General: Home Energy Model

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- CommentAuthordjh
- CommentTimeDec 15th 2023
Will in Aberdeen spotted the Home Energy Model initiative: https://www.gov.uk/government/consultations/home-energy-model-replacement-for-the-standard-assessment-procedure-sap but can't start an appropriate thread. So here is one.
- CommentAuthordjh
- CommentTimeDec 15th 2023
Posted By: WillInAberdeenBut problems are still there:

- software has to cope with design phase where key inputs are not known yet, like airtightness results, shading, occupancy for new insulated homes where solar/incidental gains make big difference to whether heating is needed or not.
Every modelling system for new builds has to deal with that issue; it's the nature of the beast. Shading and solar gains are indeed important but they are known in advance so aren't a problem. Occupancy and airtightness are dealt with in PHPP by standardisation and requirement respectively; I expect HEM could do similar - I haven't looked yet.
- CommentAuthorWillInAberdeen
- CommentTimeDec 16th 2023
Thank you!

I said previously:
SAP is to be abandoned and replaced by something called the Home Energy Model... The new Home Energy Model software that is going to replace SAP is apparently more detailed and has been tested against PHPP..

But problems are still there:

- software has to cope with design phase where key inputs are not known yet, like airtightness results, shading, occupancy for new insulated homes where solar/incidental gains make big difference to whether heating is needed or not.

- same software has to cope with EPCs for old houses where key inputs are not visible, like AT or insulation buried in walls or under floors

Apparently they tested the new software against real world data measured from a passivehouse, it did ok until the occupant started closing the blinds, so keeping the sun out.

Elsewhere they said that SAP10, PHPP and Home Energy Model performed pretty much the same, when fed with the same standardising assumptions. However they all have different default assumptions built in, which they say explains the differences seen between SAP and PHPP.

Home Energy Model apparently doesn't yet deal with blinds and curtains, maybe to be added later (as they are on many houses!).

One change is that Home Energy Model makes energy balances over half-hour slots, versus SAP which balanced per month. So daytime PV will be less useful, as cannot be used so easily to offset evening electricity use. Thermal mass will be more important to model correctly to carry daytime gains into the evening.

They also seem to have increased the Primary Energy Factor of grid electricity from 1.5 to nearly 2, which is a penalty on choosing direct electric heat or DHW, compared to gas and coal at 1.1. However the grid CO2 intensity has been halved, and when heatpump CoP is applied on top of that in the notional house spec it sets an ultra low TEr, pretty much impossible to meet with fossil or direct electric heat or DHW.
- CommentAuthorborpin
- CommentTimeDec 16th 2023 edited
Posted By: WillInAberdeenOne change is that Home Energy Model makes energy balances over half-hour slots, versus SAP which balanced per month. So daytime PV will be less useful, as cannot be used so easily to offset evening electricity use.
Batteries of course solve that!

Posted By: WillInAberdeenHome Energy Model apparently doesn't yet deal with blinds and curtains, maybe to be added later (as they are on many houses!).
But that is not simple as (for instance) it depends on the aspect of the house. If the 'public' rooms are on the North, then passive solar will have less impact on the rooms often heated the most.

Anything that depends on the sun/passive gain is always fraught with danger.

For a mass builder, with a standard house design, that could be sited in any orientation, something like PHPP will never work.

[edit]
There is also a thread running on Twitter of a CFSH5 certified (I think) flat that has so many flaws in the insulation is is a struggle to heat. It might be airtight, but if the insulation behind the AT membrane is faulty, it will still be cold (as shown by the TI images).
- CommentAuthordjh
- CommentTimeDec 16th 2023
Posted By: borpinBatteries of course solve that!
But at a cost. Both monetary and embodied carbon.

For a mass builder, with a standard house design, that could be sited in any orientation, something like PHPP will never work.
Then such a builder needs to adapt to the real world and start to design buildings that bear the orientation in mind. Much as there is already pressure to orient roofs to have north and south slopes for PV.

