Interpreting Heat Loss calculations and designing systems for a new build

[Don D]

Hi,

We're considering a self-build and looking at how to get to a point where we have a reasonable idea for the space and water heating systems, ahead of engaging/paying for any professional advice.

 

Our current build approach is based on the MBC insulated raft and timber frame package with triple glazed windows, they're local to us and we know the product well.

We THINK a correctly sized ASHP combined with UFH, an oversized pv array with battery capacity and a hw tank are likely to be our main system components and it's this we need to work on.

 

We know the u-values for the key fabric elements and we have the volumes, we even still have a copy of Jeremy's Fabric and Ventilation Heat heat loss calculator (from 2015)… the problem is, I am not sure which are the key output figures from it and how to use those to help size the heating requirement.

 

So there's a few things there, but if anyone has used a calculator like this and can advise or equally is there another way of getting to the same outcome, we'd be interested to know.

 

Cheers

 

 

[JohnMo]

1 hour ago, Don D said:

We THINK a correctly sized ASHP combined with UFH, an oversized pv array with battery capacity and a hw tank are likely to be our main system components and it's this we need to work on.

PV and battery are separate to heating requirements, so not really relevant to question asked.

 

There are some requirements for an ASHP often over looked and that is defrost - UK is pretty bad for it depending on exact location.  Inland being better than coastal or near a big body of water. So generally it wise to oversize for the typical -3 design in the UK.  Second is DHW generation, a small heat pump at design temp can take a while to heat a big cylinder as in a couple of hours.  So you are better to look at a 6kW heat pump but make sure it has good modulation so it runs well for long periods of times for best CoP.

 

The heat calculation, needs your ventilation system air changes per hour I would assume 0.5 to 0.3, and with MVHR efficiency circa 85 to 90%.  You need to find average temp in your area for the monthly heating needs. You need window and door sizes, general roof, floor and wall areas and thats about it, with U values

[Conor]

A better way to do it is to aim for a total heat demand / load per m² and make sure your build hits that through design choices and build quality. E.g. PH standard of 10W m² and take if from there. Decide what you want from your house, how you want it to feel and perform, and design accordingly.

[Don D]

4 hours ago, JohnMo said:

PV and battery are separate to heating requirements, so not really relevant to question asked.

 

There are some requirements for an ASHP often over looked and that is defrost - UK is pretty bad for it depending on exact location.  Inland being better than coastal or near a big body of water. So generally it wise to oversize for the typical -3 design in the UK.  Second is DHW generation, a small heat pump at design temp can take a while to heat a big cylinder as in a couple of hours.  So you are better to look at a 6kW heat pump but make sure it has good modulation so it runs well for long periods of times for best CoP.

 

The heat calculation, needs your ventilation system air changes per hour I would assume 0.5 to 0.3, and with MVHR efficiency circa 85 to 90%.  You need to find average temp in your area for the monthly heating needs. You need window and door sizes, general roof, floor and wall areas and thats about it, with U values

Thanks for the response, especially on the heat pump.

 

With the heat calc, I think I've input everything that's needed but it's understanding the result the spreadsheet is giving I need advice on. I've uploaded it here if you want to have a look.

Fabric_and_ventilation_heat_loss_AVETTE.xls

[SBMS]

Easy: build yourself using ChatGPT. 
easier: use jeremy spreadsheet. 
easiest: https://heatpunk.co.uk

 

check each. They were all within a few percentage of each other (which is good enough for heat pump sizing to size to nearest pump). 

[Gus Potter]

6 hours ago, Don D said:

We're considering a self-build and looking at how to get to a point where we have a reasonable idea for the space and water heating systems, ahead of engaging/paying for any professional advice.

Hi Don.

 

Admire the effort you are putting in, you will be rewarded. 

 

Jeremy Harris is an icon on BH, I've learnt a lot from his posts. His spread sheet is a good aid, a great tool to use as a sense check. 

 

I've opened up the spread sheet to see what inputs you are using. I do a quick sense check, mainly to see if the numbers look ok and where you are pushing the U values to think.. how much is this going to cost, get a supplier and so on. Your inputs are screen shotted below:

 

 

So to compare with the Scottish target U values, which are quite stringent but will serve you well if living further down south. The objective of my comment is to get you to think.. how much is this going to cost us to build, get local contractors interested in competing for you job and, can we spend our money in a better way. 

