Real world passive house heating demand figures?

[Smallholder]

Hi,

 

As per the title, does anyone have any real life data, about how much heating energy is needed per m2 in a passive house?

I realise there will be variation between one property and another, even within the scope of PH spec houses. But I could really do with a starting point.

If it makes any difference, I am in Somerset.

 

Many thanks in advance.

[Conor]

Can highly recommend a subscription to passive House + mag. They quite often have energy usage data with their case studies. Not sue of they have them online. I'm in to year 2 of a free 6month subscription trial lol.

 

My architect advised me to assume double the demand figures when deciding on a heating strategy. Double of not very much is  still not very much.

[Nickfromwales]

39 minutes ago, Conor said:

My architect advised me to assume double the demand figures when deciding on a heating strategy. Double of not very much is  still not very much.

Architects can be dangerous things...... What they should say to you is to do the sums and get on top of the M&E coordination as early on as possible. It is such a holistic equation, made up from so many relative ( but often overlooked ) facets, that such a statement seems to be popular in buying these professionals all the insurance they need to survive without undertaking such ( or sometimes ANY ) due diligence.

One 'eco friendly' chap had actually designed a heated floor to be sat on cold strip foundations without any thermal break......

Taking advice from anyone other than qualified M&E's, or testimonials from folk who have built, and then moved into, and are now living in such a dwelling is pretty dangerous practice and often costly.

Another failed to specify a heat pump for a dwelling that had a mandatory requirement for NZEB certification as part of the PP!

 

"Double the demand" is huge in PH terms. My 2 cents would be to stop taking advice from the architect as their approach is worryingly non-robust in this particular respect. 

Edited November 6, 2021 by Nickfromwales

[SteamyTea]

51 minutes ago, Smallholder said:

how much heating energy is needed per m2 in a passive house

Because it is so variable, design it the other way around.

How many kWh/day (or week, month, year) do you want it to use and what peak power, the kW.m^-2 can you live with.

Remember that there is a difference between delivered energy and primary energy.

Edited November 6, 2021 by SteamyTea

[ProDave]

Mine is not a passive house. I have applied a lot of passive house principles but it has not been modelled with PHPP.  150 square metre house uses about 1600kWh to heat it each year which is about 10.6kWh per square metre per annum.

[Smallholder]

11 minutes ago, ProDave said:

Mine is not a passive house. I have applied a lot of passive house principles but it has not been modelled with PHPP.  150 square metre house uses about 1600kWh to heat it each year which is about 10.6kWh per square metre per annum.

 

Thanks very much, that's a good starting point.

I guess if you're in Scotland, then I might use a bit less down here in Somerset.

[Gone West]

@Smallholder I designed and built our 120m2 PH using PHPP and the calculations were done at 20C and came out at 13kWh/(m2a). We actually ran the house at 23C 24/7 and the calculations for that were 21kWh/(m2a) which turned out to be pretty accurate after living in it for three years. We heated our house with a towel rail in each of the three bathrooms and supplimented that with an EASHP providing warm air when very cold. We also had a solar gain area which helped in the spring and summer. Our U factors and air tightness were much better than the minimum PHPP requirements. We were near the coast in Kent. You have to be very careful comparing PH energy values as climate and interior/exterior temperatures make a big difference to energy requirements.

[Smallholder]

1 hour ago, Gone West said:

@Smallholder I designed and built our 120m2 PH using PHPP and the calculations were done at 20C and came out at 13kWh/(m2a). We actually ran the house at 23C 24/7 and the calculations for that were 21kWh/(m2a) which turned out to be pretty accurate after living in it for three years. We heated our house with a towel rail in each of the three bathrooms and supplimented that with an EASHP providing warm air when very cold. We also had a solar gain area which helped in the spring and summer. Our U factors and air tightness were much better than the minimum PHPP requirements. We were near the coast in Kent. You have to be very careful comparing PH energy values as climate and interior/exterior temperatures make a big difference to energy requirements.

