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.

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.

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.

I think the education thing is worthy of serious debate.  In most fields of consumer choice we expect to encounter a range of product quality from almost useless to excellent in all respects.  Consumers educate themselves by reading reviews (for which there is a while industry), talking to others and developing an understanding.  This applies to the most technically complex consumables such as cars and computers.

Yet somehow we think that construction projects are different.  Consumers apparently don't need to know or understand anything, not even something simple like how many showers they want in quick succession .  Why is that the case when it isn't in other fields?

I grant you that In the ideal world every product should be fully fit for purpose, but we live in a world run by humans, some of whom are frankly crooks.  That isn't going to change, ever.  So perhaps we need to get off our high horses and accept the world for what it is, imperfect.  All the changes that you (and I) say the industry could make are very fair, but why are they going to bother(and, frankly, do the people 'in charge' have the intellectual capacity to bother, or are the very best people just running their own businesses despite the industry bodies rather than because of them?)

The dodgy bits of the construction industry (just like the dodgy bits of, say, the finance industry) relies on consumer ignorance and wrapping everything in technical mumbo jumbo.  We facilitate that by supporting the argument that consumers 'shouldn't' have to enquire. Maybe, just maybe, consumers should enquire to the same extent as they would if buying a car or a computer.  This doesn't mean try get have to be able to design either, they just have to know the right questions to ask and how to interpret the answers!

The industry won't change unless forced to by government, and possibly not even then.  This takes time, certainly more than 3 years.

Consumer education and consumer protection are the only ways to improve things in the absence of forced change.

The future of consumer protection in the renewables sector is dependent on only one thing, namely which party is in power after the next election (because anything the current government changes will take the rest of the current term to implement, and a government hostile to renewables will help inevitably find ways to frustrate it.)

The future of renewables more generally depends on only one thing, namely which party is in power after the next election.

Thus consumer education is the only non- political thing we can do which changes things.  

In our increasingly polarised discourse it's now as simple as that for me, and everything is filtered through that lens.  Either educate people (or not depending on outlook) or campaign, directly or indirectly, for a party that embraces your chosen approach to renewables.

Moderators please feel free to delete this post if you feel it violates forum rules.

How has it worked out for you.

I bought one for my small workshop a few years back. I ran a line directly to the heating oil tank outside. The workshop is draughty but it took the chill off.

I was advised to run it from a battery or at least have a battery connected with enough power to let it shut down properly.

As a power cut can result in a fried heater.