I'd be interested in the twitter thread or links to any external references it contains that explains how it came to be certified with such defects. Where is the flat? And who certified it? And so on.
- CommentAuthorWillInAberdeen
- CommentTimeDec 16th 2023
>>>"Both monetary and embodied carbon."

Home Energy Model doesn't consider capital cost, or embodied carbon, neither do SAP or PHPP.

The question is: why don't they?

In particular, the carbon from heating under the HEM input factors, has reduced to a very low level, so the focus on carbon impact of buildings needs to shift to include embodied carbon.

Building regulations are going to have to start regulating embodied carbon, as are private standards such as PH and AECB. To do that they're going to need a standardised software model to compare the embodied carbon of different designs against a standard. The new HEM software should (eventually) be the single point software, we don't want two incompatible software products for regulating energy carbon and embodied carbon.

Example based on LETI primer and HEM assumptions:

Carbon from heating over building lifetime 35kWhth/mÂ²/a Ã· COP3 x 0.09kg/kWhe grid intensity x 100 years life = 100kg carbon per mÂ²

Carbon embodied from construction - now typically 1000kg per mÂ², 2030 best practice target 300kg/mÂ²

-> embodied carbon is 3x to 10x more significant than heat losses.
- CommentAuthorborpin
- CommentTimeDec 16th 2023
Posted By: djhBut at a cost.
Everything has a cost. I simply pointed out a solution to the problem presented.

Posted By: djhI'd be interested in the twitter thread
Went back and it wasn't passive but CFSH5 and supposed to be Zero Carbon. https://twitter.com/jimmybb/status/1733051913027949027

Posted By: WillInAberdeenHome Energy Model doesn't consider capital cost, or embodied carbon,
My bug bear. Why do we still use brick and block skins? Because Planners very often demand it. I've said it before, if planners had been around longer, we'd still be in mud huts.
- CommentAuthordjh
- CommentTimeDec 16th 2023
It's unreasonable to expect an energy model to consider costs. It is reasonable for a designer or client to consider costs, and in particular buying on-site batteries to apparently shift PV generation time may very well be not justified in many circumstances at present. So they are an alternative rather than a solution. The energy should allow for batteries being present in the model in so much as they change the values it does care about.

It's similar to the heat leakage from a hot water cylinder contributing to the warming of a building.

Posted By: borpinWent back and it wasn't passive but CFSH5 and supposed to be Zero Carbon. https://twitter.com/jimmybb/status/1733051913027949027
Whew So it's another demonstration of why PHPP certification is valuable, rather than some ramshackle gov.uk scheme

In a PH the internal surface temperature should be at least 17Â°C, which is checked in the design by the materials and components used, and that it was built to design by the certifier using photographs and other records.
- CommentAuthordjh
- CommentTimeDec 16th 2023
My bugbear is that PHPP should be allowed as an alternative model, or preferably adopted instead of developing yet another UK-only model. How they account for pre-existing buildings is a whole different kettle of fish whichever way they do it.
- CommentAuthorWillInAberdeen
- CommentTimeDec 16th 2023
Home Energy Model will apparent allow both Heat batteries and Electrical batteries and neither require PV to be fitted. So you can import off-peak electricity, or self generate, and use it later. I think storing your PV will be very favourable to your score in HEM, because the Primary Energy Factor for consuming electricity yourself (1.97) is set much higher than the PEF for exporting PV (-1.0) and reimporting it later. So using a battery would score highly, and allow you to score less highly in other areas (insulation etc) and still pass.

If HEM calculated the embodied carbon of the PV and batteries (it doesn't) then it would be easy for the designer to run the same house design with and without batteries, and check which has the lowest net carbon.

Likewise the designer could run the house with and without extra layers of polystyrene insulation, or concrete thermal mass, and see which thickness has the lowest net impact. As it is, HEM will favour infinite thicknesses of both.
- CommentAuthorWillInAberdeen
- CommentTimeDec 16th 2023
Edit to add - PHPP works the same, but more so afaik, so storing and self consuming PV (primary energy factor = 0.9 iirc) scores very much better than exporting it and reimporting it later (2.7). I don't know if PHPP explicitly recognises battery storage yet, or only heat storage in cylinders or slabs.

