@mrj are you talking about the external surface? I was only thinking internally.

The other variable you can alter is the size of your emitting surfaces. UFH is excellent for this but you can also up radiator sizes to achieve the same effect. The larger the emitting surfaces the cooler you can run your heat pump and the more efficient it will be hence cheaper to run. That said I would insulate internally somehow.

@mrj those are HT pumps - warning flag -  meaning your engineer may be sizing it for smaller radiators with higher running temp.  Better not to rely on a high temperature, better to put in more emitters then you gain in many ways:  run at lower temperature with much higher efficiency -> lower run cost, usually the actual max output improves as well, and a lower water temp lessens the impact of defrosts in cold damp weather .

MCS method is notorious for overestimating based on over-high air change rate, it could easily be 14 or 16kw loss with a better air change, then your easily into one pump. you're setting yourself up for a world of complications in your system design if you go with two. You are obviously paying care and attention to your retrofit, so suggest you zoom into the heat loss calcs and look at the air change.

James, I can't offer advice on the ASHP but I may be useful for your insulation, understand the reservations regarding cavity insulation, but it's not clear how you are insulating the walls? 

My approach to this would be get the space as well insulated as is possible before the heat loss calculation, this then reduces the need for more heat/ASHP/energy etc. there will be ways. 

@alan22 Id be happy to pick your brains on that Alan. Best option for insulating a stone and lime/mud? wall? Cornerstone insulating lime plaster worked well on another wall but its very expensive and not as good an insulator as other materials. I was thinking of trying building a frame off the wall and using some type of insulator between/over it.

@iancalderbank @dunlorn, I think I have a little more understanding, having attempted to give myself a crash course in ASHP. Very much a case of unknown unknowns until now (and I'm still just scratching the surface).

Potentially unhelpfully the consultant described the spec as 2 18kW ASHP's, however they appear to be badged as 'series 18' with a max capacity of just over 12kW. These units also are capable of cooling, should we install a/c in the future which is a consideration and will also provide DWH too. 

Having had a crack at the MCS calculator (laborious and enlightening in equal measures) the Space Heating load is 17kW, which is not far off the 20kW the consultant referenced. I take note many mention that MCS overstates air changes, so this would decrease the value. I may have been too optimistic on the u-value of the new floors, windows, doors and existing roof, so this would increase load.

 I still get the impression that a second opinion is worthwhile.

In the meantime, any recommendations for questions I can push the consultant on?

Hi @mrj. I certainly wish I'd spent some time really understanding my heat loss situation, and how the system as a whole was being designed but - as with many here - I've gained all my knowledge since installation.

Getting another view on your heat demand sounds like a good idea. Whilst you obviously need to make sure the system can deliver at your design OAT (-2 or whatever), it's worth running the numbers under more normal winter conditions for your area. Even where I am (Scottish Highlands) we don't have months of sub-zero conditions (honestly 😁) and for much of the heating season we're at around 5-9°C. Out of the defrost range this is a real sweet spot for operation so the system needs to work well in this range. So, establish heat demand in a 'typical' winter day and make sure (a) your heat pump can modulate down this far as you don't want it cycling on and off rapidly and (b) your emitters can deal with the low water temperatures as you'll have a very low delta-T between rads and room.

Last point I'd say is to understand how your emitter side is being designed and that it's not based on an unduly high water temperature. Assuming a high LWT (say 45° or more) will result in smaller/fewer rads so may seem like an attractive cheaper option. However, the reality is you may not reach 45°C at low OATs (because of defrosts) and - even if you can - it's going to be costly. Based on my experience here, I would aim for the system to deliver the required design output at a LWT of around 40°C to the rads. My heat pump will generate a bit more than this in very cold conditions but then it loses a few degrees going through the buffer tank. 

Posted by: @bontwoody

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@alan22 Id be happy to pick your brains on that Alan. Best option for insulating a stone and lime/mud? wall? Cornerstone insulating lime plaster worked well on another wall but its very expensive and not as good an insulator as other materials. I was thinking of trying building a frame off the wall and using some type of insulator between/over it.

