@ian-s 

I can understand where you are coming from but I do think that, particularly given what you have already told us, the risks are being overstated. 

There are basically 3 recurring errors that we see on this forum namely

Grossly oversized heat pump

Buffer tank

Poor controls or poor control

These account for the overwhelming majority of the problems.

You have estimates of around 6-7kW.  That needs refining a bit (to work out if you can get away with 5 mostly) but you aren't going to be fitting a grossly oversized pump because, even if your house is only 3kW, you are going to want a 5-6kW one for the and anyway you can't buy anything less than about 5.   So this error won't apply.  Exact sizing at the 6kW level is unnecessary.

You can simply tell people you won't accept a buffer tank.

You can simply tell people you won't accept third party controls other than those specifically designed for heat pumps.

If you do the above you will get a directly connected pump running open loop on its native controls delivering, assuming the flow temp is say 45 or less (if it isn't just upgrade a couple more rads) a scop of around 4 (which should mean cheaper bills) and a higher level of comfort than you get at present.

For houses like yours it really is that simple.  There are some workmanship matters, eg insulation, to observe as the work proceeds, but nothing compoex.

The industry makes it more complex than it needs to be because they try to cover all bases with a single process, which is fine at one level but overcomplicates it (and introduces potential for error) for the simple cases.

+1

#4 Primary pipes too small so not enough flow, but this one is easy to catch

Posted by: @bobflux

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+1

#4 Primary pipes too small so not enough flow, but this one is easy to catch

Yes and yes, although I haven't seen many here which are down to this, so I wouldn't count it as a top failure mode based on what we see here.

Most installers would, I hope, catch this when they commission the pump and it complains about low flow.  More will probably have already upgraded unnecessarily when existing would do, particularly if they had grossly oversized the heat pump in the first place.

My real point here is that, with a house that is in fact sub 7kW, it's actually very easy for the customer to filter out the mistakes.  If only the industry could organise itself around this it might actually get somewhere.

Mine only complains if flow is catastrophically low, otherwise it simply works at higher deltaT, up to 10°C or even 20°C, with a corresponding drop in efficiency of course. But it runs fine, so if you don't actually look down the menus and check the numbers, it's not obvious there is an issue. It would be easy for an unscrupulous installer to pretend it isn't happening...

Agree 7kW is not difficult, it's within the capabilities of cheap pipes... even if repiping is needed, 26mm MLC is cheap and easy. Above 12-14kW, 32mm MLC becomes too small so it has to be 35mm copper or bigger, a large step up in cost!

My neighbor renovated their house, they put in a new gas boiler. I saw the installation: all brand new 14mm copper to the rads. It's a shame to do it on a new installation, they could have used 20mm MLC which is cheaper and much more heat pump friendly.

Posted by: @bobflux

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Agree 7kW is not difficult..

And that is the whole point really.  The ofgem standard house (medium) consumes 11.5MWh of gas per year so is well less than 7kW.  The fact it's the ofgem standard tells you are lots and lots of them.  This is easy territory which we make unnecessarily difficult by designing a process that works equally well (or badly) for mansions as it does for a 3bed semi, instead of focussing on where the volume is and making that easy enough for your local plumber to do.

Octopus have done what needs to be done the question is why does the rest of the industry not follow suit.  If one were being cynical one would suggest that the monopoly low volume high complexity business suits the encumbants, but I wouldn't be that cynical.

@jamespa I used 9MWh of gas but the heat loss was originally calculated at around 8.5kWh and a 9kW heat pump installed.  It was subsequently recalculated to 7.5kWh but I actually think it is better than that.  The house is very well insulated - attested by the 32c sleeping conditions recently!!  I think anyone having a installation greater than 7kWh should be querying it for sanity.

Posted by: @andrewj

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 I think anyone having a installation greater than 7kW should be querying it for sanity.

Absolutely spot on IMHO. 

Anytime someone posts here that they are installing a 10,12,16kW heat pump or worse still a cascade I am immediately suspicious.  Of course it happens occasionally that this is needed, but extremely rarely.  

Several jokers offered to install 16kW for my 7kW house!  Eventually I just told them that 7-8kW was what I was going to accept (and provided the evidence to prove it) and if they didn't want to do that then please don't bother bidding.  This forum deserves a lot of the credit for my beigg able to do that.

Hi all

Many thanks for your input, it is very appreciated.

Heat Geek have been round and worked out our heat loss as 6.5kW which tied in with Aria, what I found interesting is the recommended heat pump is a 5kW Vaillant aroTHERM plus (Anthracite) which I could swap for a 7kW Vaillant aroTHERM pro (Anthracite) for £5 extra. The installer said the 5kW was more than capable of doing the job and it was the premium heat pump offering. Aria quoted a 6kW heat pump.

