ASHP for small stone cottage?

[Caddy]

I live in a small (60m²) stone built cottage in the Somerset coutryside. There is no mains gas (in the whole village) and heating is currently either a solidfuel stove or electric convection heaters.

 

I am trying to decide whether to go down the route of installing a ASHP, potentially trying to get it in before the RHI deadline (a silly idea I know).

 

The house is poorly (I think) insulated being solid (about 0.5m thk) stone walls and solid floors, but I think being so small it would not take a lot to heat. I made a start on increasing the insulation, replacing the tatty loft insulation with 100 PIR (got a load cheap) with 170mm normal loft inslation on top. I will also be insulating some internal walls where it do not make too much of a change in room size. I think the house is fairly air tight as there is only 1 door, covered by a enclosed porch, and 4 pretty small windows. The only other penetrations to the walls are 1 x soil pipe, 2 x waste pipes and 1 x extractor fan. I have checked these for leaks and they seem ok, although I will replace the extractor with something more modern I think. I also have a 3.5kW solar PV system on my south facing roof.

 

I'm thinking of a simple system, just a rad in each room (4 or 5 depending if I put one in the kitchen) with no UFH or DHW from the heat pump. I have minimal HW requirements and either the PV heats the water or its cold if its cloudy, I'm not too bothered. An hour on the immersion heats enough water for me to have a bath normally anyway. The house averages about 11-13°C internal temp at the moment. This is fine for me and I only put the stove on if it gets below 10°C but this low temp must be bad for the buildings fabric and increasing the risk of damp and mould. It is also pretty uncomfortable for any guests who normally don't like to live as cold as I do. I would only be wanting the rooms to be around 17-18°C, as 20-21°C feels far too hot to me.

 

Do you think I am a good candidate for using ASHP for heating? I think I would need a small compact system and I can keep the flow temp down pretty low as no DHW heating and low requirement for room heating, so hopefully should get a good COP. I can install the rads and pipework myself (sized correctly) and any of the monobloc that I'm allowed to do to try and keep costs down. Would shooting for the RHI be advisable or wait for the "grant" from April onwards? Any advice appreciated.

[ReedRichards]

To qualify for the RHI you need an MCS accredited installer who can complete the job before the deadline.  They may be in short supply but I don't know this.  You also need an EPC that does not recommend additional loft or cavity wall insulation.  Otherwise you don't need good insulation to have a heat pump, you just need a bigger one (and bigger radiators) if your insulation is poor. 

 

If you can get the RHI I'm not sure if you would derive any benefit from not heating your water with the heat pump.  Otherwise the running cost will be at  least half that of your immersion heater and, like with the immersion heater, you only have to use it to heat your water when you want to.     

 

    

[SteamyTea]

10 hours ago, Caddy said:

but I think being so small it would not take a lot to heat

Do you know how much you use at the moment?

If not, find out, it will help no end and take most of the guess work away.

10 hours ago, Caddy said:

replacing the tatty loft insulation with 100 PIR (got a load cheap) with 170mm normal loft inslation on top

There may be a condensation problem if the PIR restricts vapour transmission too much.

10 hours ago, Caddy said:

I'm thinking of a simple system, just a rad in each room (4 or 5 depending if I put one in the kitchen) with no UFH or DHW from the heat pump.

Have you considered Air to Air Heat Pump/s.  Depending on the house layout they may be the cheapest option for space heating, and as they do not get RHI, it takes the time pressure off you.  They get a better CoP than a wet system.

 

10 hours ago, Caddy said:

I also have a 3.5kW solar PV system on my south facing roof.

You say you use little DHW, but have you thought of using a PV diverter to the cylinder to optimise self generation usage?

[J1mbo]

Does the house have a damp proof course?

[Cpd]

@Caddy interesting to read your post as I am having to plan something very similar with a small stone cottage that I want  “ready” for my ageing parents should the need arise in the next few years……. I won’t ramble on about my situation as it’s a mirror image of yours and I am still in the late  stages of contemplation…… I will be adding DHW to the ASHP and just insulating the hell out of the tank as my parents are also minimal hot water users but want to future proof the system. I got my ASHP of eBay some years ago so will be doing a self instal (where possible) and not claiming RHI. 
 

[saveasteading]

17 hours ago, Caddy said:

poorly (I think) insulated being solid (about 0.5m thk) stone walls and solid floors

I have been looking into exactly this so can help  a bit.

 

Thick stone walls turn out to be better insulated than was believed. This is for a few reasons but mostly when the wall is made of 3 skins, as the central core is rubble pus 40% mortar.

If the outer rock is of dense rock, well built with large stones and minimal mortar it sheds water quickly and there isn't too much heat-sucking evaporation.

 

However it still isn't great on its own. Do you have/intend to have an inner lining?

 

The floor will be a big heat  sink, but do you have enough height to insulate above it, remembering that a higher floor may take you above door lintels.

 

Whatever the heat source, you will lose a lot if the insulation is poor.

 

Damp-course : There is unlikely to be  one, but damp doesn't rise much in these walls. More on this later perhaps.