I'd be happy to see PHPP used for building regs compliance but can't see why PHI would want to agree to that, as gov would presumably want control to tweak it.

At some point as DJH mentioned the software will have to spit out EPCs for existing houses, based on 20-minute 'surveys', so it will need a lot of robust assumptions and default values.
- CommentAuthordjh
- CommentTimeDec 17th 2023 edited
Scoring PV consumed directly versus exported to the grid battery and then re-imported rapidly gets complicated, with all kinds of exceptions and what-ifs. At one point I think I had understood it (the PHPP version?), but I no longer can remember the details and I'm not sure I can be bothered to re-learn it. It all feels like angels dancing on a pin.

IIUC (If I understand correctly) the way HEM is organised is as a base physics model, with various wrappers around it to incorporate policy (or political) judgments. PV export scoring ought to be part of whichever wrapper IMHO. I don't know whether it is?

Clearly the PHI would not allow the UK government to control it, which is all to the good IMHO. But I don't see that as a valid reason for the UK government (edit:) NOT to adopt it in building regs, as some in the Scottish parliament are trying to do.
- CommentAuthorfostertom
- CommentTimeDec 18th 2023 edited
<blockquote><cite>Posted By: WillInAberdeen</cite>the focus on carbon impact of buildings needs to shift to include embodied carbon</blockquote>
or even make it the primary consideration - this is really the message of the moment, according to brilliant and authoritative new book
https://www.amazon.co.uk/Build-Beyond-Zero-Carbon-Smart-Architecture/dp/1642832111/ref=sr_1_1?crid=1ALNK5MEHXU0Q&keywords=build+beyond+zero&qid=1702891707&sprefix=build+beyond+zero%2Caps%2C193&sr=8-1
quite hard to swallow for all us eco-builders who've been thinking energy-in-use for so long.
- CommentAuthorWillInAberdeen
- CommentTimeDec 18th 2023
Think I've been saying that a couple of years here, not sure how well I went down!

The best time to build a passivhaus with thick insulation and thermally massive floors, was probably ten years ago, when saving heating was a very good way to save carbon. The new round of regulation for 2025+ are for a different time, when electric pumped heat has very low carbon (per the new HEM carbon intensity data) but concrete and polystyrene are still high carbon.

It's actually quite breathtaking how fast the world changed, and the draft regs have not really kept up - still requiring U values 0.1-0.2 with carbon-intensive insulation, when U= 0.5 might be the new sweet spot for lowest lifecycle carbon. Unfortunately the new HEM model is another Energy Model, not an Embodied Carbon model or a Lifecycle model.

For something like a house which will last (hopefully) 100 years, a question is whether it should be heavily optimised for the situation of the next decade (low carbon energy, high carbon materials), so risk becoming sub optimal a few decades in future (-> poss demolished like much 60s/70s high rise). If not, then what should it be? Adaptable, modifiable, extendable, perhaps.
- CommentAuthortony
- CommentTimeDec 18th 2023
I would rather look at thing in terms of energy used/saved and the costs and make sensible decisions based on economics.

For me u-values below 0.1 for walls and ceilings/roofs and 3g as standard air tightness as Canada
- CommentAuthorborpin
- CommentTimeDec 18th 2023
Posted By: WillInAberdeenstill requiring U values 0.1-0.2 with carbon-intensive insulation, when U= 0.5 might be the new sweet spot for lowest lifecycle carbon.
I like the look of straw as the insulation material.

If I was going again (I wish) I think insulated slab, Straw insulated walls, recycled plastic 'weather board' or the panels made from Volcanic rock.

Does that offer the lowest embodied carbon footprint I wonder?