We would upgrade our cylinder, currently have a 180L slimline to the Vaillant uniSTOR PURE 175L Slim. If we kept our current rads the guaranteed efficiency would be 365% we could change 2 rads from type 11 to type 22 for 375% change another 2 from type 21 to type 22 for 380%.

The flow temperature would be 49°C which to me seems a little high so would be interested in people’s opinions.

Posted by: @ian-s

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The flow temperature would be 49°C which to me seems a little high so would be interested in people’s opinions.

49 is quite high, I would prefer 45, but if they are guaranteeing scop it doesn't really matter, and anyway it will in practice be what it will be, dependent only on your radiators and actual, as opposed to calculated house loss.  Furthermore the evidence from heat pump monitor is that, while ft definitely matters as thermodynamics says it does, operation (including not oversizing) and installation dominate actual performance.  If, nevertheless, you want it lower, just change some more radiators!

It's interesting that they are offering either the 5 or the 7kW.  Vaillant heat pumps tend to have 'sticker' capacities lower than actual in many circumstances, however it might also suggest that they think the calculated loss may be higher than the actual, but can't prove it within MCS rules.  They could well be right.  If you have previous fuel consumption then you can get a steer from that.  Did you ever tell us that figure?

In the end 5 or 7 probably won't make a vast difference so long as it doesn't turn out to be undersized.  You could probably get them to guarantee that if the one they are recommending is undersized, they will replace it.   However first I would compare with existing consumption. 

Basically I think you are pretty much there as much as the inevitable uncertainty in determining loss (other than by actual measurement) allows, with the one caveat of doing the sense check against consumption.

@jamespa many thanks for your thoughts, my gas usage for last year was 9309 kWh, 2024 was 9655 kWh, 2023 was 9011 kWh. Most of this would of been heating as we use air fryers and slow cookers a fair amount.

@ian-s that would point to a 5kW heat pump being sufficient assuming you want the same temperatures across the house.  These ought to be considered 15 year lifetime devices and the only thing I would add is that as you get older, you definitely feel the cold more and may well find 23c is a more comfortable temp.  We are two adults in our 60s in our house and historically I’ve felt comfortable at 20c but no more - 22c/23c is definitely our desired temp.  The MCS “standard” temps designed to are mental for older people: my wife would have my jiggly bits if the bedroom could only reach 18c when it is -3c outside.  In other words, a 7kW unit might give you some headroom.

@Ian-S, I think @JamesPa’s latest comments are probably the most useful way of looking at this.

His point that most of the problems we repeatedly see here come back to oversizing, unnecessary buffers and poor controls is worth keeping in mind. You’re already nowhere near the territory of somebody being sold a 12kW or 16kW machine for a modest house, and if the proposed system is direct, sensibly controlled and properly commissioned, you’ve removed a lot of the obvious ways these things go wrong. 

The 5kW versus 7kW question is interesting, though. Like James, I wouldn’t automatically choose the 7kW just because it’s another £5. If Heat Geek are confident enough to recommend the 5kW against a calculated 6.5kW heat loss, I’d want them to explain why. James also makes the useful point that Vaillant’s nominal capacities don’t necessarily tell the whole story, and that the real question is what the unit will actually deliver under your design conditions. 

Your historic gas use makes that discussion even more interesting. You’ve been around 9-9.6MWh a year for three years, which at least gives you some real-world evidence to sense-check the calculated heat loss against. As James says, it doesn’t magically give you the answer, but it is useful context before deciding whether the 5kW really is enough.

On the 49C design flow temperature, I’m with James again. I’d prefer to see it lower if that can be achieved sensibly, but I wouldn’t get hung up on the number alone. If a few relatively inexpensive radiator upgrades pull it down towards 45C, that seems worth exploring. If you need to start replacing half the house to gain a tiny efficiency improvement, perhaps less so. 

@AndrewJ also raises something I hadn’t really considered… what temperatures you actually want to live at over the next 10-15 years. If the heat loss calculation assumes bedrooms at 18C but you realistically want 21-22C, I’d ask Heat Geek to show you what happens to the design if you change those assumptions rather than simply buying the bigger heat pump for headroom.

So I think I’d now ask Heat Geek three fairly simple questions. Why they prefer the 5kW, what the design flow temperature becomes with each radiator option and whether the 6.5kW heat loss figure changes materially if you design around the temperatures you want.

From where you started with all the doubts, you actually seem to be getting pretty close to a sensible, explainable design rather than just being handed a quote and told to trust it. Well done.