We suspect your home heat-loss is closer to the 4kW at peak, I am looking forward to the data the next winter against a -3 external temperature, the 5kW (5.7 actual at -1.7) leaves a little room for defrost recovery and intermittent use if required, this system will nicely run very low. 

We complete another on Friday and the customer said I quote " my flue was twice as loud as the Vaillant ASHP " so another happy customer.

KR

AAC Group Ltd  

Work was done by AAC Group who have recently moved from Ashford to Folkestone in Kent.

Well, absolutely no surprises here because AAC Group is owned and run by the excellent @ashp-bobba!

Work was done by AAC Group who have recently moved from Ashford to Folkestone in Kent.

As stated earlier they are only working in Kent at the moment. Quality seems to take priority over quantity with them.

I don’t take up various invitations I receive to praise companies on trust websites etc. as I think such forums are subject to possible abuse and as such, do not read or heed them myself. RHH is quite another story however and I feel confident that recommendations aired here have a far higher degree of authenticity and value! Regards, Toodles.

Sadly the firm who I would be very happy to recommend if it were not against forum rules only work in Kent.

I’d echo what @editor has said, but I’d also like to make a minor correction. Naming good installers is not only not against forum rules; it’s actively encouraged. We can’t in fairness name and shame cowboys without also naming and rewarding those who give good service.

Name and recommend away, @wanttobegreen, and let them bask in a bit of deserved praise.

I’d push back slightly on the idea this is just a 'nice positive story'. What you’ve actually exposed is how far off some of the mainstream approaches still are. An 8.8kW vs 4.5kW heat loss isn’t a small miss! That’s the difference between a system that runs continuously at low temps and one that ends up cycling and running hotter than it should.

The key bit for me is the process you went through. A proper room-by-room calc, time spent on site and full transparency. That aligns with how systems are supposed to be designed... heat pumps are sized close to the design load, not oversized "just in case", because they don’t have the headroom to recover like boilers do. I'll caveat that by saying it is contingent on the brand and whether it really delivers its rated kW at your design temperature.

https://youtu.be/Uw1SpYnIrEI

Also interesting on the ventilation losses. 1 ACH is a standard assumption, but in the real world it’s often lower, so you may well find your actual heat demand sits closer to that 3-3.5kW you suspect. That wouldn’t surprise me. We see that all the time.

Overall, solid example of what "good" looks like, fingers crossed!

1) Daily COP including hot water ranges between 3.4 and 4 in the last 7 days since installation, still learning how best to control it. One cold night with frost about -1 and other days up to 14 degrees and bright sunshine all day with solar gain heating house higher than set temperature. Would be reluctant to draw any conclusions from such little data other than the system is working well so far.

2) Pipes from unit comes out turns 90° runs along the back of the unit in flexi pipe and then straight for 30cm, through drilled holes in wall of house (I wanted to minimise external pipework) total length approx 1.3m. Hot water cylinder just the other side of wall.  All well insulated with closed cell insulation, anti-freeze valves, system flush valve, shut off valves incorporated into that 1.3m. 

1.  What range of COP is the system giving so far and at roughly what ambient temperatures?

2.  Can you describe the flow and return pipework and insulation from the external unit to the house walls - pipe lengths, fittings, valves, flex, insulation thickness etc etc.  I ask as my own system, installed in Sept, seems to be rather more complicated than I expected and, to some extent, not well executed.

After two years of thinking about it, reading everything I could about heat pumps on this site, reading the Guide to Heat Pumps cover to cover and having three different installers visit I have finally taken the plunge and committed to a heat pump GULP!

Story so far

I live in a 120m2  three bedroom 1930's detached house in East Kent that has been much mucked about with in its life including by me.

Gas central heating boiler updated in 2013 but much of the system looks like it was based on an old back boiler (probably coal fired) from nobody knows when.

House insulation was much upgraded in 2013 with a new roof and wall insulation.

Octopus came and did a survey and suggested a 8.8 kW heat loss and needing giant K3 radiators in the lounge (er no thanks) and a Daikin 8.8kW heat pump known to be a limited larger version that is poor at modulating down with a design flow temperature of 50°C.  (I have been running my current condensing gas boiler at 48°C for a couple of years now, would be lower but hot water is then not hot enough to reach the shower.)

My own Heat-punk attempts at survey had come in at between 4 and 5 kW so there was no way I was going to accept that.

Then I invited Aira rep round, we had the chat, I asked questions such as what was the outside design temperature, now maybe I did not word the question quite right but he did not seem to have a clue what I was talking about.

So no confidence there and the gas boiler was still working fine so  I resumed my masterly inactivity.

Made contact with a local firm based in Folkestone and  was invited to view their premises/ demo rigs.

The gentleman was very generous with his time, answered all my questions and supplied extra information such that I had a great deal of confidence that he knows what he is talking about.

No high pressure sales techniques, no bullshit answers (that I could detect anyway)

So he came and did survey  and spent some three and a half hours (unusual house and a client that kept asking questions)

Conclusion was a 56 page report using Spruce software that let me check every tiny detail of the calculation, which I did very carefully.

Recommendation was for  

three radiator changes recommended replacing  two which are probably over 40 years old and well overdue for replacement and one extra rad squeezed into a bathroom with two external walls and a solid floor to supplement a nice towel rail.

Total of 4.5 kW heat loss but as nearly 2kW of that is from ventilation (assumed 1ach, 1.5 in bathrooms ) I strongly suspect that actual heat loss at design temperature will be around 3 - 3.5kW

So a Vaillant 5kW heat pump was installed in March 2026 paired with a new 150 litre cylinder. Single zone using existing 22mm and 15mm copper pipe 10 radiators in a three bedroom house of 120m2.

Everything I have read supports the design, now to find out if the real world agrees.

What I have learnt along the way

never, never accept a heat loss estimate without fully checking it out.

A Vaillant 5kW  heat pump is incredibly quiet even when going full out doing domestic  hot water heating.

I am going to have to wait until winter for a full test but a couple of frosty nights so far give me great confidence that all is working as designed.

The firm have offered to come back when it gets cold to do a final balance of the radiators an offer that I have gladly accepted.

Sadly the firm who I would be very happy to recommend if it were not against forum rules only work in Kent.