 

 

I wonder, where did you get the values you are using for your inputs?  and can you post some details on how you are going to construct the basic elements of the fabric. 

 

The U values for the glazing look a bit optomistic. Best to be pragmatic and conservative at this stage to avoid disappointment later. 

 

 

[Don D]

Thanks Gus. The floor, wall and roof values are direct from the MBC passive frame & foundation, the ACH similarly are what they will regularly achieve in the blower test. They are probably unique in the UK in offering the foundation and frame, it's one of the main selling points for us, reducing/removing the heat bridging issues and the risk of using separate contractors.

 

The windows are from Rationel, their Auraplus series and the advertised values. 

 

 

[JohnMo]

Cell B2, will be -3 in England, -5 Scotland.

Cell B4 use 6 degs if well insulated 

Cell B2 is ground floor area only

Cell B45 will give you your heat loss 

[JohnMo]

1 minute ago, Don D said:

ACH similarly are what they will regularly achieve in the blower test

The figure has nothing to do with that, it's your ventilation through put. Spreadsheet already assume very airtight build.

[Gus Potter]

16 minutes ago, Don D said:

Thanks Gus. The floor, wall and roof values are direct from the MBC passive frame & foundation, the ACH similarly are what they will regularly achieve in the blower test. They are probably unique in the UK in offering the foundation and frame, it's one of the main selling points for us, reducing/removing the heat bridging issues and the risk of using separate contractors.

 

The windows are from Rationel, their Auraplus series and the advertised values.

Ok what I can tell you is that I have designed as an SE a few insulated rafts over the years, once you get down to the engineering there is no ideal world, there is a trade off between engineering cost and satisfying the window suppliers " best performance". Building houses is an art where you have to make informed compromises. 

 

 

You have to recognise that if you go for the best value it will dictate everything else you do and that can come at and often nasty hidden cost. 

 

If you pair everything down now when doing you basic costing it will come back to bite you. 

 

The best advice I can give is to think about who might build this for you and sound out MBC on their caveats on the ground conditions, if you need steels and so on, point loads from the structure etc. 

MBC are by all accounts good at what they do but they will charge you for every extra! be that funny loads or funny geometry of you house. 

 

Also, sound out local Contractors, they may tell you, keep it simple and stupid, spec stuff we can source easily, and we will deliver an air tight structure. 

 

Oh and on a light note. If you are a doggy person then leave an unheated bit in the floor so your dog goes there for respite! UFH is not good for dogs!

 

 

 

 

[SimonD]

On 01/12/2025 at 21:44, Gus Potter said:

Ok what I can tell you is that I have designed as an SE a few insulated rafts over the years, once you get down to the engineering there is no ideal world, there is a trade off between engineering cost and satisfying the window suppliers " best performance". Building houses is an art where you have to make informed compromises. 

 

 

We had quotes for Rationel windows. Interestingly the Declaration of  Performance gives a specific window size for this best performance value. I was also a bit intrigued that the declaration of performance gives the same value for different types of window and openings at that same size. Our triple glazed windows from a different manufacturer all have a different whole window U-value that was given within the quote and design stages. Whole window value average out at something like 1.03 across the whole house in the end with the detail not differing much from the above image although our frames are a bit more chunky.

 

On 01/12/2025 at 20:30, SBMS said:

Easy: build yourself using ChatGPT. 

 

No, definitely don't do that and rely on it. I've been playing around with using AI for heat loss calcs and system design using several AI models and each of them have got some fundamentals wrong.

On 01/12/2025 at 21:13, Don D said:

Thanks Gus. The floor, wall and roof values are direct from the MBC passive frame & foundation, the ACH similarly are what they will regularly achieve in the blower test. They are probably unique in the UK in offering the foundation and frame, it's one of the main selling points for us, reducing/removing the heat bridging issues and the risk of using separate contractors.

