Heat Pumps

It helps if you think of temperatures in Kelvin, rather than Celsius.

Extracting some energy from air at 270K to warm some water to 310-330K (depending if you have UFH or rads) sounds more reasonable ...

You need a property with above thermal characteristics (not guaranteed by just slapping on more insulation) before you should even think about. If you are on mains gas then stick with it (and still upgrade the insulation).

If you have a bungalow with a solid floor then you already have a possibly worse perimeter to area ratio, especially if it has had hotch-potch extensions in the past and this means a lot of heat loss through that 1 metre strip of floor all around the perimeter, next to external walls, which cavity wall insulation will not improve.

Refer to Part L of the buildings regs for details

Thats a fallacy. All heat sources are improved by more insulation, it isnt just heat pumps. A well designed and fitted ASHP should have a SCOP of 3.5 or more, making it , currently, the same price for use as a gas boiler. In future, I can see the gas prices rising more than electric, which will make the HPs cheaper to run, but, at the moment, there is a very artifical political decision to keep gas prices at the same ratio with electric prices, even though there are now some very low cost ways of producing electricity.

And fitting any type of heat source will have the same problem of losing too much heat through the fabric of the building. Gas has been cheap for the last 50 years, those days are ending, so we cannot carry on wasting it as we've been doing. With ASHPs, proper design has to be done for domestic heating, something that has hardly happened for the last 40 years, and being as gas was cheap, the fitters have got away with it.

Only if you conveniently ignore the extra cost of the infrastructure to support them and the need to supply backup generators when they fail to work when needed most.

And the problem still exists both with gas and ASHP. A lack of skill in the industry to correctly design the system. With so many cowboys entering the solar/ASHP market in order to suck up all the grants the chances of having a correctly designed ASHP system may be less than having a correctly installed new gas boiler.

On 25/02/2024 15:19, Jeff Gaines wrote:

Its a long explanation. Andy previously has said it really. Basically, the idea is to suck in loads of air, the refrigerant is cooler than that, it gets warmed by this hotter air, expands, is then compressed, releasing the energy it has abosrbed.A heat exchanger then warms the water passing through the outside unit. ( there are some that have indoor and outdoor units, these are split units, though they all work the same way).

Currently you can get a £7500 grant to take out a gas boiler and have a G/ASHP fitted. Usually this £7.5k is not enough to pay for the full install. Mine has cost around £8k for the parts (full heating system), there is easily 2 weeks work for a full CH system to be fitted, so labour will be £3k or so, hence a total cost to the customer of around £3 -4k.

Octopus have been fitting a lot recently, and giving such good prices that it's daft to not take up their offer. However, there are some downsides to their offerings. From anecodatal evidence, they are designing them to run at 50 degrees. This will get the house warm, but, it will cost around 25% more to run than one designed for 40 degrees. British Gas also do the same thing, they guarantee that your house will be warm. Thats easy, if its too cold, they turn up the flow temperature, and for each 1 degree increase in flow temp, you'll be paying 2.5% more for your electric.

Design is the key. Get it designed to run at 35 degree flow temperature, and you'll be seeing COPs of 4+. The downside to that is that most of the radiators in an existing installation will be too small. Radiators in the 60's/70's were designed for a radiator temperature to room differential of 50 (DT50). Even now with new gas boilers, this will not work using the quoted figures for radiator output, as new boilers are now limited to a flow temperature of 55 degrees. If you have a room temperature of 20 deg. then your DT will be around 25 (55 flow, 35 return = 45 mean -20 = 25), so the quoted radiator output is roughly half of the actual figure. With a well designed HP running at 35 degrees, with a return of 30 deg, the Mean Water Temperature is 32.5 deg, minus the room temp of 20 gives a radiator DT12.5, so the radiator quoted at 1000 watts at DT50, now only gives out around 250 watts (it isnt quite as straightforward as that, there is an equation to get the output correct). So from that, you can see that your radiators will probably not be big enough to run really efficiently at 35 degrees flow, and be able to warm the house, as they are just not big enough to emit the required heat. In the real world it normally isnt quite so bad, as plumbers just chucked any rads in that fit, and they were mostly over sized, as, well, gas was cheap, and no one moans if the house is too hot, but they are on the phone immediately if its too cold. If you are going for one, your house needs a proper heat loss survey, not a 10 minute look around. A proper survey should take at least a hour to measure up everything in the house, then at least another hour working out the results. Each room needs to be assessed to see what the heat requirement is for that room. To get the Grant, the house also needs to meet some insualtion requirements. An open barn will not get a grant.

For anyone who wants to go into it any deeper, I found that Stiebel Eltron have a few hours of videos and tests you can sit through to improve your understanding ... you get a pretty worthless certificate too!

Are they? My "new" gas boiler had a default flow temperature or 75C but easily changed to be a lot lower.

Just to say . . . I had a quote a month or so ago - £15k. And that was the basic pump and hw tank install only - no other work or alterations. Totting up the parts came to about £6k - so £9k labour.

I was at a gathering of about 100 retrofit people the other day, and anecdotally there's a number of companies taking, er, liberties with the pricing in view of the grant. And some deep cynicism around the MCS certification/qualification, with painters/decorators jumping on board etc.

FWIW I'm not going with an ASHP for now - going to concentrate on insulation and mechanical ventilation. I'm also looking at solar, but I'm not fixated on economic returns. I'll look at ASHPs again once (and if) that little lot's sorted, and I can afford it.

You can guarantee the labour is going to be at least £7k to "soak up" the jobs for the boys grant ...

Many thanks for all the replies on this.

Is seems to me that sticking to gas is probably the best bet for now, I'm

You've got me a bit frightened with all the calculations! It reminds me of the first CH system I was involved with at an uncle's house when I was 11 years old. I was given a book showing the heat loss per hour in BTUs for all the materials found in a house and depending on the direction they were facing. I don't know how well I did with the calculations but I do know that house always felt blooming cold!

I believe these days the regulations with regards changing a gas boiler also require a house heat loss calculation to be performed in order to fit the correct size of boiler.I'll bet that most installations will be a like for like capacity boiler which may not result in the best fuel efficiency with modern condensing boilers.

I'll have to brush up on my BTUs (or is it BThUs?)!

Even when I first calculated my heat loss, perhaps 40 years ago, the U values for various walls/roof/windows etc. were quoted in Watts per square metre (per degree C)

Btu to kW, divide by 3412

I'm talking 66 years ago!