The 12M swimming pool might be a problem though...
- CommentAuthordjh
- CommentTimeDec 18th 2023
Posted By: borpinIf I was going again (I wish) I think insulated slab, Straw insulated walls, recycled plastic 'weather board' or the panels made from Volcanic rock.
The reason for lime render has a lot to do with fire resistance. But certainly Ecococon panels or similar look interesting. I'd also look again at a timber floor structure if I was building again, although a lot depends on the site. Looking at the heave I see in our garden, I'm glad our floor is pretty solid!
- CommentAuthorwookey
- CommentTimeDec 20th 2023
Posted By: WillInAberdeen>>>"Both monetary and embodied carbon."

Home Energy Model doesn't consider capital cost, or embodied carbon, neither do SAP or PHPP.

The question is: why don't they?

To be fair energy modelling is not the same as emissions modelling. But getting both answers from one model makes a lot of sense. There is a PHPP addon that does embodied energy: PHribbon
https://aecb.net/aecb-phribbon/

Building regulations are going to have to start regulating embodied carbon, as are private standards such as PH and AECB. To do that they're going to need a standardised software model to compare the embodied carbon of different designs against a standard.

Agreed and they have had one for a few years. It's all annoyingly proprietary, but it does exist.
- CommentAuthorwookey
- CommentTimeDec 20th 2023 edited
Clearly the PHI would not allow the UK government to control it, which is all to the good IMHO. But I don't see that as a valid reason for the UK government (edit:)NOTto adopt it in building regs, as some in the Scottish parliament are trying to do.

Neither I, nor the UK govt, are very keen on adopting a proprietary bit of software/spec controlled by a foreign private entity as the basis for national legislation.

If PHPP was an open standard then it would indeed be a good choice for such adoption, but it isn't (although it is inspectable (effectively 'source-available'), which is better than most software). The govt are clearly considering open-sourcing the HEM (the code is in a git repo - well it will be 'in a few days' it says at: https://dev.azure.com/BreGroup/Home%20Energy%20Model ), which would be a very good idea IMHO (we are paying for it after all).

Then they can probably do away with the current Â£1000 certification fee people have to pay BRE before their software is deemed an accurate implementation of SAP (because everyone will just use HEM). Legislation can specify particular releases if total repeatability is needed, and the model can develop over time.

This is an interesting development, which could be very positive (I have been arguing for more open design software for about a decade now). I wish I had more tuits to properly look into the model.
- CommentAuthorWillInAberdeen
- CommentTimeDec 20th 2023
The previous building regs set out the rules (eg "U value shall be 0.11 or better" and just referred to SAP as a procedure by which to do the calculations. Anyone could write software to do the SAP procedure, various free spreadsheets have emerged.

The new regs will apparently just say "building must pass in the government's HEM software". The actual rules such as U values are buried in the HEM software spec, which seems to be a 166page spreadsheet. Not as good for usability IMO.
- CommentAuthorWillInAberdeen
- CommentTimeDec 20th 2023
Edit: sorry that sounded grumpy! not intended, I'm missing the GBF edit function.

It's good that HEM is open source. Would be good if people might integrate it with their own energy model software, to make a one-stop software model using 6 one set of input data. for building design, regulations compliance, EPC update, heating system. EG

- MCS require that heat pumps are designed using their proprietary heat loss model spreadsheet, would save duplicate inputting if this could feed input data to/from HEM and use same heat loss calculation

- There's some good heat loss model software called Heat Engineer which boiler installers can use to survey a house, calculate its heat losses, select radiator sizes and pipe runs, estimate radiator temperature hence CoP. Has good feature to scan dimensions of each room using your mobile phone camera and make a 3D model. https://youtu.be/LdzLT-7vid4
- CommentAuthordjh
- CommentTimeDec 20th 2023
Posted By: WillInAberdeenThere's some good heat loss model software called Heat Engineer
I just took a look at their site. I Had a go with their heatloss estimator and it came out with a reasonably accurate estimate when I fed in details from my PHPP spreadsheet, so that's good. The software appears to be proprietary so I've written to ask them how it's been audited.

