat losses / typical u-values?  I realize there are lots of  variables involved but real world examples would be very useful. I searched for a thread to that effect but couldn't find anything.

 

[Mr Punter]

I have a SAP here for end terrace 115m2 that says the total space heating requirement is 30.89 kWh/m²/year.

The December and January heating requirements are about 730 kWh/month.

 

Another is a large mid terrace and 197m2 the total space heating requirement is 27.66 kWh/m²/year.

The December and January heating requirements are about 1150 kWh/month.

 

These were built 3 years ago.

[ProDave]

Yes our 150 square metre new build has similar U values and similar total heat loss.  It has been running fine for a couple of years heated with a 5kW ASHP and under floor heating.  Peak heat loss at +20 inside and -10 outside is about 2.3kW.  And being in the Highlands, -10 is normal in winter.

 

As built SAP came out at A94.  Annoyingly, (just a point of interest really) the SAP calculation estimates the electricity needed to run the ASHP at about 3 times what it really uses.  That makes me wonder what the SAP score would really be if we knew why it over estimated so much and could correct that.  Jeremy's spreadsheet gave gave a heat loss prediction much closer to reality than the SAP calculation.

[Bramco]

We had a room heat loss analysis done by a consultant which checked out very well with Jeremy's spreadsheet. Similar U values to yours. Our peak hat loss is expected to be just over 5kW at -3.6C so roughly in the same ball park I guess.

No experience yet as the TF is only just nearing completion.

 

Simon

[saveasteading]

47 minutes ago, ProDave said:

what the SAP score would really be if we knew why it over estimated so much

I have been involved with SAP/EPC  (as a targeted building designer) since the very first talks by BRE.

Along with most others present I left at half time when they couldn't explain where any numbers came from.

 

I think it was rushed through before it was ready, like Breeam and air tests, because it was a money-maker. Lots of approximate assumptions were included.

 

Then once the programmes were written there was little desire to change them.

The professional assessors can't change the formulae in the core programme, so you chuck in some numbers and tick boxes and out comes a rating.

We bought the core programme and played with it to see where the anomalies were, which was fun but annoying.

There were lots of tick box items which didn't make any sense to us, and I think the experts learnt which give the best outcome.

 

For your own interest you can insist on the dozen or so back pages of the assessment. There you will see the assumed power consumption when your ashp goes into cooling mode for three months for the Highland summer, and the power used by heat recovery.

Then deduct that manually for an academic A++

 

[ProDave]

10 minutes ago, saveasteading said:

For your own interest you can insist on the dozen or so back pages of the assessment. There you will see the assumed power consumption when your ashp goes into cooling mode for three months for the Highland summer, and the power used by heat recovery.

Then deduct that manually for an academic A++

I do have all the "input" calculation pages, by input I mean all the hoards of data presented to it to make it spit out a number.  It has all the wall and floor areas, U values of each surface, Actual Uw values of each window, even the exact model of mvhr and ASHP and actual air permeability test result.  So no excuses for not being accurate.

 

One day if I am bored I might scrutinise that and see if I can spot where it went wrong.

[saveasteading]

1 minute ago, ProDave said:

I do have all the "input" calculation pages

I mean the output pages. There are lots of histograms and pie charts telling you where you are using notional electricity, including cooling and heat exchangers. 

I can't remember if there is even any allowance for where you are in the country.

Your assessor should be able to copy it all to you. It is genuinely interesting, though annoying.

[SteamyTea]

32 minutes ago, ProDave said:

I do have all the "input" calculation pages, by input I mean all the hoards of data presented to it to make it spit out a number.

Are they emaillable?

Could make interesting reading.

 

I am not sure how much difference it makes where you are.  I looked at the difference between mean temperatures in Newcastle and Cornwall once, seem to remember it was under 1°C.

That does only tell part of the story though.  Cornwall tends to have cool spring times as the ocean is cold.

[IanR]

2 hours ago, RomyD said:

The problem I have is that in my head it just seems so low. Fabric heat loss comes out at just over 2kW. The re-design u-values are walls 0.14, floor 0.21, ceiling 0.10 and windows and doors at 1.2. Does the heat loss sound sensible?

 

Seems low for a refurb. What's you internal floor area? what Air tightness value did you use? and what inside to outside temp delta have you based the calc on?

My total energy loss at a 30 degree delta T is just under 15W/m². My U values are Walls 0.12, Roof 0.11, Floor 0.09, Windows and Doors 0.65, no/minimal cold bridging and air tightness is around 0.1m³/m².h@50Pa

 

Edited to add:

 

Actually, that's my calculated energy loss which used an air tightness of around 0.4m³/m².h@50Pa iirc, rather than my actual air tightness.

