I installed a ground source heat pump some years ago and the adventure, that I enjoyed, is documented on this forum. Now I have the idea that I could install an Air Source Heat Pump in a UK domestic situation.

I need about 15 kW output and ideally 70 degrees for rads and hot water, from a single phase supply. At this early stage I am thinking that I remove the existing gas boiler and put the ASHP in its place. I have an Evohome controller which I hope I can continue to use.

These are my questions. . . .
1. Who are the best manufacturers to look at?
I have found a Daikin product can provide 70 degrees water output
2. Where can I find a list of experienced installers?

We had a Daikin 'HT' ashp in a previous house. It was effectively two heat pumps in series with different refrigerants. The first is mounted outside as usual and heats to ~30-40deg, the second is mounted inside and boosts the heat level up to ~70deg. We had it as a drop-in replacement for an oil boiler so it fed the existing radiators.

We were overall pleased with it. The two heat pump circuits are fairly lightly loaded, so overall the CoP was pretty good for the times, though better is available now. There was plenty of heat even in deep Scottish winters. It was cheaper to run than oil, got RHPP and RHI and paid back its cost in savings, and slashed our CO2 footprint.

We had insulated the house so the existing radiators were bigger than necessary, so the heat pump only ran at 70+ degC when heating the place up from cold, then modulated to a lower temperature.

The indoor circuit was a bit noisy to have inside the house. It failed after 7years which was not that long, we were moving out so didn't replace it.

For our next house, we are watching how the market develops. There are some coming out with new refrigerant mixtures that give reasonable CoP even at radiator/DHW temperatures (someone linked to a Valiant one). The market for air-air heat pumps is also interesting as it is marginal whether its worth retrofitting underfloor heating in our place. We won't bother with another two-stage heat pump but would look at another Daikin.

To claim RHI or GHG if you are eligible, you need to use a MCS installer, they have a list, but others have found it cheaper to DIY. Legally you need a aircon technician with a F Gas certificate to do any work on the refrigerant circuit but a plumber or sparky could do the water or electric connections.

Hi WillInAberdeen,

Very useful, thanks.

Do you need to use an MCS installer to get the £5k grant?

I'm with jfb here, unless the property is really large I'm skeptical that 15 kW and 70 °C is really needed.

In this somewhat leaky pile of rocks I'm renting in Scotland I've been logging various parameters for a while, including the flow temperature to the radiator in the small bedroom I use as my study. The maximum flow temperature is typically just over 50°C and the worst average for a whole day last winter (Nov, Dec, Jan, Feb, Mar) was 37.54 °C on Boxing day (2019-12-26). For all other days it was less than 36 °C.

Since the heat output from a radiator is more-or-less linear with excess over room temperature this implies that the same heating effect could be got from running the heating continuously at, say, 40°C rather than intermittently at 50°C.

The nominal output of the boiler [¹] is 18 kW but even on Boxing day it only actually ran for 34.6% of the time so for continuous operation a 6 kW boiler would have done fine. Mostly even that would modulate or switch off at least some of the day; for the climatological winter (Dec, Jan, Feb) the overall average boiler run time was a hair under 25%.

Caveats: this is a small (two-bed Victorian/Edwardian 1.5-storey semi) house, I keep the “study” cosy but the rest of the house might be a bit cool for others' tastes. OTOH, it has decent DG but the rest of the insulation is a bit perfunctory.

From what I've seen plumbers tend to over spec boilers; they're much more likely to get a callback if the boiler's not powerful enough than if it's too powerful, the cost increment is not large and they can make a small markup on that anyway.

With heat pumps a lot more care is needed to pick an appropriate power and flow temperature hence, I think, the horror stories of vastly over-expensive installations and/or installations which don't perform well either because of the design or the way they're operated - e.g., people expecting them to heat the house to very short order.


[¹] Why TF do we call them boilers? If they ever boil anything something has gone dreadfully wrong.

WillinAberdean, Ed, jfb,

Thanks all. Very useful comments.

I am learning. I think I overstated the heat required. I was being simplistic and just finding an equivalent to the gas boiler. Wrong.

Posted By: Ed DaviesWhy TF do we call them boilers?

I would guess that the modern devices can trace their lineage back to the large-bucket-over-an-open-fire devices that really did boil water and indeed were used to produce steam at pressure.

To add: 'Meaning "vessel for boiling" is from 1725; specific sense "strong metallic structure in which steam is generated for driving engines" is from 1757' according to https://www.etymonline.com/word/boiler

Americans call their domestic boiler 'furnace' - image of half-naked stokers shovelling coal into the behemoth!

Thanks Paul for the cultural update!

Technically correct is the best sort of correct, as they say but i think they're usually being sarcastic

Posted By: gravelldcan you explain this? Sounds like learning material...

Well, if you think about it, it is pretty obvious. Q: What does a boiler do? A: Boils things. QED

Specifically: A: a boiler boils a liquid, to turn it into a gas.

Heat pumps and fridges work by boiling cold liquids, to absorb heat energy. Then they compress and condense the resulting gas, to give out the heat energy at a warmer temperature.

Oil boilers atomise their liquid fuel into tiny droplets, which can boil in the flame, to burn as cleanly as possible.

Gas boilers are the odd ones, as no liquids are actually boiled. The fuel arrives as a gas already, and the CH water remains a liquid.

However, they do condense some of their combustion exhaust gases, so you get physical tangles such as: "A condensing gas boiler.. !"

As Ed gently points out, this discussion does score quite high on the pedantometric scale.... but his question did start it off!

The temperature in a flame is sufficient to boil kerosene (not just to evaporate it). But the droplets pass through the flame rather quickly, so they have to be small, so that they can get completely heated and boiled before they pass on through as soot. The short time scale and length scales, mean there are all kinds of other surface effects and temperature gradients and pyrolysis going on, to make it more complex than 'simply' boiling as I described it.

In contrast, the pool of molten wax at the top of a candle is not hot enough to boil, it's actually at its melting/freezing temperature. It can evaporate from its surface, but not enough to sustain a flame. The wick gives the molten wax some extra surface area, and lifts it up closer to the flame, where things get hotter and evaporation goes faster.

"Pedantometric scale" - like it! Must remember that one!

Posted By: Paul in MontrealBoiling is not talking about the fuel or the refrigerant :)

On your definition, it's not talking about anything at all
boiler and furnace are just two arbitrary strings of letters used to name two classes of device.

And it doesn't answer Ed's original question:

Posted By: Ed DaviesWhy TF do we call them boilers? If they ever boil anything something has gone dreadfully wrong.

Next week I have an appointment to see a Domestic Renewable Heat Incentive installer. I have done the rough estimate on my house and the amount is £960 pa for 7 years. I remember reading about a lump sum, has that now gone?

I wonder if anyone had done the calculation of installation cost, versus annual savings with Gov heat incentive to see how long it takes to break even.

Any tips on subjects that I should be sure to mention gratefully received.