Posted by: @old_scientist

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On the flip side, had we not replaced the radiators and had chosen to reuse them, how many installers would have remove them to power flush them before connecting them up to a shiny new heat pump. I wouldn't want 20+ years of sludge from reused old radiators passing through my expensive new heat pump, so there is also a cost implication to undertaking that cleaning work, or perhaps most installers just view that as not our problem?

All of which makes the complexity and cost of a heat pump installation considerably more than a straight boiler replacement IMHO.

I suspect that the hex in a boiler is as susceptible to being blocked by sludge as that in a heat pump.  A powerflush would be a good idea in both cases!

Using he heatgeek design flow temperature of 47°, 42° return, some of my radiators are oversized. the most extreme has a heat output of 610W in a room with a heat loss of 276W .

In my spreadsheet if I reduce the return temperature of that radiator to 21 degrees giving a temperature drop of 26° across the radiator, then its output reduces to match the room heat loss. Is this what would actually happen and how would it be achieved.

Posted by: @david-2

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my spreadsheet if I reduce the return temperature of that radiator to 21 degrees giving a temperature drop of 26° across the radiator, then its output reduces to match the room heat loss. Is this what would actually happen and how would it be achieved.

Yes that's what would happen by turning down the lsv to restrict the flow.  This is part of commissioning and called 'balancing', in this case for room temperature as opposed to for equal flow (which is how balancing is classically understood)

What would also happen is that some of the excess heat will end up in adjacent rooms. 

In summary oversized radiators are not a problem!

I had the same problem with heat geek.

My heat loss was 6.4kW (similar to other surveys). All my radiator are Type11 single panel convection ones on 10mm microbore on a 140 sqm 2004 property.

We were given 2 options for heat pumps 5kW Vaillant aroTHERM plus or 7kW pro. We were considering the plus for the added warranty. 

We were given 0 radiator changes at 355% efficiency or 1 radiator change at 365% efficiency. 

No flow temp given and Installer didn't have the info.

All my radiators were deemed sufficient based on their delta T50 outputs. Most of my radiators barely meet my heat loss at Delta T50 so was confused as to how they would work with heat pumps. When asked the installer advised to trust the algorithm and the guarantee is there of We need it.

I did a bit of digging and the 5kW unit can do 6.4kW when at or under 40 deg C flow temp.

However my radiator outputs are considered at Delta T50, implying a 75 deg c flow.

How can a 5kW heat pump work with my radiators for a 6.4kW heat pump is baffling. This doesnt even consider defrost cycles and DHW reheat. 

Interestingly 1 of my rooms is already not meeting the heat loss at Delta T50 yet it is still sufficient according to heat geek.

Someone online commented that heat geek look at the whole house as 1 big Emitter as air flows which I dont agree with as many doors stay shut at night for bedrooms, toilets etc.

But entertaining that idea (even if I dont agree with it). My total radiator output for my whole house is 9.6kW at delta T50. So my flow temp has to be atleast 57 deg C for it to output 6.4kW. Even with this approach the 5kW unit is undersized. 

This is again ignoring my 22 year old radiator condition and sticky TRVs. 

@jo2255 

Firstly welcome to the forum.

I think you may be getting a bit mixed up here as some of what you say conflates different things.  Heat Geek generally have a good reputation, in fact I cant remember a single complaint here about an actual instal of theirs in over 2 years.  That said I don't like 'trust the algorithm' either.

If you would care to post the heat Geek proposal (with anything sensitive redacted) I am sure someone here will comment on things you should insist on answers to and things that are in fact OK.  I am presuming you haven't yet had a heat pump installed. 

Perhaps you could also clarify what you mean by the following and how you know it to be the case.

Interestingly 1 of my rooms is already not meeting the heat loss at Delta T50 yet it is still sufficient according to heat geek.

1. @jamespa attaching info of the room in question. Heat loss 461w, existing radiator output at delta T50 423W. This room should need a new radiator under every circumstance 

Adding some more info on the other rooms

All noted.  There are a few observations worth making here.

• A 140sq m 2004 house should not have a loss anything like as high as 6.4kW.  I suspect heat Geek know that and may have taken it into account in their design, but are constrained by the MCS calculations in what they write down, hence the guarantee approach
• Looking at the first room you posted the ventilation loss assumes ACH = 1 approx which is an outdated over estimate.  ACH=0.5 for habitable rooms and ACH = 0 for non habitable rooms are the more recent assumptions.  I haven't checked the other rooms you posted
• In a sense a SCOP guarantee which, SOFAIK, only Heat Geek give, is much better than a figure for the FT.  You dont ask (and are not very interested in) how hot your car engine runs, you ask how many mpg (or mpkWh) it does!
• Heat Geek do have a good reputation.  I suspect, but dont know, that they are using accumulated actual data from previous installations to 'calibrate' survey results, which are inevitably whatiffery and have a high degree of uncertainty in reality even though they look scientific,  I also suspect they are playing a bit of a numbers game with this, know that perhaps 1 in 10 will require a rad upgrade or two that they haven't directly costed for, but have built that into their pricing on average.  This is, I admit, pure supposition but consistent with their approach of guaranteeing SCOP (which is what matters to you) not FT (which sort of doesn't).

Nevertheless the mismatches are a concern and questions should be asked and answers received in writing.

It really would be useful to know your annual gas consumption if you have it!  If this confirms that the loss is in fact a lot less than 6kW, which I strongly suspect it will, then there is one set of questions to ask.  If it doesn't confirm this then there are more serious questions to be asked. 

Thank you. Annual gas consumption about 19000kwh over the last 2 years.

2004 ideal classic non condensing original gas boiler.

Range gas hob

The Heat geek installer sent their own proposal from Spruce with heat loss of 6.2kW 

British gas survey shows heat loss of 6.1kw at -1.8deg c outdoor 21 indoors.

Octopus at 7.9kw heat loss.

Posted by: @jo2255

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Thank you. Annual gas consumption about 19000kwh over the last 2 years.

If you mean 19000kWh per year that is indicative of 6-7kW loss, unless your DHW demands are very high or your gas boiler has degraded efficiency (which is possible given its age).

I am assuming the other suppliers are proposing to change more rads?  Are they also proposing a higher capacity heat pump, how do prices and any other specs compare?  All other things, including price, being equal bigger rads are always better.  Higher capacity heat pump not necessarily so (oversized can be bad for efficiency)

@jamespa 

Yes annual of 19,000kwh. Dont think high hot water demand as it's only 2 of us with a 1 year old. 

Gas boiler would definitely be degraded as its non condensing but no issues as such but hard to estimate efficiency.

Heat geek quote is around £8k with no radiators and 5kW

Heat geek installer own quote is £9.5k with 12 new radiators , 7kw Vaillant arotherm plus

British gasis £7.4k, 15 new radiators, 7kw Vaillant aerotehtm plus

Octopus 9kw Daikin, 11 radiators, £5.5k

Main concern is heat geek numbers dont add up for radiator suitability and 5kw heat pump sizing.