 

There's no doubt that Jeremy's heat loss sheet is great value tool, but heat loss calculation methodologies have moved on a bit since that sheet was built. With and MBC passive frame and foundation, whoever is doing the heat loss modelling should be provided with the thermal bridging psi values which are used to input into a full room by room heat loss calculation. Also the heat loss nowadays includes a whole house fabric ventilation loss together with a room ventilation loss which is derived from blower tests because it now considers losses due to wind loads (thus pressure differences across the building). Essentially, the previous way to do heat loss calcs would lump ventilation and infiltration losses into one figure, nowadays these are separated in the calculation, to hopefully reach a more accurate heat loss.

 

On 01/12/2025 at 21:15, JohnMo said:

The figure has nothing to do with that, it's your ventilation through put.

 

It does. As I mention above, the old way of using ACH is heat loss calcs is to lump infiltration and ventilation losses together. So the 0.45 in the spreadsheet must be assumed to be a combination of the 2. The 0.45 for a very airtight house with MVHR I'd suggest could be a bit high with a well detailed build. Mine where I have designed a natural ventilation system (yes @Gus Potter I'm with you on natural ventilation 😉 ) comes out at 0.38 ACH even when using intermittent extract ventilation in bathrooms, for example.

[JohnMo]

Only thing I would add, the Jeremy spread sheet is designed for a high performance and pretty much airtight house. So if you are not that kind of house the calculation are pretty different and yes you do need to take into account natural ventilation as well as forced ventilation.

 

So the build looks to be MBC which normally comes with a leakage rate below passivhaus levels. So at any point in the building life, the natural infiltration is next to nothing. So can be ignored to the gross heat loss pretty well sorted. 

 

But to do heat loss correctly you need to do the room by room approach 

 

25 minutes ago, SimonD said:

The 0.45 for a very airtight house with MVHR I'd suggest could be a bit high

Building regs drive you to 0.5 ACH throughput of the MVHR system. Passivhaus drive you to 0.3. Passivhaus is closer to where you need to be in winter so as not to over ventilate.

[SimonD]

13 minutes ago, JohnMo said:

Building regs drive you to 0.5 ACH throughput of the MVHR system. Passivhaus drive you to 0.3. Passivhaus is closer to where you need to be in winter so as not to over ventilate.

 

Not quite - but we might be talking cross purposes due to different building regulations. I'm referring to England and Wales as I'm not familiar with the Scottish ones.

 

So in England, Part F provides a minimum rate per m2 of floor area, or alternatively a minium rate per number of bedrooms. I'm sure with some properties, depending on the height of rooms, this will be approx 0.5, but in my case it comes out at 0.34 ACH for continuous whole house mechanical ventilation, so the ventilation is going to vary by property. And, of course then Part L says you need to design not to over ventilate. Part of the problem is that people see the 0.5 ACH given in category C of Table 3.8 of the CIBSE Domestic Heating Design Guide, for example, and assume this is a specified/required amount. It isn't, as it says clearly in the guide that houses that have been designed and built with high levels of airtightness require specialised advice to avoid over ventilation. So in these cases, reduced ventilation rates can be used with sufficient justification, usually in my experience to just do the basic calcs using the figures given in the approved documents.

 

[JohnMo]

Scottish building regs, just state a simple number, 0.5 ACH of whole house internal volume for MVHR (balanced inlet and outlet). Boost is above this rate. So no if buts or maybe, pretty simple, but leads to massive oversized ventilation on anything bigger than a shoe box.

 

MVHR is also mandatory for any house that has an air test result of 3m³/m² @ 50Pa.

 

The actual air test result is at 50Pa equal to gale force wind acting on all surfaces of the building - which can never happen, as there are always sheltered sides to a building. So actual air test result is factored down depending on your location etc. So in OP case 0.45 ACH at 50Pa is equivalent to 0.04 air infiltration, so pretty much meaningless in the calculation. Hence say that isn't the figure for air test result isn't the figure to add to spreadsheet.

 

 

[Nickfromwales]

On 01/12/2025 at 15:44, JohnMo said:

PV and battery are separate to heating requirements, so not really relevant to question asked.

