150mm + PIR will one day be considered standard, it's worth it, but it has to be installed carefully and thoughtfully, if you have window and door reveals consider Superquilt, the closer you can get to passive the less heat you need to generate to heat the space.

I have seen new builds by reputable construction companies that install the minimum they can get away with, and the installation can be shocking, people don't look at wall cavities and loft space when buying a house, it's a bit nuts really  builders put a lot of effort into 'finish' over what is essentially out of date insulation for a non cheap gas future. 

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

I fully agree with all the previous poster's comments.

The starting point should be to insulate as much as possible to the highest standard, since it should be a win - win. No need for a second heat pump and much lower running costs.

From my limited knowledge of UFH, I would suggest keeping the pipe to pipe distance as small as possible, thereby providing greater heating surface, so able to run at lower Leaving Water Temperature (LWT), therefore higher efficiency and lower running costs.

Any radiators should have the highest output capacity possible, whilst maintaining the overall balance between rooms, so that they all require approximately the same LWT to keep the various rooms at their desired temperature. In rooms where space for radiators may be limited, there are fan assisted radiators available, which although more expensive, could mean the whole system could operate at a LWT several degrees lower.

If you already have a mains gas supply, you could consider a hybrid system, with the heat pump performing the bulk of the heating, with the gas boiler being a back-up for heat pump failure and/or extreme cold weather conditions.

There are also hot water tanks available which include a small ASHP. The larger heat pump would therefore benefit from not having to stop heating the home to spend time heating the DHW, and would also not need to run for much of the Summer when hot water production is the main job of any heating system.

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.

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%).

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. 

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.