Fabric and ventilation heat loss calculator

[LiamJones]

It’s the first post of this thread

[SBMS]

Have been having another play around with the heat loss calculator.  Have a couple of questions - I have plugged in the following numbers:

 

 

We have a fair amount of glazing, but to be honest it makes little difference if I remove all the glazing.  

 

This generates an annual heat energy input per annum of about 14,000 kWh. If I take this and divide it by internal floorspace (circa 397m2), the energy per sqm is about 35 kWh per annum per sqm.  Passivhaus stipulates less than 15 kWh.  Even if I removed all glazing doors and rooflights, this shifts it to about 24kWh per sqm but still significantly out.

 

Am I missing something? How would this figure be achievable? Am I reading the heat energy input values (row 84) correctly? I just added them all to get to the 14,000 kWh figure.

 

Be really useful if anyone can share their calculations @TerryE think you mentioned on an earlier post an 11,000 kWh energy input. Am trying to figure out what moves the needle to get such a low input value?

[JohnMo]

29 minutes ago, SBMS said:

Have been having another play around with the heat loss calculator.  Have a couple of questions - I have plugged in the following numbers:

 

 

We have a fair amount of glazing, but to be honest it makes little difference if I remove all the glazing.  

 

This generates an annual heat energy input per annum of about 14,000 kWh. If I take this and divide it by internal floorspace (circa 397m2), the energy per sqm is about 35 kWh per annum per sqm.  Passivhaus stipulates less than 15 kWh.  Even if I removed all glazing doors and rooflights, this shifts it to about 24kWh per sqm but still significantly out.

 

Am I missing something? How would this figure be achievable? Am I reading the heat energy input values (row 84) correctly? I just added them all to get to the 14,000 kWh figure.

 

Be really useful if anyone can share their calculations @TerryE think you mentioned on an earlier post an 11,000 kWh energy input. Am trying to figure out what moves the needle to get such a low input value?

ACH is well high. Reduce to something realistic, your MVHR will be set to 0.3 to 0.5 they are better figures.

 

-10 OAT, short event, what is the design temperature for your area?

 

85m2 glazing in your living room - check your figures, 2m high and 40+m wide?

 

Big front door also 4m tall and 2m wide. 

[SBMS]

1 hour ago, JohnMo said:

ACH is well high. Reduce to something realistic, your MVHR will be set to 0.3 to 0.5 they are better figures.

 

-10 OAT, short event, what is the design temperature for your area?

 

85m2 glazing in your living room - check your figures, 2m high and 40+m wide?

 

Big front door also 4m tall and 2m wide. 

Thanks. 
 

Glazing wise that includes a vaulted rear window, 2 storey glazed entrance and 13m of sliding doors. I put all the glazing into a single column in spreadsheet. As said though, even removing glazing doesn’t make a massive difference.  2 sets of French doors and a side door make up the doors.

 

is 0.75 bad for air tightness? I know 0.6 is passivhaus so thought I’d target just above?

 

Am I correct with the kWh per square

meter figure?

[JohnMo]

1 hour ago, SBMS said:

0.75 bad for air tightness

It's not your airtightness figure, it's your ventilation flow. Hence asking for it's efficiency.

[SBMS]

10 minutes ago, JohnMo said:

It's not your airtightness figure, it's your ventilation flow. Hence asking for it's efficiency.

So the ‘air changes per hour’ column is not the airtightness figure?

[Sparrowhawk]

2 minutes ago, SBMS said:

So the ‘air changes per hour’ column is not the airtightness figure?

No. Airtightness is how much can come in/out without your control. Air changes per hour is the rate you expect to change the air in your house at (via MVHR or other means). Try 0.3 as a starting point (see Approved Document F or the Passivhaus guides for why)

[SBMS]

10 minutes ago, Sparrowhawk said:

No. Airtightness is how much can come in/out without your control. Air changes per hour is the rate you expect to change the air in your house at (via MVHR or other means). Try 0.3 as a starting point (see Approved Document F or the Passivhaus guides for why)

Ah! So the passivhaus level of 0.6 is not the target here? I assumed that ACH on the spreadsheet is the target measure of airtightness (or, I guess, a result of the level of airtightness?)

Edited June 4 by SBMS

[JohnMo]

1 minute ago, SBMS said:

Ah! So the passivhaus level of 0.6 is not the target here?

No, remember air test is completed at 50Pa, this  is like a gale force wind in all directions, which isn't possible in reality.

[SBMS]

18 minutes ago, JohnMo said:

No, remember air test is completed at 50Pa, this  is like a gale force wind in all directions, which isn't possible in reality.

Got you thanks. 
 