 

There are some requirements for an ASHP often over looked and that is defrost - UK is pretty bad for it depending on exact location.  Inland being better than coastal or near a big body of water. So generally it wise to oversize for the typical -3 design in the UK.  Second is DHW generation, a small heat pump at design temp can take a while to heat a big cylinder as in a couple of hours.  So you are better to look at a 6kW heat pump but make sure it has good modulation so it runs well for long periods of times for best CoP.

 

The heat calculation, needs your ventilation system air changes per hour I would assume 0.5 to 0.3, and with MVHR efficiency circa 85 to 90%.  You need to find average temp in your area for the monthly heating needs. You need window and door sizes, general roof, floor and wall areas and thats about it, with U values

With your love of buffer tanks, what’s the method of defrosting in the absence of one? 
 

I’ll go get my tin hat ready……

[SimonD]

1 hour ago, JohnMo said:

leads to massive oversized ventilation on anything bigger than a shoe box.

 

MVHR is also mandatory for any house that has an air test result of 3m³/m² @ 50Pa.

 

This is a problem from my perspective as it's too prescriptive. The English system at least gives provision for multiple ventilation strategies. Natural ventilation really has to be a choice even in very airtight buildings - and despite what some many in the industry say, it with good design works in those building too. Making MVHR mandatory just gives the industry a golden invitation to cream as much as they can out of it.

[Nickfromwales]

3 minutes ago, SimonD said:

Making MVHR mandatory just gives the industry a golden invitation to cream as much as they can out of it.

As a blanket yes, but after 3ach not so much.
 

People in affordable of HA housing will switch MVHR off and happily choke to death, whilst drying wet clothes in 3 rooms and typing out a complaint about the mould and condensation issues.

 

You can’t educate pork, but also there are some terrible handovers conducted, or none at all, so what people don’t know they simply won’t know.

 

MVHR and airtightness are both the best things since the slicing of the first loaf, but it needs to be done correctly, in a dwelling that sympathises, and be owned by an informed occupant / end user. 

[JohnMo]

1 hour ago, Nickfromwales said:

With your love of buffer tanks, what’s the method of defrosting in the absence of one? 
 

I’ll go get my tin hat ready……

Just the normal way, I just let the heat pump do its own thing - you don't need a buffer for defrost, you just need the minimum prescribed volume available, as detailed in the installer manual - you never did need a buffer, it just hang on from fixed duty heat pumps.  Heat pump just moves the 4 port refrigerant valve to cooling mode, refrigerant does the defrost. Water circuit just provides the heat. It's only in defrost mode a couple of minutes.  Attached from the other day when it was cold and defrosting. The attached is the temperature and heat flows. Green line is return temp (from heating system/volumiser), red line is flow temp, yellow is heat pump thermal output, blue area is electrical input. - no hat needed😀

 

[Nickfromwales]

1 hour ago, JohnMo said:

Just the normal way, I just let the heat pump do its own thing - you don't need a buffer for defrost, you just need the minimum prescribed volume available, as detailed in the installer manual - you never did need a buffer, it just hang on from fixed duty heat pumps.  Heat pump just moves the 4 port refrigerant valve to cooling mode, refrigerant does the defrost. Water circuit just provides the heat. It's only in defrost mode a couple of minutes.  Attached from the other day when it was cold and defrosting. The attached is the temperature and heat flows. Green line is return temp (from heating system/volumiser), red line is flow temp, yellow is heat pump thermal output, blue area is electrical input. - no hat needed😀

 

Thanks for that, as nowt beats a bit of raw data!

 

Interesting to see how it's still performing admirably, even immediately before deciding it's time for a defrost.

 

The ASHP that's just gone in at one of my current projects defrosts by using some of the buffer volume, I believe. When we had the bitter cold snap recently it defrosted in probably 30-45 seconds; just a big cloud of steam and an almost instant runoff of water from the melted ice.

 

A few minutes at not much more energy consumption doesn't seem to be the worst thing, so your graph offers some assurances, so 👍.

 

The hat's got a few holes in it, may not have taken another hit tbh......  

[JohnMo]

The latest Viessmann ASHP do it slightly differently - on approach to defrost, they move an internal valve and charge a small buffer to about 50-60 degs and the whole defrost energy comes from that.  Slightly more efficient to the normal.