Some recent discussions have reminded me how cheap small ASHPs can be. It got me to reconsidering whether it might be appropriate for our house, and I'd appreciate any thoughts anybody has.

We currently use electrical resistance heating for our space heating and our water heating and have an E7 meter. For most of the year, we don't need any space heating and our water heating is provided by PV diversion. April through September, our grid consumption is between 4 and 7 kWh/day. November through February is our heating season. March and October are transitional. Daytime usage during the heating season is about 8 kWh/day and nighttime E7 usage, which is when the DHW heating and most of the space heating occur, is around 20 kWh/day. Heat diverted to water is typically maybe 6 kWh/day when PV is heating it, so I suppose about the same or maybe a bit less is provided from the grid during the winter. Which leaves about 14 kWh/day as the space heating demand. Times say 150 days is 2100 kWh per year, which at 7p /kWh is £147 per year.

So what are the costs of a small ASHP? Purchase? Installation? Maintenance? Electrical running costs? These are numbers that anybody with an ASHP probably knows a lot more about than me!

Posted By: tonyHow long a period do you want to cost it over?

I'm looking for advice from people with knowledge of what is sensible. What is the guaranteed and expected lifetime of the devices you are talking about?

Professional installed a single typical single split ASHP can cost £5k, there are diy ones available for between 500 and 1000.

What specifically?

Assume a 3.5 CoP so

I ass U me. No, I would rather have people's actual experience.

Your 7p per kWh looks too low

You're right. It's 7.6p

I would install solar thermal and a shower timer and take the pain.

What would the solar thermal do that the PV doesn't? Why would I want all the extra complication?
Why would I want a shower timer? I don't want to compromise our experience (wear a hair shirt).

Up side is you gain air cooling

Agreed, but that rather contradicts your hair shirt suggestion and cost basis.

Are you looking at Air to Water or Air to Air? I'm guessing the latter.

Posted By: Jeff BWill the ban on the installation of gas boilers in new builds from 2025 accelerate the development of more efficient ASHP's?

I doubt it. I don't think the UK market has much influence on ASHP development. Places like Asia and America where there is a lot more demand for air-con have a lot more influence.

I don't think banning gas boilers, if it happens, will have much influence on insulation either, sad to say. I don't think mass builders will go beyond what they're required to by building regs etc and the SAP/EPC system is not fit for purpose in measuring heat requirements either, IMHO.

@owlman, I would guess A-A as well, but that's the kind of thing I hope and expect other people to know better than me.

@will, yes lack of return was why I didn't really consider ASHP when we built but there do seem to be some cheaper small units around now, or maybe I missed them 5-10 years ago. There's lots of brands I've never heard of as well as a few like Mitsubishi & Daikin that I know of but don't know much about. So I'm hoping somebody might be able to give me a few pointers.

I don't expect I'd qualify for any subsidies, and our low demand means we would need to minimise standing costs, so knowledge of which brands if any don't require maintenance contracts etc would be useful.

Our space heating is by heating the ventilation air and by a radiant heater in the hall. Basically as long as they dissipate enough heat energy somewhere in the building each day, the temperature stays fairly constant and reasonably evenly distributed (the inside walls are less well insulated than the exterior walls and internal doors are normally open). There might be a bit of a conflict between heating and cooling with an ASHP. I put the radiant heater in the hall because its on the north side of the building and the space has immediate access to both floors, so there's a reasonable chance of the heat getting distributed some useful. So that would be a good place for the inside unit for heating. But equally, it's the south side of the building that gets hottest in summer - the living room and dining room specifically, so there would be better for an inside unit for cooling.

To some extent the need for cooling is self-inflicted and is very marginal in any case. I still haven't built the pergola I plan to use as the brise soleil shown on the plans shading those two rooms, so they get more solar gain than was designed. It's not been a serious problem because we both like it warm, and my wife is concerned that plants on a pergola will attract mosquitos etc.

Many thanks, Mark. That's very helpful. Sorry to hear of your wife's problems. It's amazing how much difference to comfort even a single degree can make in some circumstances.

