New boiler - Air source heat pump vs mains gas - future proofing

Bathsheba309

We need to replace our hopelessly inefficient old gas boiler - it's probably about 50 years old. We are on mains gas, so I know that it would likely be cheaper to get a new condensing boiler, but with gas prices rising and pressure on the government to meet environmental targets I'm wondering whether it makes sense to put in another gas boiler, so we're thinking an air source heat pump. We have a pretty well insulated 1960s bungalow, about 250m2, and we have a couple of log burners. At the moment we pay around £1600 on gas a year - all used during the colder months. We bought the house as a doer upper and plan is to install wet underfloor heating a room at a time - it will probably take 2-3 years. Our EPC says our water and heating needs are well in excess of 20000 space and 2000 water, so our RHI repayments would not recoup the whole cost of installing an air source heat pump, but we should get over 11 grand, so would be much better to do it before March 2022 when it changes to a blanket 5 grand grant. Installing a new gas boiler would cost a few grand so taking the RHI into account the boiler replacement is likely close to cost neutral - it will cost us 4-6 either way. We've been told we will need a 14KW air source heat pump and will cost between 14-17 including some radiator upgrades. We're almost convinced but I'm dithering because of all the scare stories about ending up with a permanently cold house in winter and lukewarm water! I would much appreciate any advice / comments etc and experiences from those of you who've got an air source heat pump - are you happy with how they heat your house?

matelodave

We've had an 11kw unit for eleven years - our EPC for a 140m2 reasonably well insulated bungalow reckons we need about 12600kwh/pa to heat it and 2750kwh/pa for hot water. Our average leccy consumption over the eleven years is around 7500kwh a year (and that include heating, hotwater, cooking etc etc). The cost of our heatpump and tank came to around £7.5k eleven years ago and we got £5k back over seven years with the RHI. (The cost of the u/f heating was also around £7k on top)

So far we've got away with pretty cheap leccy at between 10-12.5p/kwh, this is the first year we've breached £1000 a year. HOWEVER now leccy has suddenly jumped in price I expect to be paying around £1700 for the forseeable future unless it starts dropping again next spring

As we've got an overlay wet underfloor system we run it at around 30-40 degrees whch keeps the house temperatures pretty even unless we turn it down to much or turn it off, so it runs more or less continuously when it's everso cold outside. The lower the flow temperature the more efficient to heatpum is

You can have the hot water temperature as hot as you like but an ASHP can usually only manage around 55degrees so to get it hotter you need to use an immersion heater for the xtra. We heat ours to 45 degrees which is fine for showers and other stuff, dishwashing goes in the dishwasher.

IMO most of the scare stories are from people who have either just bunged it onto an existing system, havent had it properly designed or set up properly or dont know how to use it. The heating system as a whole has to suit lower flow temperatures.

A heat pump wont save you money despite all the hype, it's cheaper to run than storage heaters or ordinary electric heaters, probably on a par or a bit more than oil, cheaper than LPG but significantly more expensive than mains gas, even at todays's inflated prices. Unless there's some re-adjustment of the costs between gas and leccy then a heatpump will still be more expensive to run than mains gas. TBH going green isn't cheap

If you assume that gas is around 5p/kwh and leccy is 21p/kwh and you manage to acheive a COP of 3 - 3.5 (four is really pushing it) then 1kwh from a heatpump is around 7-8p/kwh. Dont forget if you go to electric, you'll have to cook electric as well to avoid paying a standing charge for a very low gas consumption.

BTW because I was in the first tranche of RHI my COP is a deemed 2.5 and thats what my RHI is based on. I guess we actually get a little better than that most of the time, although when it's everso cold it's probably a lot less.

lohr500

You are probably aware of this but I thought I would mention it anyway. If you plan to sell the house after you have done it up, then the RHI repayments transfer to the new owners. They don't keep going to you. Not an issue if you plan to stay there for the 7 year payback period.

Reed_Richards

I replaced an oil boiler and radiators with a heat pump and a new set of radiators (there is no mains gas where I live).  I can't really tell the difference, the house was perfectly warm before and perfectly warm after the heat pump was installed.  It has not been quite a year yet but it looks like the running costs of the heat pump for its first year will be comparable to what I had been paying for oil.  But the price of both heating oil and electricity has gone up a lot recently.  I heat my hot water tank to 50 C but there is a mixer on the output that makes it a bit cooler.  This is very safe (from the possibility of scalding) and just hot enough for washing dishes.  

My radiators are set up for 50 C flow and 45 C return.  If instead I had underfloor heating I could run the heat pump to a lower output water temperature, making it more efficient.  If I had a mixture of radiators and underfloor heating I would want them all to operate at the lower temperature of the underfloor heating or there would not be any economy.  There is no way you would achieve this without replacing all your radiators.  I'm sure at least half the scare stories are from properties where the radiators were not upgraded or not properly so.  If you have a 14 kW heat pump you need perhaps 12 kW of radiators (before you start installing your underfloor heating)  and that is the heat output at whatever mean water temperature you chose to use.  So if you were to aim for a mean water temperature of 20 C above the room temperature you would need 12/0.304 = 39.5 kW worth of radiators based on their standard rated output at Delta T = 50 C.  So that might be 14 radiators with an average rated output of 2.82 kW.  A double panel double convector radiator that is 600 mm high by 1800 mm wide would be a little more than sufficient (just over 3 kW at delta T = 50). 

