Posted by: @jo2255

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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. 

Posting here as I have a very similar situation, although different type of house.

1900s end terrace, solid brick walls, double glazing, some insulation in a 20 year old loft extension, suspended timber floors with no insulation - 125sqm.

The pre-survey estimate heat loss from heat geek was around 9kw.

The survey came back with 6.3kw - the guy surveying was great and talked through the whole thing, about how he was changing some of the default assumptions and somewhat gaming (not his words but my understanding) of the app to give a heat loss figure that was in his experience closer to reality.

Despite the heat loss, I was presented with the same two options: 5kw arotherm plus or 7kw arotherm pro, the latter being £50 more expensive.

No radiator changes, 210L cylinder - £8,200 total cost (which includes £900 additional pipework recommended by the surveyor, which was explained and would be necessary), and a 415% efficiency guarantee.

A lot of aspects of this slightly surprising to me:

1 - my pre-survey estimate from HG was £6,050 - for an 11kW Vaillant AroThermPro, 210L cylinder, and 1 radiator change. Even with the extra plumbing required, I don't really understand how the price has increased so much for a smaller ASHP and fewer radiator changes

2 - as others have said, no flow temperature mentioned anywhere. But as also discussed, I'm happy with this given the guarantee - I do feel like this should be the target of the industry, "if we set it up right you will achieve this outcome" rather than just "this is our recommended set up".

3 - one room where no radiator change seems odd, and its the largest room. A kitchen diner which is 21sqm and heat loss measured at 1350W. There is a currently a large column radiator there which assessed at 1400W dt50. My understanding is that would only cover the target heat loss (21inside, -2 outside) if the flow temperature was 71, which it is never going to be. Using gemini to do the maths, at a 55 degree flow temperature, this radiator wont provide enough heat to the room once the external temperature drops below 8 degrees. I have reasonably sized column radiators and type 22s elsewhere, all providing at least double the heat loss wattage at dT50 so understand why these don't need to be replaced - maybe the heat geek model sees the radiators as a whole being sufficient to heat the house and isn't looking too closely at one room? But I am concerned that this is the largest downstairs room and there are only 3 radiators downstairs. But then also, Heat Geek are guaranteeing the internal temperature as well as the efficiency.

4 - given some of the above, surprised that the guarantee is so high. I was expecting around 350-370%.

As I say I was very impressed with my heat geek firm - I want to go with them, they seem very knowledgeable and have been responsive to many of my questions. But wanted to lay out my experience here for others to see and comment on, particularly as it seems to fit with a pattern.

Perhaps HG have got enough real world data from their installs and models that they're more relaxed about the bounds of what they can offer, that in many parts of the country -2 degree periods are vanishingly rare, and a good modern heat pump set up to just tick over will be more than good enough!?

@donciccio thats interesting. 

I really cant understand how the radiators would work in that case.

What is the total output of all radiators combined for your house ?

It will be interesting to see if the total radiator output can supply the heat loss and at what flow