The air changes per hour in the leakage values is the most guessed parameter. Assessors have guidelines based on the house build date. If you have an actual measurement then the best designers can use that (or at least after they’ve asked the help desk or a colleague). That changed a default 3 ACH to the measured 0.5 in our case.

The worst assessors refuse to take into account what they can’t see such as retrofit cavity wall insulation. 
We knew our annual gas consumption so that was a great sanity check both for us and our chosen installer. That’s how it should be used, as a check to the figures, using a different calculator such as different tools doesn’t help if the same garbage gets entered.

Posted by: @editor

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Posted by: @drei

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To be honest every single heat loss in the UK is not fit for purpose, because a proper heat loss survey should combine airtightness testing with thermal imaging to provide a comprehensive understanding of energy loss. This is why, I would take the highest heat loss and go with that. As I mentioned, having oversized radiators is actually a great thing, as it allows for much lower flow

I wouldn’t go as far as saying every single heat loss in the UK is not fit for purpose because there are some excellent designers and engineers out there doing very thorough work, but I do take your broader point.

I actually filmed a podcast about this on Friday because the inconsistencies we’re seeing are really concerning. We’re also having a full heat loss survey done at our house next week with Heat Engineer to dig into this properly and see where the discrepancies lie. More education needs to be done on this to protect homeowners from crazy heat loss calcs.

It’s disconcerting how often these calculations are being done incorrectly or with such wide variations between companies. For something that underpins every aspect of system design, that’s not good enough.

Mandatory sense checks based on measured consumption (or something else if measured consumption not available).  Only way forward IMHO.  If the sense check and fabric assessment stack up to within the accuracy necessary for the design then proceed, else work out why there is inconsistency and reconcile it before spending thousands of pounds on installing the wrong kit.  

It will never happen however whilst the industry is in charge.  What a mediocre or poor installer requires is armour clad certainty that they are immune from rebuke.  One, and only one, mandated method provides this to perfection, the introduction of any kind of sense check destroys the armour.  Thus householders have to be taught how to do it themselves.

Posted by: @editor

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I actually filmed a podcast about this on Friday because the inconsistencies we’re seeing are really concerning. We’re also having a full heat loss survey done at our house next week with Heat Engineer to dig into this properly and see where the discrepancies lie. More education needs to be done on this to protect homeowners from crazy heat loss calcs.

@mars it seems madness giving you advice but I feel I must.  Whatever figure(s) you get somehow find a way to check them against your oil consumption or previous Heat Pump heat delivered.  Its a sense check and a safety measure and if its OK then proceed.  If its not find out why, if necessary by doing an air leakage test, reviewing U values particularly of any unusual materials and delving into the calculations. 

The physics of fabric heat loss calculations is totally sound, but the assumptions are, in some cases, pure speculation.  Furthermore many surveyors simply dial numbers into pre-constructed tools and have no idea what the tools are actually doing.  Consider mine, where room to toom losses were counted but not room to room gains, just one component of a miscalculation that led a 7kW house to be assessed as 16kW separately by two surveyors each following a 3 hr survey.

For a house assessed at 6kW it doesn't really matter much because you are going to install a 5-6kW machine whether the loss is 3 or 6.  For your house, with pretty high loss, it really, really matters. 

Posted by: @editor

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Posted by: @drei

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To be honest every single heat loss in the UK is not fit for purpose, because a proper heat loss survey should combine airtightness testing with thermal imaging to provide a comprehensive understanding of energy loss. This is why, I would take the highest heat loss and go with that. As I mentioned, having oversized radiators is actually a great thing, as it allows for much lower flow

I wouldn’t go as far as saying every single heat loss in the UK is not fit for purpose because there are some excellent designers and engineers out there doing very thorough work, but I do take your broader point.

I actually filmed a podcast about this on Friday because the inconsistencies we’re seeing are really concerning. We’re also having a full heat loss survey done at our house next week with Heat Engineer to dig into this properly and see where the discrepancies lie. More education needs to be done on this to protect homeowners from crazy heat loss calcs.