Is my understanding of the passivhaus recommendation for kWh per sqm correct? I wasn’t sure whether the kWh per sqm figure was adjusted for the heat source (eg if using an ASHP then the energy input would actually be circa 3.5x lower). If not I’ve no idea how one could get to a 15 figure using Jeremy’s spreadsheet anyway. It’s not that I’m arbitrarily aiming for that figure but it’s something that gives some context to energy requirement when assessing u values, insulation depth etc. 

[JohnMo]

5 minutes ago, SBMS said:

understanding of the passivhaus recommendation for kWh per sqm correct?

Yep heat input into the house. Not the net gain via a heat pump etc. the spreadsheet does over read the actual input required a little, as no account is taken from human inputs or other equipment in the house or solar gains etc.

[TerryE]

BTW "Internal" refers to the inside dimensions of the external surfaces: the walls, floor slab, top ceiling or warm roof.

The main reason that your totals are greater than mine is that your house is quite a bit bigger than mine.  My internal floor area is under 80m², though we do have 3 floors as my son's bedsit is in our warm loft space.

 

[SBMS]

2 minutes ago, TerryE said:

BTW "Internal" refers to the inside dimensions of the external surfaces: the walls, floor slab, top ceiling or warm roof.

The main reason that your totals are greater than mine is that your house is quite a bit bigger than mine.  My internal floor area is under 80m², though we do have 3 floors as my son's bedsit is in our warm loft space.

 

Thanks @TerryE, makes sense why your overall energy input is lower. We just seem to be coming out with a high energy requirement per sqm which I’m struggling to reduce down. Even dropping walls to 0.1 and removing all glazing doesn’t help. One figure that does shift the needle is the internal volume as it’s 950m3. But I would expect as your floor area increases as does your volume so wouldn’t expect to get 390 sqm in a smaller volume (we optimised the house design for this, with rooms in the roof). 
 

ultimately it probably doesn’t hugely matter as the energy input is acceptable. Am just intrigued as to how a reasonably large well insulated, airtight house can hit the PH energy per sqm target. 

Edited June 4 by SBMS

[JohnMo]

15 minutes ago, SBMS said:

Thanks @TerryE, makes sense why your overall energy input is lower. We just seem to be coming out with a high energy requirement per sqm which I’m struggling to reduce down. Even dropping walls to 0.1 and removing all glazing doesn’t help. One figure that does shift the needle is the internal volume as it’s 950m3. But I would expect as your floor area increases as does your volume so wouldn’t expect to get 390 sqm in a smaller volume (we optimised the house design for this, with rooms in the roof). 
 

ultimately it probably doesn’t hugely matter as the energy input is acceptable. Am just intrigued as to how a reasonably large well insulated, airtight house can hit the PH energy per sqm target. 

Difficulty getting PH loss may be down to form factor. The closer you get to a cube the better the form factor. A good form factor needs less insulation to achieve a good heat loss figure. Our house is long and thin single storey, all ceilings are vaulted, we have a poor form factor and would stand no chance being PH, with all U values well below 0.1. But generally our heat loss is good.

[TerryE]

IMO the PH certification is OTT.  We wouldn't  get it for our house.  Our planners imposed a cottage style to fit in with the village street scene, so no solar on roof and smallish windows on principle (SSE facing) elevation and no windows on gables because of overlooking neighbours, so very little solar gain.  However, with now 7 years in the house, I actually think they were right: we like the look and feel of the house.

 

What is more important is that house has low energy loss, and a high internal thermal capacity, so it works well as a single zone and we can keep the whole living space at a pretty constant comfortable temperature 24×7.  No zones, no zone controls and valves; nothing complicated, pretty much no moving parts to go wrong and therefore no maintenance costs. 

 

In my case I have 4 power relays in my Consumer Unit Expansion and a Grundfos circulating pump, all of which are simple and cheap to replace on failure.   The system works well with Octopus Agile so the house is cheap to heat using resistive heating.  Having an ASHP would save me maybe £400 a year -- less the £200⁺ maintenance contract -- but the discounted cost of procurement and installation would be at least 30× that of the annual saving, so doing this makes no economic sense.

 

I do have 3 oiled filled <1kW electric rads (~£60 each) that I do use for time dependent top-up in the worst winter months and these are controlled by my HA through Tasmota smart plugs (~ £15 ea).  Again my HA handles these automatically to make best use of cheap Agile rates.  The running cost of these is in the above figure.

 

Edited June 5 by TerryE

[SteamyTea]

20 hours ago, SBMS said:

Is my understanding of the passivhaus recommendation for kWh per sqm correct

Passivhaus use primary energy, not delivered energy.

So if all electric, and you import 1 MWh per year, and the factor is 2.4, then you divide 2.4 MWh by your area.

