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Now then boys & girls, I am looking for advice on heat pumps.

We live in our 14 yr old self-build house, it is extremely well insulated and has all the bells and whistles that you would expect.

There is no heating upstairs, and has UFH downstairs.

The underfloor heating is provided with heat from a 500 Litre thermal store upstairs which in turn is heated by immersion heaters and a small wood stove back boiler. The Thermal store also provides domestic hot water.

The UFH is embedded in a 150 mm thick screed which in turn is directly on top of 100 mm of concrete slab, the slab sits on 300mm of EPS.

I have a desire to install an ASHP to provide heat directly to the UFH manifold and hence further reduce my electricity consumption.

The UFH manifold is located within a downstairs coat cupboard so presumably it would make sense to install a cylinder in this space and heat this directly from the proposed heat pump.

The UFH manifold temperature controller is set to its minimum flow temperature of about 35 C and this seems adequate to heat the house.

Due to the thickness of the slab/screed it takes perhaps 2 days to warm up, but once warm, the house will bumble along and on a full winters day perhaps consume upto 15 kWhrs of heat. For most of the winter the heat requirement is more like 5-10 kWhrs of heat

Given that I would look to provide this energy on a TOU tariff I assume that I might have a little as 5 hrs of cheap electricity time to run the heat pump, for the sake of conservatism I assume that I might need a heat pump with a rating of 3-5 KW output.

To heat the existing thermal store from the proposed new heat pump would require a lot of messing about but in principal it is possible.

I understand that this project probably makes little financial sense but there is the Government ASHP grant to consider.

If I am not entitled to the grant, or the costs in excess of the grant are excessive, then it might make sense to install Air2Air systems.

Questions.

1. What kind of turn-down ratio am I likely to get from a small ASHP ? Will a small unit be happy to bumble along delivering very little heat.

2. Is there a brand/model of unit that is recognised as being the most reliable at this kind of size ?

3. How large a cylinder should I install next to the UFH manifold ?

4. Will I be able to obtain to the Government Heat Pump grant ? does the grant require that the DHW is supplied by the heat pump ?

5. Am I missing anything obvious ?

Thanks in anticipation.

The grant is only available afaik if you are removing a source of heat that uses combustion, as its to incentivise the change to electric heating rather than gas/oil boilers etc I'm afraid.

You probably don't need a cylinder for heating. So you may just need to swap the thermal store for a smaller unit just for hot water and use the heat pump directly to heat the slab during cheap hours then keeping the slab to a chosen temperature in the day/evening. Buffering reduces the CoP of the heat pump vs direct heating.

I'm not sure on which units are best. Vailant seem to score well. I like the look of the Ebac unit, but there aren't many in the wild.

pudding wrote: ↑Fri Jan 31, 2025 2:55 pm The grant is only available afaik if you are removing a source of heat that uses combustion, as its to incentivise the change to electric heating rather than gas/oil boilers etc I'm afraid.

If I remove the wood/coal burning stove would that eliminate this problem ?

Sounds like a very suitable application for an ASHP. I doubt the grant will be suitable because there is a prescribed one-size-fits-some installation that is required to get the grant and that probably wouldn't work in your situation.

To answer your questions:

newtoallthis wrote: ↑Fri Jan 31, 2025 2:49 pm
1. What kind of turn-down ratio am I likely to get from a small ASHP ? Will a small unit be happy to bumble along delivering very little heat. Normally c. 25%. Varies by model and often the smallest models (that you're going to need) turn down less than the bigger ones.

2. Is there a brand/model of unit that is recognised as being the most reliable at this kind of size ? Have a look on here: https://heatpumpmonitor.org/. Valient Aerotherm dominates the best-performing installs.

3. How large a cylinder should I install next to the UFH manifold ? You don't need one! A buffer reduces efficiency and is a fudge.

4. Will I be able to obtain to the Government Heat Pump grant ? does the grant require that the DHW is supplied by the heat pump? No and Yes.

5. Am I missing anything obvious ?
Keep it simple. There is a guy on Youtube who fixes inefficient heat pump installs by chucking: zone valves, mixing valves, supplementary pumps, buffers, etc. in the bin. Watch a few of his videos: https://www.youtube.com/@UrbanPlumbers. If I were you I'd fit a 4-6kW HP, run it 24/7 set a 25℃ flow temp and ramp that to 30℃ in the cheap period.
Thanks in anticipation.
No worries!

pudding wrote: ↑Fri Jan 31, 2025 2:55 pm The grant is only available afaik if you are removing a source of heat that uses combustion, as its to incentivise the change to electric heating rather than gas/oil boilers etc I'm afraid.

Looking on the government website it states:
1. Overview
Through the Boiler Upgrade Scheme, you could get a grant to cover part of the cost of replacing fossil fuel heating systems with a heat pump or biomass boiler.

Fossil fuel heating systems include oil, gas, electric or LPG (liquefied petroleum gas).

So no reason you shouldn't be eligible with the above statement and it is also a coal fuelled boiler that helps to heat your home.

I doubt the thermal store will be of any use as the temperature in it will only be what you require to keep the house warm and therefore no use for hot water if it is supplied through a coil in the thermal store. If that is the case then you would need a hot water cylinder which could be placed where the thermal store now is or in the coat cupboard where the UFH manifold is.

I would as Stinsy says remove all controls on the UFH manifold, my installer said to do this and it all works very well without temperature control, zone valves and the UFH pump.

Hmm, maybe there's a chance then to get the grant, but how could they define an electric heating system as a fossil fuel heating system, seems contradictory? What exact system could that be?

pudding wrote: ↑Fri Jan 31, 2025 4:59 pm Hmm, maybe there's a chance then to get the grant, but how could they define an electric heating system as a fossil fuel heating system, seems contradictory? What exact system could that be?

I guess they mean SHs. But yes. Lazy language.

pudding wrote: ↑Fri Jan 31, 2025 4:59 pm Hmm, maybe there's a chance then to get the grant, but how could they define an electric heating system as a fossil fuel heating system, seems contradictory? What exact system could that be?

Because our grid is always consuming gas, the highest heating demand probably coincides with highest grid demand during harsh cold weather - electricity at these times tends to be dominated by gas.

Stinsy wrote: ↑Fri Jan 31, 2025 5:55 pm

pudding wrote: ↑Fri Jan 31, 2025 4:59 pm Hmm, maybe there's a chance then to get the grant, but how could they define an electric heating system as a fossil fuel heating system, seems contradictory? What exact system could that be?

I guess they mean SHs. But yes. Lazy language.

In Scotland it does mention old and newer style storage heaters in the type of heating being replaced, I suppose a few second-hand SH's could be pressed into service prior to the application...