It’s disconcerting how often these calculations are being done incorrectly or with such wide variations between companies. For something that underpins every aspect of system design, that’s not good enough.

In all seriousness, if the engineer doesn't use thermal imaging and a air tightness measuring unit, then it will never be accurate, just best guess estimate, and none of them will be correct, except the one that made the most accurate guess:)

Imagine that I am asking everyone here to guess what number I am thinking of between 1 and 100. And you all come up with a number, and I say, Mars is the best guesser here, he got it correct, or almost correct, use him from now on for all guess work.

The other thing that could be done if you have 3 or 4 quotes with different heat loss numbers, (that hopefully you didn't pay £500 a pop for, thus resulting in a £2000 bill before the work even commences), would be to just do the average W per room based on their numbers, or go for the worst one so even if you oversize your radiators, it's all good, just don't oversize the heat pump:)

@editor I can understand why. But it’s a shame they don’t give you the details that go into the outcomes. One would assume the formulas they are using are the same. So the variables they put in must differ. But I understand why they don’t go into that detail as it would be too much for most homeowners. But if they do share those details I could probably say well that variables you put in isn’t correct knowing my home etc.

Posted by: @downfield

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Posted by: @drei

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What's the Jaga warranty on the fans, and do you know if they can just be replaced, or does it require a new radiators?

Not sure about the warranty, but they are purchased separately and use a number of individual fans so probably could be repaired easily.  See 

https://jaga.com/uk/dbh/

I just found this blog where the poster is building his own fan units and putting them into the Jaga rads.

https://www.vanwerkhoven.org/blog/2021/diy-jaga-dbe-dbh/

The Jaga fan units look very similar but of course include the control gear that senses when the flow temp rises and turn on the fans.

OK so from what I see now, the fans look similar to fan units used in data centres on servers, routers, network switches, storage device etc. Assuming these fans are powered, so need to be plugged in, I would definitely not have power near every radiator location.

Saying that, I purchased a Heat Fan, Stove Fan, no idea how it works, but once the fan gets hot, for example placed on a wood burner, it starts spinning spreading the heat:

https://www.vonhaus.com/vh_en/6-blade-double-stove-fan

This is the one I purchased a while back:

https://www.amazon.co.uk/Operation-Fireplace-Increased-Efficiency-Friendly/dp/B07K8R42WF/?th=1

If you could get fans that start spinning once they reach a certain temperature, and require no power, now that would definitely be something good, but these would most likely only work on gas boilers as they seem to operate around 60c.

Posted by: @jords

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@editor I can understand why. But it’s a shame they don’t give you the details that go into the outcomes. One would assume the formulas they are using are the same. So the variables they put in must differ. But I understand why they don’t go into that detail as it would be too much for most homeowners. But if they do share those details I could probably say well that variables you put in isn’t correct knowing my home etc.

You could go with the average, but that would also be inaccurate.

I won't be taking credit for this one, just ran it through an AI with slight amendments:

What causes installers to produce different results

Installers commonly differ on:

What is the correct approach

You want one heat loss report that is:

✅ Based on actual property data, not guesswork
✅ Uses consistent design temperatures (e.g., 21–23°C for living areas)
✅ Uses realistic U-values for your building
✅ Uses the correct outdoor design temperature for your region
✅ Uses correct ACH based on your fabric and leakage

If one engineer does this properly, their result will be more accurate than averaging four sloppy ones.

Very useful rule of thumb

If three or four quotes produce different results and one of them also “feels” right from lived experience (pre-heat-pump heating pattern) — that is often the one that matches reality.

In your case — because you lived in the house and know:

• You comfortably heated it to 22°C or whatever temperature it is comfortable for you now
• You know how much gas you use
• You know which rooms bleed heat

— those observations help verify whether a heat loss number makes sense.

One heat loss properly done Manual J / EN12831 calc with correct inputs is the most accurate one.