[JohnMo]

14 minutes ago, SteamyTea said:

Passivhaus use primary energy, not delivered energy.

So if all electric, and you import 1 MWh per year, and the factor is 2.4, then you divide 2.4 MWh by your area.

Not sure that is correct 

 

From Passivhaus 

 

The Passive House criteria allow buildings to go by either criterion - the 15 kWh/(m²yr) heat demand OR the 10W/m² heating load. So heat source doesn't really matter, the actual energy delivered to house that matters.

[SteamyTea]

3 minutes ago, JohnMo said:

Not sure that is correct

It used to be.

Would make my house a Passivhaus as I only use 8 W/m².year for heating.

[MikeSharp01]

On 04/06/2024 at 19:41, JohnMo said:

completed at 50Pa, this  is like a gale force wind in all directions, which isn't possible in reality.

Not sure 50Pa will result in gale force wind, it is only 0.5 millibar pressure difference after all.  I dimly recall that 4mb pressure difference gives 10mph wind, so 0.5mb = 1.25mph, although I could be very wrong on that as fluid dynamics not a strong point with me. To be fair I did our first suck out air test the other day and took the house down to 25 pa and I was hard pressed to open the outward opening front door. Will go down to 50 pa this week and see how the extract rate is. 

[JohnMo]

15 minutes ago, MikeSharp01 said:

Not sure 50Pa will result in gale force wind, it is only 0.5 millibar pressure difference after all.  I dimly recall that 4mb pressure difference gives 10mph wind

 

See the equivalent wind speed on the chart half way down this page 

 

https://efficiencymatrix.com/how-can-air-leakage-testing-help-you/#:~:text=As seen in the graph,to a 5km%2Fh wind

 

So the equivalent of a 32km/h wind, but the point was it's applied to every surface at the same time - when it's windy the wind normally comes from one direction, not all at once. 32km/h is a fresh breeze, so pretty windy. 

[MikeSharp01]

8 minutes ago, JohnMo said:

 

https://efficiencymatrix.com/how-can-air-leakage-testing-help-you/#:~:text=As seen in the graph,to a 5km%2Fh wind

Interesting - will need to clear out my dusty rules of thumb draw or maybe I mixed up the pressure generated by a wind rather than the wind velocity generated by a pressure difference meteorologicaly.

[Gus Potter]

Hiya all.

 

I totally get your enthusiasm.

 

But you are all spending a lot of time an effort juggling numbers, could be the JH spread sheet could be some nice high end software.. I too have some fancy software in my office but recognise that it's just a tool to give you a heads up. 

 

Some folk on BH actually think this will happpen in real life for no extra cost and you will be able to russle up a builder who will play the game.. you are living cloud cuckoo land.

 

The reality is that you need to be able to get a local builder to execute your output and get material off the shelf.. Honestly as a self builder if you get so anal at this stage you have little chance of making your self build a success. I have self built my self and also built houses for othe self builders..

 

My advice is to use your calculations as a concept model and then adapt to what can be built at a reasonable cost to your specification. A good 50% of the above looks good on paper but is complete bollock in real life.

 

If anyone has a problem with this then you can find my mobile number on my web site or just give me a call on 0771 308 1597.

 

 

 

 

 

[joe90]

Even JH didn’t get it completely right, if I remember correctly his calculations didn’t take into account the local weather patterns (valley that trapped heat) and he ended up needing less heating and more cooling than the figures projected. 
 

I on the other hand (being a Luddite) did no calculations, copied other people’s principles and with paying attention to the detailing ended up with a comfy house that was cheap to run.

[MikeSharp01]

6 hours ago, Gus Potter said:

A good 50% of the above looks good on paper but is complete bollock in real life.

Perhaps a little harsh, we are, after all, trying to learn from one another - nobody is an expert at everything, evolve ideas where can and help others make choices in their projects. You are correct decision support software is just that it helps people make choices and designs need execution during which process yet more choices are made. There are, after all, choices everywhere in self building. All that is somewhat over simplistic because, for instance, during his time here @Jeremy Harris added vastly to the communities knowledge while also proliferating a myriad of choice points. Also, in the end, its good to talk.

[SteamyTea]

"All models are wrong, some models are useful"

 

I like to take the reductionist view, break down the parts until they are easy to calculate, then intergrade those results into a coherent mathematical model.

 

Yesterday's More or Less had a bit about the sampling size to get accurate results.

It is worth listening to the first 18 minutes and 40 seconds to get the the interesting bit.

 

The easy way to test a model is to change 1 parameter and see what the results are, then change that parameter back to the original value, and change another.

Keep the results and use them to create a distribution chart.