Edited November 2, 2021 by IanR

[RomyD]

1 hour ago, IanR said:

 

 What's you internal floor area? what Air tightness value did you use? and what inside to outside temp delta have you based the calc on?

 

 

Floor area is 134sqm

I used .43 as per the Jeremy spreadsheet. Its very difficult to know how leaky a refurb will be and I still need to get my head around its real world effects.

Temp delta is 24.6C (21c and -3.6C)

 

 

[RomyD]

3 hours ago, ProDave said:

Yes our 150 square metre new build has similar U values and similar total heat loss.  It has been running fine for a couple of years heated with a 5kW ASHP and under floor heating.  Peak heat loss at +20 inside and -10 outside is about 2.3kW.  And being in the Highlands, -10 is normal in winter.

 

That's reassuring. Suffolk is a bit warmer in winter though. Or rather its probably better to say its not as cold. We've had quotes for ASHP and U/F and new HW system......12kW to 8kW pumps and over £20K...8/. It makes no sense to me.

 

And now RHI is unlikely so need an alternative heating strategy.

[IanR]

17 minutes ago, RomyD said:

 

Floor area is 134sqm

I used .43 as per the Jeremy spreadsheet. Its very difficult to know how leaky a refurb will be and I still need to get my head around its real world effects.

Temp delta is 24.6C (21c and -3.6C)

 

 

 

Your energy loss also works out at 15W/m². You've done exceptionally well on a refurb if you've achieved that.

 

0.43 ACH is very low. Specialist builders (or self-builders), doing a new build with lots of tape and gaskets for all penetrations would be very happy with that level of air tightness.

Have you got mechanical ventilation? you need it with that level of air tightness in order to bring fresh air in. If you've done lots of working fixing all the leak paths then I'd go with 3ACH on a refurb, and somewhere between 5 and 10 if you haven't.

 

Better to over-estimate the energy loss, rather than under spec the heating system.

 

18 minutes ago, RomyD said:

We've had quotes for ASHP and U/F and new HW system......12kW to 8kW pumps and over £20K...8/.

 

Without the UFH you can expect to pay £10K - £12K for the install from a decent MCS installer.

With the cost of the UFH on top, it depends on the work and the spec, but the price sounds a little expensive. Have you tried Eco East Anglia?

 

The RHI is to cover the cost of the ASHP & UVC, over and above what gas combi would cost. Are you saying that's unlikely due to you not being able to get it commissioned by 31.03.2021?

[RomyD]

1 hour ago, IanR said:

 

Your energy loss also works out at 15W/m². You've done exceptionally well on a refurb if you've achieved that.

Which is why I was a bit dubious but the original build exceeded UK regs as it was an imported Swedish TF kit, with 3G and 0.233 walls to start with for example. re-design u-values have been calculated as a result of upgrading floor insulation 20mm>70mm, adding IWI and lots of loft insulation plus decent glass. ProDaves experience is encouraging though.

 

Quote

0.43 ACH is very low. Specialist builders (or self-builders), doing a new build with lots of tape and gaskets for all penetrations would be very happy with that level of air tightness.

Have you got mechanical ventilation? you need it with that level of air tightness in order to bring fresh air in. If you've done lots of working fixing all the leak paths then I'd go with 3ACH on a refurb, and somewhere between 5 and 10 if you haven't.

ACH heat loss is a bit perplexing to me. With the Jeremy spreadsheet and no MVHR (0% efficiency) and ACH say of 5, I see a ventilation loss of 11.4kW? I find it hard to accept that in a real world scenario there will be so much heat loss via ACH than via fabric, more than 5 times as much. I do not understand why. We will have MVHR though absolute.

 

Quote

 Have you tried Eco East Anglia?

Yes. No response which appears a common theme at the moment generally. Everyone is extremely busy.

 

Quote

 

The RHI is to cover the cost of the ASHP & UVC, over and above what gas combi would cost. Are you saying that's unlikely due to you not being able to get it commissioned by 31.03.2021?

I think there is very little chance of meeting that deadline and the new scheme is ridiculous as far as i can tell.

Edited November 2, 2021 by RomyD
made it better

[ProDave]

4 minutes ago, RomyD said:

ACH heat loss is a bit perplexing to me. With the Jeremy spreadsheet and no MVHR (0% efficiency) and ACH say of 5, I see a ventilation loss of 11.4kW? I find it hard to accept that in a real world scenario there will be so much heat loss via ACH than via fabric, more than 5 times as much. I do not understand why. We will have MVHR though absolute.

