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Advice for ground source heat pump in new build with PV and batteries

gcon45 · 2025-05-06T14:00:33Z

Hi everyone. We are currently building a house and garage with a total of 406 square metres that will have under floor heating. Our heating engineer initially quoted us for an ASHP but came to the conclusion that even the largest single phase one they could supply would be working too hard due to the size of the property that it would be heating. We own the surrounding field and lease it for farming so using an area of it for geothermal pipe work isn’t an issue. We have been given 2 options for a GSHP. The cheaper option is a Warmflow GS03 4.5kw - 18kw. The more expensive option is an Ecoforest B1 3 -12kw. I know the Ecoforest unit has more features. What should I go for? What soil considerations should we have? We have a pretty extensive 18.9kw solar setup in place with an estimated annual output of 14978kwh (please note we have an EV too). It will feature a Gateway, 10kw inverter (pending G99 approval), 42 All black panels with in-roof system, an eddi for water heater and 4 x 8kw batteries (upgradable to 6). Any advice / recommendations welcome. Thanks in advance.

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Grounds Source Heat Pump (GSHPs)

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gcon45

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Topic starter 06/05/2025 10:20 pm

@johnmo

Very good point.

It’s something I should have given a lot more consideration.

I will contact my contractor to ask if it’s possible at this stage to change the open fire in this room to a sealed stove. I know it has been built to accommodate a fireback and has the lintel etc in place.

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Mars

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06/05/2025 10:21 pm

Even though GSHPs are more expensive upfront, they generally offer a longer lifespan than ASHPs and significantly better SCOPs when well designed.

@barry-sharp, for example, has a system in Scotland achieving a SCOP of over 4.5, and if your budget allows, it’s definitely worth considering ground source seriously.

Yes, heating oil is currently around 50p per litre, but as we head into late autumn and winter, it’s likely to climb back toward 70p or more. In that context, a well-designed GSHP will almost certainly be far cheaper to run in the long term.

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Anonymous 5011

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06/05/2025 10:28 pm

Posted by: @gcon45

↑

open fire in this room to a sealed stove.

When looking at room sealed stoves, you need to ensure primary and secondary air are taken from outside - some only take primary air from outside and secondary from the room.

Room sealed should only need a 100mm hole through the wall and this is then ducted to stove.

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JamesPa

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07/05/2025 12:32 am

Posted by: @gcon45

↑

I can try and find any bits of info that anyone thinks would be beneficial though?

The useful bit is the bit that specifies the heat loss at the design temp ( presumably around -3).

It's your choice of course but I genuinely fear that whoever is designing your heating system may be specifying something that you will not be happy with.

I have no axe to grind, I'm not involved with the industry I'm just an interested amateur with a background in phisics who happens to have evaluated carefully what's been said here for the last two years and, as a result, has ended up with a very satisfactory ashp experience mosty by avoiding installers that frankly were talking BS and being very careful about system sizing.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Judith

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11/05/2025 3:01 pm

Posted by: @gcon45

↑

Yeah the flue for the fire will be sealed up for about 360 days of the year with a draught excluder.

It’s a traditional house so it’s in keeping with the style.

I’d imagine the fire will only ever be actually lit around Christmas time.

We have a cassette style inset stove, much more efficient than an open fire, which is less than 5kW so doesn’t require any deliberate ventilation. It is a Stovax but the model isn’t important. With the door closed there was no discernible air leakage when we had the whole house leakage measured.

Our MVHR endures ample ventilation if we were nervous about CO with the fire lit. The fire really is just a visitors on special occasions usage and when we feel like it, since the novelty wore off a while ago back.

So your multi-fuel stove will have no leakage to speak of leaving only the open fireplace extra to a well insulated modern house. Our 200m2 50s house needs a 7kW heat pump which keeps us nicely warm. You are unlikely to to need much above 12kW!

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC

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Anonymous 5011

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11/05/2025 3:52 pm

So is that with external air supply?

This post was modified 5 months ago by Mars

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Judith

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11/05/2025 9:29 pm

@johnmo no external air supply, just the room air. We do have CO detectors and never had any alarms.

But the measured flow on the MVHR system vent provides enough air flow

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC

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Anonymous 5011

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11/05/2025 10:03 pm

Posted by: @judith

↑

@johnmo no external air supply, just the room air. We do have CO detectors and never had any alarms.

But the measured flow on the MVHR system vent provides enough air flow

Braver person than me. No way I would go down that route.

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