Welcome to the forums @mrj. In very quick reply (because I'm heading out the door) the 2x18kW units seems to be on the overkill end of the spectrum. We are currently heating an area exceeding 300sqm in our property with just a single 18kW heat pump, and it's proving more than sufficient. Prior to installation, we sought several quotes, and almost every installer recommended a unit in the 16-18kW range. Interestingly, there was one exception who suggested two 16kW Ecodans. His decision to text us the quote rather than providing a more formal proposal was quite telling.

Blown bonded eps beads in that cavity, that’s definitely something you should do.

are you active on buildhub?

@editor Thanks for the welcome!

I'm slightly paraphrasing, along with potential for failing memory, but the way it was explained to me was that the the stated output on the ASHP's was not the actual output. From each 18kW ASHP you could expect c.12kW to give a total of 24kW. Whilst the heat loss calcs for the house are c.20kW, he likes to add a buffer hence the over spec.

If in actual fact all of the above is nonsense, then I have one of two scenarios. First I have completely misunderstood what he was saying (distinct possibility). Second is that he has completely over spec'd the system and two 12 kW or a 16kW and a 4kW would be absolutely sufficient..? If it's the latter then my level of confidence has dipped somewhat

@hughf I'm not active on buildhub.

In truth I'm somewhat put off by the stat that 1.5m brits with retrofit cavity wall insulation are unhappy with the result. I appreciate that this covers the full spectrum of unhappiness, but thats 25% of all retrofit cavity wall jobs. I'm just not happy with that type of risk.

@mrj Hi James, I would definitely advocate some wall insulation to keep running costs lower, what about insulated plasterboards internally, your pocket will thank you later 🙂

alternatively if you can run to it, external wall insulation!

@bontwoody I had initially thought insulated plasterboard could be a good solution, albeit my architect dissuaded me from this. I think it might be something I need to look at in a little more detail.

EWI was the suggested route. However this would require planning and detract from the exterior (it's a limestone exterior with cavity to masonry).

I guess what I'm trying to understand is my worst case scenario. If I didn't insulate the walls, would I be left with an inadequate system that ends up costing a fortune in energy bills?

@mrj  obviously do all youi can on insulation. others have said that well. 2x 18 sounds enormous even for 300m2. especially if you are making a serious effort on insulation. couple of things to be aware of

•  if the actual capacity is significantly below the "nominal plated number" that is a warning flag. Study in detail the capacity tables of any proposed unit. What you DONT want is one of the ones where the capacity goes down as the air temps gets cooler and the emitter water temp gets hotter (which is what you need when it gets colder). Some brands do not have this as an issue.
•  with two units you will need a buffer tank. which brings its own complications in terms of trying to control and modulate between two units at full power vs one unit at low power (in mild weather). there's one other regular poster on here, @dunlorn, with a two unit system, and I think he finds it hard to control.
• consider the low modulation point (mild weather) of the system as well. what can it turn down to?

I'm in a 1973 property which is 210m2, no deep retrofit just the crappy 20mm cavity insulation it was built with, I need 9kw at -6C (as proven by this weeks weather), I have a 16kw, which is more than I need.  I'd have thought that 1x16 (making sure its one that maintains capacity all the way down the temp chart) would be enough for you.

@mrj Its difficult to say without an in depth heat loss survey. You could do one of these yourself as there is free software out there or if not some companies will charge you up front for one if you ask for a quote.

What reasons did you architect give for not liking insulated plasterboard? A less effective method I have tried is insulated lime render for a wet stone wall. It seems to be dealing with the moisture fine so far but its not cheap at £20 a small bag. Its called cornerstone if you wanted to have a look. Also you could go down a timber frame route and use either kingspan like boards, mineral wool or real wool if moisture is an issue.

Hello @mrj. @iancalderbank is correct. I've 2 x 12kW Samsung units operating as 'master' and 'slave' which aren't particularly efficient and the system as a whole doesn't see a great deal of benefit from the second unit.

I'd avoid having two units combine in a buffer tank, it makes secondary side flow matching a challenge and I end up losing 2-4.5°C across the buffer. If you really do need two units then I'd give some thought to dedicating them to separate heating zones, or upstairs and downstairs say and controlling them independently. 

Incidentally, my 12kW Samsung doesn't deliver that once you factor in regular defrost cycles. In cold and damp conditions mine can be entering a defrost cycle every 30-40mins. They only last 5 minutes but their effects are felt for longer and the heat pump can struggle to reach set point LWTs above 42° before the whole cycle starts again. My observations of generated output tell me I'm only achieving around 9kW in these circumstances.

I don't want to get LWTs too high (it's too costly) but if - in practice - you are limited to a peak of 42° or so then you need to make sure your secondary side (emitters) are designed on this basis. 

Hi @iancalderbank @dunlorn, the units are Daikin altherma H HT, which from what I can see are a good quality pump.

Vague recollection of the BUS scheme feeding in to the overall spec, but in truth I need to have a more in depth conversation with the consultant. On initial presentation and conversation my knowledge was as close to zero as you can get. I guess slowly building up so let's see if I can't push him on an explanation.

There could be an element of simply trying to understand why a home with heat loss calcs of 20kW requires 36kW of ASHP's, and that's what I need to get to the bottom of.

If I'm at 20kW heat loss, then it looks like I need two. As @dunlorn noted then maybe one to run the UFH on the GF and the hot water tank and the second the rads upstairs.

@bontwoody It was a concern regarding damp. More that we would need to do a lot more work on understanding how all the different elements interact with each other which adds additional risk to the design. We're looking at a mortar slurry wash, albeit there are planning restrictions over what I can do so it can't be of the same thickness of a render as that would apparently take away from the character of the area. At least according to the planners. Unfortunately everything I do has to received planning approval. Yay