I guess the first question is do you want to cover it? you can insulate and board with a vented cavity. 

I can show you an example of sandstone, stud wall(in my case steel) with a 50mm air gap, PIR over, it looks like a cage within the building, only thing touching the outer stone wall is wall ties keeping the frame rigid, the air gap is ventilated from the roof space, an insulated box within the building, you loose room space to the thickness of the frame and insulation and need to be meticulous with airflow in the cavity, and of course you lose the authentic old wall. 

@alan22 Actually thats exactly the thing I had in mind although I was thinking of using the wood i have just removed from the adjacent wall for the frame. It was up for several years and is in pristine condition so damp doesn;t seem to be an issue. Its a bungalow so venting to the loft is easy enough. Thanks

@bontwoody you can reuse timber for a stud wall no problem, with a wooden stud you lose less space by cutting PIR to fit into the stud then make up the depth by overlaying, makes plaster boarding easier also.

@alan22 Just what I was thinking. I should get 100mm of kingspan over most of it and 50mm at the studs. Thanks

@mrj any ashp will drive a DHW tank, and most will also provide cooling, so these are non-factors in selection. Other brands and heating engineers are available.

A good starting place is anyone with a "HeatGeek" certification. 

cooling is a factor in system design obviously. don't think any of us can help you there.

Its completely common to gain the understanding post install. Mine is about 50/50. I self-installed, make some mistakes that I would change if doing again (one of which being system size).

massively echo @bontwoody, if you allow an emitter design for 45C when -3C, the HP will spend a lot of time defrosting at these temps, so will actually struggle to achieve 45C. whereas if the emitter design is 40C it will be able to get there much more stably and hold onto its output. And you obviously gain massively at mild temps it'll run in the mid 30's and be really efficient (4+ COP).

I think we're being fairly unanimous on the recommendations : Don't overspec it. 1x 16kw (if getting a 16kw, get it from a brand that really holds onto close to the 16kw at low temps) should be enough (perhaps more than enough) given all the effort you're making on insulation. One with a published (or empirically proven by other users) downward modulation to no more than 4kw (25%).

Posted by: @mrj

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Hi,

I would really appreciate the advice, if anyone has the time or inclination to humour an extreme novice in renewable energy.

We're about to undergo a full reno of a late 70's property, which is just shy of 300sqm in the South West.

As part of the works we're very seriously considering switching from gas to ASHP and have appointed a recommended engineer to design the system (and ultimately use an experienced installer too). The system would be for both heating and water.

My concern is less of whether we are considering enough capacity (from the looks of it, two 18kW units is on the very large side) but more of real world running costs.

The reno works will include insulating the ground floor and installation of a slab with wet UFH, replacement of all windows, doors and garage doors (currently visible air gaps through all, so extremely leaky) and upgradings the rads upstairs and UFh to the bathrooms. We're not installing cavity wall insulation as the property is prone to driving rain, so there is too much concern about water ingress, damp and cold spots.

I don't have the heat loss calcs to hand, (I have asked for them so hopefully I will receive in a few days), but recall 20kW as the output of his calcs.

Am I crazy for considering ASHP, is this far too little information for anyone to begin to pass judgment on? Any advice would be greatly appreciated.

Thanks,

J

 

As others have said 2 18kW units sounds like a massive overkill, even allowing for the overstated outputs.  Someone suggested 2 16kW Mideas (which have overstated outputs) for my house (about 200sqm, 1930s solid walls/suspended floor with about 30% 1980s extension, partial IWI, double glazing and good loft insulation).  The actual measured consumption is 7.5kW.  

Have you got any measurements of actual consumption, I presume the answer is no because you haven't done the renovation.

Personally I wouldn't touch the proposal you have quoted without a lot of interrogation.  If you care to post some more detail here I am sure I an others may be able to add some more (hopefully) useful comment.