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: @drei

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Assuming these fans are powered, so need to be plugged in,

Yes they need power - but very little IIRC. Perhaps 20W per rad.  You wouldn't get much air movement without it.

I had 13A sockets nearby and put the necessary spur inside the top part of the casing - which is empty - before they were fitted.

@jords were you not given a copy of the full heat loss report from any of the 3 quotes? 

I use heat engineer software to carry out heat loss reports and I send the full report to customers which has all details to how I have come to the conclusion I have with the heat loss.

from air change rates, u values for all materials within the house, radiator current sizes and proposed radiator sizes and lots more. 

I think no two people will be exactly the same on a heat loss report as it is an interpretation of a house and the materials used to build the house. 
You will probably find the heat loss reports on the higher end have stuck to CIBSE’s design guide on air change rates resulting in a higher heat loss. 

As for the placement of the cylinder the heat geek installer may have allowed for a pump for the Temp & Pressure relief to allow the cylinder to be located where he suggested.

It seems you’re doing your own research which will hopefully result in getting a great install at some point in the future. 

On the subject of heat loss. Today I called an Essex based company called Dynamic Insulation, mentioned that I had a heat pump installed and my house doesn't seem to retain the heat which doesn't help with my undersized radiators and other issues caused by the heat pump.

They asked me if the installers of the heat pump performed thermal imaging and checked inside the walls, loft etc before confirming that my property was fit for a heat pump. I told them that as far as I was aware, they did not, and made a booking with them.

This company is doing this for free, where the heat pump installers charged for the heat loss. I will have to see how it goes, maybe compare their feedback with the heat loss provided by EPC and Regenertec, which I will print out to have on hand and share with them.

It does sound to me like more or less, they will be performing some kind of a heat loss assessment, but called insulation assessment instead?

@chas-b no unfortunately not. The most detail I received was just the loss per room.

Posted by: @jords

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@chas-b no unfortunately not. The most detail I received was just the loss per room.

If you paid for any of the assessments try asking for the full calculation.  I did for the one I paid for, very useful.

I have recently gone through the whole process. I got quotes from Octopus, Aira and from a local heat geek installer. Octopus wanted to supply a 9kW unit, Aira a 6kW unit and the heat geek chap 3.5kW.

I carried out a deep retrofit 10 years ago and only the heat geek installer took any real notice of the actual U values and the fact that I have an MVHR system and low infiltration rates. My heating and hot water was provided by a gas combi and my annual usage was around 2200kWh so very low. Hence I didn't trust the Octopus quote at all. Also only the heat geek installer listened when I said I was in a conservation area. The others proposed a location for the outside unit which would have entailed planning permission which I didn't want to do. One even said I wasn't in the conservation area.

I went with the Vaillant aerotherm plus 3.5kW with a heat geek mini store but have only had it going for 2 weeks. The mini store works really well but all the other suppliers insisted I had to have at least a 90L HW tank which I would have difficult to put anywhere.

I was very pleased with the heat geek installer as he really understood what I was trying to do and involved me in the design, explained everything and supplied me with the Spruce calculations. This gave me a lot of confidence and the installation is very tidy.