 

That's really useful, many thanks indeed

[tonyshouse]

I am on 42kWh/m2/a all in , heating is almost never needed, -3C need 60W, -6C 300W 

 

I think I am probably OSB about 7kWh/m2/a for heating but I have difficulty separating getting out HW demand 

[Stones]

5 hours ago, ProDave said:

Mine is not a passive house. I have applied a lot of passive house principles but it has not been modelled with PHPP.  150 square metre house uses about 1600kWh to heat it each year which is about 10.6kWh per square metre per annum.

 

Is that input to ASHP or output from ASHP (ignoring the top up from your WBS)?

[ProDave]

Just now, Stones said:

 

Is that input to ASHP or output from ASHP (ignoring the top up from your WBS)?

Electrical input to the ASHP.  I have an old dual rate electricity meter set up to meter heating and DHW electricity usage by the ASHP.

[Iceverge]

2 hours ago, tonyshouse said:

I am on 42kWh/m2/a all in , heating is almost never needed, -3C need 60W, -6C 300W 

 

I think I am probably OSB about 7kWh/m2/a for heating but I have difficulty separating getting out HW demand 

Is your interseasonal thermal store making a significant contribution?

[tonyshouse]

No but it is contributing, winter starts mid December and ends at the end of Feb when the sun starts warming the house up again anyway 

 

downward heat losses are greatly reduced , basement 19C this year at end of Feb, I do use some heat from a small heat pump down there now though as have family staying and they are not as hardy as me, 19C is ok with no draughts, but when inactive I need a proper woolly jumper. 

[Smallholder]

21 hours ago, tonyshouse said:

I am on 42kWh/m2/a all in , heating is almost never needed, -3C need 60W, -6C 300W 

 

I think I am probably OSB about 7kWh/m2/a for heating but I have difficulty separating getting out HW demand 

Do you know what COP you're getting from the heat pump? The only figures I've got to go on for PH is the max 15kWh/m2 for heating.
But I don't know what internal temperature that's based on.
So assuming a COP of 3, it could be around 5kWh/m2
Not yet sure what's realistic in terms of average COP

[tonyshouse]

PH is based on average outdoor temperatures in your location over a year to calculate the heating demand, different every day and summed up over a year 

 

 

————-

 

re my interseasonal store:- 
 

No but it is contributing, winter starts mid December and ends at the end of Feb when the sun starts warming the house up again anyway 

 

downward heat losses are greatly reduced , basement 19C this year at end of Feb, I do use some heat from a small heat pump down there now though as have family staying and they are not as hardy as me, 19C is ok with no draughts, but when inactive I need a proper woolly jumper. 

[Gone West]

14 hours ago, Smallholder said:

The only figures I've got to go on for PH is the max 15kWh/m2 for heating.
But I don't know what internal temperature that's based on.

PHPP defaults to an internal temperature of 20C but that is an input variable so you can run it at any temperature you like.

[TerryE]

You really need to do the sums for a sketch design.  Search for Jeremy Harris spreadsheet on this site.   I have a 3 storey passive house with underfloor heating.  Our heating days are quite high but the actual amount of heat is quite low (we heat the house with a Willis -- which is essentially a 3 kW immersion heater in a jacket.)  At the moment it's on for maybe 3-4 hrs a night at E7.  However, we have little solar gain because of the house orientation and planning restrictions limiting the area of windows facing SE (and non on our S/W gable). 

 

The same house with larger windows on a south facing principle axis wouldn't need any supplemental heating at the moment.   You really can't generalise, but a properly constructed passive house should be within 10-20% of this rough-cut estimate.  The caveat here is the "properly constructed" as the devil in the detail.  If you've got building flaws which result is thermal bridges, missing insulation, significant air leaks, etc. then your actuals could be way off.  