It really is that bad.

 

We need fresh air to breath, so we need a certain level of air changes, we can't live in a completely sealed box (for very long)

 

If you just have air changes without heat recovery, whether by mechanical means (fans) or just trickle ventilation, all the air leaving the building is taking heat out with it. And that spreadsheet shows the ventilation heat loss can easily exceed heat loss through the walls etc.  so controlled heat recovery ventilation becomes crucial to getting a low energy building.

[RomyD]

But the two studies, on very ordinary housing stock, that I linked to in my first post show ach via leaks, extractors and vents as generally less than 0.5, which suggests much lower ach numbers should be used even if there has been little attempt at airtightness improvements. what am I missing?

 

I totally get that uncontrolled air leaks are bad - and get worse the older i get but the less said about that the better.....

 

 

 

 

Edited November 2, 2021 by RomyD

[IanR]

59 minutes ago, RomyD said:

But the two studies, on very ordinary housing stock, that I linked to in my first post show ach via leaks, extractors and vents as generally less than 0.5, which suggests much lower ach numbers should be used even if there has been little attempt at airtightness improvements. what am I missing?

 

Just read a bit of one of the articles you linked to and I believe the point it was making is that the air permeability tests measuring at 50Pa are over-estimating "general" permeability as the pressure difference is "typically" between 3Pa and 6Pa.

 

The article didn't appear to be considering the max energy loss, but rather what the background ventilation rate is, so it would be correct for it not to consider worst case. (But I have not read the whole article.)

For your max energy loss calculation to size your heating system to, you should use air permeability at 50Pa (windy day), 30 degree temp delta and ensure there is some head-room for hot water production.

Edited November 3, 2021 by IanR

[RomyD]

Thank you for your responses all. Very helpful and now I have a bit more thinking to do.

[Bramco]

12 hours ago, IanR said:

Without the UFH you can expect to pay £10K - £12K for the install from a decent MCS installer.

 

I've never understood why companies ask £10-£12k for an install when the parts are about half that.  What value does an 'MCS install' provide?

 

Simon

[Moonshine]

4 minutes ago, Bramco said:

What value does an 'MCS install' provide?

 

allows you to get the RHI payments

[IanR]

Just now, Bramco said:

 

I've never understood why companies ask £10-£12k for an install when the parts are about half that.  What value does an 'MCS install' provide?

 

Simon

 

MCS provides the entrance to RHI.

 

My pricing is for Nibe, which are certainly not the cheapest, and I don't believe it would be possible to purchase a Nibe 12kW ASHP, Nibe 500l UVC, Nibe 200l Buffer, Nibe SMO 40 controller + all required brass and copper for £5.5K.

 

My Install was around 5 man days, plus the back office work to fulfil MCS with detail heat loss calcs etc.

 

Mine was around £11K all in, so I can see where the money was spent. I did have several other quotes though that ranged from £15K to £19K, so some companies do try it on.

[jamieled]

9 hours ago, IanR said:

Just read a bit of one of the articles you linked to and I believe the point it was making is that the air permeability tests measuring at 50Pa are over-estimating "general" permeability as the pressure difference is "typically" between 3Pa and 6Pa.

 

When we had our air test done, one of the testers invited me to open an external door when we were at 50Pa. It was like a hurricane entering the house and nothing like 'normal' conditions. Does anyone know if SAP calculates ventilation heat loss using the 50Pa measurement, or is it just used as an index in order to calculate heat loss at a more realistic pressure difference?

[A_L]

24 minutes ago, jamieled said:

Does anyone know if SAP calculates ventilation heat loss using the 50Pa measurement, or is it just used as an index in order to calculate heat loss at a more realistic pressure difference?

 

SAP assumes that the natural unpressurised infiltration is 1/20th of the 50Pa value.

[SteamyTea]

10 minutes ago, A_L said:

SAP assumes that the natural unpressurised infiltration is 1/20th of the 50Pa value.

That is very useful information, do you have a reference for it please.

[A_L]

@SteamyTea , section 2.3, page 11,  here  https://www.bre.co.uk/filelibrary/SAP/2012/SAP-2012_9-92.pdf

[SteamyTea]

6 minutes ago, A_L said:

section 2.3, page 11,  here  https://www.bre.co.uk/filelibrary/SAP/2012/SAP-2012_9-92.pdf

Brilliant, thanks.