Yes I have used separate installations, I feel multi output single base unit would not suit retrofit.

An added bonus of the separate units is providing some sort of control to the rest of the heating which is storage heaters and wood stove

Hi there

I feel that is a jump to conclusions, I would not use the term scattered, More placed carefully and with full appreciation of the surroundings, others less inclined to give the benefit of the doubt may use the term scattered.

Now I have had experience of some cheap units in France and one of these was of the piston variety. I can understand why people would be upset with operating within 50 m of a property.
Our best unit is a 3.5kw unit Mitsubishi slim jim. This was my best purchase at <£200 I collected at 70 miles and he added a condensate pump for free. Still working 10 years later. I defy anybody to hear this style working at 10ft that is inverter scroll type very quite and neighbors do not hear it.

Now this unit has a wall controlled thermostat controller. With care this can be set up like a central heating unit with no added control by yourselves. I use this to give me overall control of a kitchen/diner temperature.
These are 2 rooms of about 6m*4m in an L shape with storage heater in the dining room and the air-con in ceiling of the kitchen. This gives me control of the temp which storage radiators are incapable of doing.
enough for now
Mark

Car-Mark: thanks for your response. Re: "scattered" - by this I meant (as per dictionary definition) "occurring or found at intervals or various locations rather than all together" and not in any derogatory sense! I asked the question because I do not have much choice where I could put three outside units in my property without one or two being rather near the neighbours' boundaries and this would influence any choices I would have to make.

I can concur with your comment about the Mitsubishi external unit - we have two at our chapel and they are very quiet, as indeed are the four indoor units (critical in a church environment).

Been there got the T shirt

Ours also had an inverter-driven circulation pump for the hot water, and a motorised ch/dhw diverter valve, both uniquely made with fancy electronics to match. It was also an oddity with two compressors. But (as you guessed!), it was a non-moving part that killed it - the main heat exchanger developed a tiny crack and lost refrigerant, and as it was mounted underneath everything else in the box, the labour costs to remove/replace everything were uneconomic.

Sadly the useful lifetime of many similar appliances seems to be more like 10years rather than 20 years now - thinking of fridges, freezers, washing machines, gas boilers etc - and kettles/toasters/computers/phones last even less, with no moving parts!

<blockquote><cite>Posted By: owlman</cite>@ Jeff B; You could stack them using cantilever brackets.</blockquote>

The problem with that is that we have a large-ish bungalow and having multiple external units in one place would mean having impractical long runs of refrigerant pipework. Any idea what the longest permissible run would be?

Posted By: Jeff B

Posted By: owlman@ Jeff B; You could stack them using cantilever brackets.

The problem with that is that we have a large-ish bungalow and having multiple external units in one place would mean having impractical long runs of refrigerant pipework. Any idea what the longest permissible run would be?

I think it's variable, dependent on manufacturer, unit size, etc., but 50M is not unusual, some bigger units go up to 75M, some smaller ones 30M

You can get skirting units I believe, but usually the option if the situation needs something different is to go for an air-to-water heat pump and use conventional wet radiators etc. You want refrigerant pipework and heat/cold emitters to be easily accessible for maintenance as far as possible.

Posted By: WillInAberdeenWhy would an underfloor grid of refrigerant pipes need different maintenance than the equivalent grid of hot water pipes?

Just curious, never seen it done so guess there must be a catch, but can't see why not. Air-to-water heat pumps have to run hotter in the refrigerant circuit than air-to-air ones, hence the better efficiency of air-to-air. 'Air-to-floor' pumps could run even cooler and more efficiently, in theory...

The testing pressure for soft copper refrigerant pipes is much higher than standard UFH so maybe problem number one, and I guess the total pipe length would preclude any such use as you describe.

Hi Jeff,
You mentioned you live in a bungalow, assuming loft space is unused can't you utilise that for your refrigerant pipe runs? Then simply separate your CH from your DHW; a good plan anyway IMO, and there's choice there.