Delta T (°C)	Correction factor
5	0.05
10	0.123
15	0.209
20	0.304
25	0.406
25.49	0.41666667
30	0.515
35	0.629
40	0.748
45	0.872
50	1

        

k_bagpuss

I’ve had a Valient Aerotherm Plus system fitted in September, with mostly new radiators , no new pipe work was needed, they just attached flexible plastic pipes to the existing pipes in the wall and the house is as warm as it was before with my old combo boiler. I have my back temperature set to 19 degrees and the water tank is set to 50. It’s too soon for the bills analysis as we’ve switched supplier and changed tariff, but so far absolutely no complaints.

matelodave

k_bagpuss said:

I’ve had a Valient Aerotherm Plus system fitted in September, with mostly new radiators , no new pipe work was needed, they just attached flexible plastic pipes to the existing pipes in the wall and the house is as warm as it was before with my old combo boiler. I have my back temperature set to 19 degrees and the water tank is set to 50. It’s too soon for the bills analysis as we’ve switched supplier and changed tariff, but so far absolutely no complaints.

Tariff is what you pay in £££'s whereas to assess your consumption you need to measure and analyse how many kwh you are using compared with what you used before.

If you;ve changed supplier and tariff you could easily be using significantly less energy (kwh) but be spending a lot more in £££'s because of the tariff

ispookie666

I'm probably an odd ball here.  Just replaced our 40 year old oil boiler with another oil boiler.  I did quiet a bit of calculations and I just could not make an economic sense to go with GSHP or ASHP(air to water).  I do have Air to Air HP(Aircon) which we have been using, but have to say it seems to struggle these days due to temperature being close to freezing.  

lohr500

@ispookie666. Not an oddball. I did the same thing at the start of the year.

The numbers for switching to an ASHP just didn't make economic sense for us either.

We went for an externally located similar sized output oil boiler to replace the ancient Myson oil boiler that was inside the kitchen. The new boiler has a 10 year parts AND labour warranty (subject to an £80 ish annual service) and is using around 28% less oil for similar timer and room thermostat settings. I am sure that part of the saving has come from having the hot water and central heating circuits converted to an S plan system.

I also spent time balancing the radiator flow and pump speed to get a return temperature below 45 deg, with a flow close to 60 degrees to help the boiler condense effectively. 
 

Bathsheba309

We have decided to go with an ASHP with radiator upgrades throughout the house. It will drastically lower our CO2 emissions, and given our current old boiler was only 65% efficient when it was new c50 years ago, we will likely save money on what we're paying now. The model they're recommending is a grant aerona 3 17KW heat pump. There isn't much in the way of reviews out there, but what there is seems to suggest that it's cheap (relatively) and very efficient. It should be big enough to heat the whole house, and once we've installed underfloor heating in the main living areas we should be able to lower its running temperature to make it more efficient still, so that's the plan! Thank you for some very helpful comments on here. There are a lot of 'experts' out there, who don't have any experience of ASHPs, prophesising doom and gloom about costs / cold houses etc. It's hearing from people who actually do have them that's reassured us that provided we do everything properly it makes sense to take the plunge!

I have a couple more questions about fitting it: should the fitters be doing a power flush of the central heating system to ensure that it's running efficiently before they fit the new radiators? And, I've read that some pipes need replacing with an ASHP because they won't be wide enough to heat at lower temperatures. In practice though I don't know what that means. What do I need to be checking to make sure that they're doing it properly?

Reed_Richards

If your old central heating system has gunge in it then that will mostly be inside the radiators so replacing those will make it a lot cleaner.  If you do want to do a power flush it would make more sense to do that after you have replaced the radiators.  You'll probably get an inline magnetic filter to keep your heating water clean as part of the installation.

Hopefully you will use an MCS accredited fitter in order to qualify for RHI payments (assuming you can complete the work before the end of March).  If so they ought to know what they are doing.  My heat pump is a single outdoor unit (as is the Grant one, I think).  My installer used 28 mm diameter pipes between the heat pump and the buffer tank and DHW cylinder.  The central heating distribution uses 22 mm pipes with 15 mm pipes to individual radiators.

This is purely my own opinion but I don't see that there is any running cost benefit in having some radiators and some underfloor heating compared to all radiators.  In both cases you need to make the water temperature hot enough for the radiators so you don't benefit from the improved efficiency you could achieve with entirely UFH.
   
  

Bathsheba309

Thank you that's really helpful. It will take a few years for us to get underfloor heating throughout the house but that's ultimately the aim. We'll prioritise the rooms we keep warm. The company we're using are MCS accredited, we got them off Green Match and they're supposed to be coming the end of Jan, though we don't have a date yet.