They say it was written using calculations from the 2014 CIBSE Domestic Heating Design Guide. It's not clear why they didn't use the 2021 guide?

Their site refers to https://www.mapcoordinates.net/en as a means of finding elevation. That seems to be a useful site that gives accurate coordinates very simply
- CommentAuthorfostertom
- CommentTimeDec 20th 2023
Nice one, along with PVGIS and Meteonorm (generates weather data for input into WUFI, except doesn't give wind direction)
- CommentAuthorMike1
- CommentTimeDec 20th 2023
Posted By: wookeyNeither I, nor the UK govt, are very keen on adopting a proprietary bit of software/spec controlled by a foreign private entity as the basis for national legislation.

If PHPP was an open standard then it would indeed be a good choice for such adoption, but it isn't (although it is inspectable (effectively 'source-available'), which is better than most software)
Apart from being published by a independent research institute with long standing integrity, PHPP remains the most accurate model that that has proven accuracy over a wide range of buildings, based on decades of scientific evaluation. Anything the UK Government comes up with is unlikely to be more accurate, and its accuracy is likely to take several years to prove in real-world use (assuming anyone puts up the cash to fund that).

I therefore can't see any technical reason why PHPP couldn't be accepted as an alternative, even if the Government feels the need to restrict its use to specific version specified in a statutory instrument.
- CommentAuthordjh
- CommentTimeDec 20th 2023
Posted By: Mike1even if the Government feels the need to restrict its use to specific version specified in a statutory instrument.
That sounds like a nightmare in practice. You need to use an old version for the government and a new one for the PH certificate.
- CommentAuthordjh
- CommentTimeDec 20th 2023 edited
Posted By: WillInAberdeenIt's good that HEM is open source.
Is it?

I see a statement in hem-tp-01-general-summary-of-core-calculation.pdf : "The Home Energy Model reference code
What: The full Python source code for the Home Energy Model and the Home Energy Model: FHS assessment has been published as a Git repository." and 'a Git repository' is a link to https://dev.azure.com/BreGroup/Home%20Energy%20Model but that says: "The code for this project will be made available in the coming days."

So unless I'm missing something, the claim that the model is open source is a lie as is the specific statement that the source has been made available.

edit: to add that I've asked them for an explanation.
- CommentAuthorwookey
- CommentTimeDec 21st 2023
Give them the requested 'coming days' Dave. The doc was published 8 days ago. I presume the source will be uploaded reasonably soon. The definition of its openness or otherwise is the licence it is published under, not its presence in a github repo - that is just a convenience. We don't actually know what that licence is yet.

The technical doc "General summary of the Home Energy Model core calculation" says "The full Python source code for the Home Energy Model and the Home Energy Model: FHS assessment has been published as a Git repository. This code is identical to that sitting behind the consultation tool. We are currently considering whether the open-4HEM-TP-01 General summary of core calculation source code could serve as the approved methodology for regulatory uses of the Home Energy Model."

Obviously it would have been better if it had in fact been uploaded before the release of that but I'm not going to hold this delay against them unless it becomes excessively long. The govt is being much more open than they usually are about this sort of thing and that is a good thing. And it's nearly crimbo - maybe someone went on holiday - give them a chance.
- CommentAuthordjh
- CommentTimeDec 21st 2023
Posted By: wookeyGive them the requested 'coming days' Dave.
The document says "has been" not "will be"! They should publish accurate documents in the correct order. I will wait and see what they say. As it stands it is a lie. It's a failure of their much-vaunted QC process if nothing else.
- CommentAuthorWillInAberdeen
- CommentTimeDec 23rd 2023
https://dev.azure.com/BreGroup/_git/Home%20Energy%20Model

HEM source code was actually published earlier this week, under MIT open license. The text about 'coming days' was just a placeholder that hadn't been removed from the front page.
- CommentAuthorWillInAberdeen
- CommentTimeDec 23rd 2023
Correction: the code has been developed in that repository for months. The latest tweaks were earlier this week.

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General: Home Energy Model