Edited November 19, 2021 by TerryE

[Simon R]

On 06/11/2021 at 15:44, Smallholder said:

As per the title, does anyone have any real life data, about how much heating energy is needed per m2 in a passive house?

Just thought I would throw in my pennies worth.

We're not passive, but designed from the start to minimise inputs. Our calculated structure value came in a 68W per degree. In practice this means we need about 2.5kW for maintain a 20 degree difference. The input is so small we don't have an ASHP and rely on a small gas boiler to heat two towel rails and provide hot water on demand.

 

What is easy to get wrong is solar gain. All our SAP calculations looked good but we soon found out that our east facing glazing was causing overheating and we have had to add solar film to the windows. We're not alone in this it has caught out several builders on the forum including Mr Harris.

 

More importantly by designing to minimise input you get a very comfortable living environment, and tiny bills. Our gas used this week works out at about 75p a day,  and we have had a couple of frosts. We are now looking at adding a small 2.5kW air-con system to provide summer cooling which hopefully we'll run from PV.

 

We used a very good service from 'SAP Box' that cost just £120 for design stage calculation, money well spent.

 

[SteamyTea]

4 minutes ago, Simon R said:

What is easy to get wrong is solar gain. All our SAP calculations looked good but we soon found out that our east facing glazing was causing overheating and we have had to add solar film to the windows. We're not alone in this it has caught out several builders on the forum including Mr Harris.

Some trees may counteract this, they can shield in the spring, summer and autumn, but allow light though in winter.

As long as they are not evergreens.

[Bitpipe]

4 hours ago, Simon R said:

Just thought I would throw in my pennies worth.

We're not passive, but designed from the start to minimise inputs. Our calculated structure value came in a 68W per degree. In practice this means we need about 2.5kW for maintain a 20 degree difference. The input is so small we don't have an ASHP and rely on a small gas boiler to heat two towel rails and provide hot water on demand.

 

What is easy to get wrong is solar gain. All our SAP calculations looked good but we soon found out that our east facing glazing was causing overheating and we have had to add solar film to the windows. We're not alone in this it has caught out several builders on the forum including Mr Harris.

 

More importantly by designing to minimise input you get a very comfortable living environment, and tiny bills. Our gas used this week works out at about 75p a day,  and we have had a couple of frosts. We are now looking at adding a small 2.5kW air-con system to provide summer cooling which hopefully we'll run from PV.

 

We used a very good service from 'SAP Box' that cost just £120 for design stage calculation, money well spent.

 

 

Completely agree - overheating is often overlooked with an obsession on heating spec.

 

We have a passive standard build, MBC 2.5 storey TF build ontop of a passive basement. Located in Berkshire.

 

We installed low temp UFH on the ground floor (suspended timber over basement) and aside from electric UFH in the bathrooms (just to warm tiles) and towel rads that is it. Nothing in basement, bedrooms or room in roof.

 

Gas boiler (as we're on mains) and it's mostly used for DHW, 4k of solar on roof with diverter to immersions when we're exporting. Pre price hike, gas usage was about £1/day and most of that was h/w I would guess.

 

UFH is rarely on outside of coldest months but overheating can occur not just in summer but anytime the sun is low in sky and can shine directly into house. We specced external blinds for the east windows and east and south facing velux. I've just installed some IKEA internal motorised blinds on the west facing sliders.

 

Do wish we'd made provision for split air con to run in summer (for free using PV) but I struggle to see where we could locate one, especially the external unit.

 

[Iceverge]

On 06/11/2021 at 15:44, Smallholder said:

real life data, about how much heating energy is needed per m2 in a passive house?

 

We have an all electric passive house. No heat pumps or stoves. Just an E7 cylinder and a single resistance heater.  

 

It's early days yet but I've been keeping a close eye on our electricity usage. PHPP has predicted 315kWh of heating for November, an average of 10.5kWh/day. So far we're on 9.75kWh/day. It's been mild so we'll see how the rest of the year goes.