Air Source or Ground Source Heat Pump

1 Mar 2016
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United Kingdom
Are there any plumbers or specialists on here who are Air Source heat pump or Ground Source heat pump advocates?

I was getting a quote for a ASHP, circa 11kw. Just want to understand the downsides of this. I have a large home where all three floors are wet Under Floor Heating. No existing boiler or radiators due to it being a large renovation project.
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You will need a concise heat loss calculation for every room in the property. And a solution for DHW that is going to deliver your requirements.
Building ventilation and fabric loss are hugely important as you will be aiming for very low flow temps and not waste any of the energy produced.
I install commercial heat-pumps. Downside is the initial outlay cost. As stated do a correct heat loss for the building. I’m assuming you have done that with your proposal for an 11kw?

Or call a manufacturer speak to a sales rep get them to size it up. Vaillant daikin nuheat.
If you have the space for a ground source then go for ground source. On a cold winter's day an air source system might not be able to collect enough heat from the cold air to keep the house warm. Worse case it may be necessary to use heat to de-ice the outside unit. Either by using electric heaters or by running the pump in reverse to pump heat from the house into the outside unit.
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Are you allowed to keep these units within an internal garage to the house given that there’s no co2 emission and the only output is cold air from the reverse refrigeration process? Or does the fact that you need a constant supply of air to extract the heat from it mean that the temperature decline in the garage for to the cold air output not make this an effective choice?
Surely you'd need it to be open enough for the volume of air movement that goes through ASHP so don't see how it could be fitted internally.
ASHP efficiency has improved a lot with R290. The outside box needs to be, outside. It cools the air passing through it.

Underfloor heating is the perfect use-case as the flow temperatures needed are lower than radiators, which means the heat pump works with higher efficiency ratios.

Re calculations - the system needs to be sized for both your heating and DHW demands. The systems will heat either but not both, at any particular point in time. 11kW is small compared to a combi boiler that will provide 30kW or more. But of course there will be a storage cylinder with the heat pump.

A system can be designed with multiple outdoor units btw and will need an internal water volume enough to avoid short cycling, i.e. 250 litres or more for 11kW system. Volume can be added with buffer tanks if required.

Are you allowed to keep these units within an internal garage to the house

With ASHP in the winter, the outdoor unit blows out cold air; very cold air. As you realise it is a giant freezer in reverse, so the temperature in the garage would drop dramatically and freeze everything inside. Once the temperature inside the garage was cooler than the ambient outside air temperature, less heat will be being recovered from it and efficiency will fall. The unit will also spend more of its time, and generated heat, defrosting itself. The 'Outdoor Unit' has its name for a reason.
If there's a gas supply in the street then use a gas boiler...heat pumps will never compare financially to boilers.
Heat pump installers conveniently forget about the ongoing maintainance costs and extortionate parts...they're a cash cow for the manufacturers.
Electricity ~15p per kWhr, gas 2.9p. Design flow 35°C, R290 systems SCOP > 4.8, effective cost per kWhr ~3.1p.

ASHP unlikely to achieve that number in practice, but neither is gas boiler 100% efficient; differential is small and that's before the government consider how to get to net-zero, i.e. tax out gas.
I've had two companies (from the Valliant website that recommends installers) so far come around to inspect my property but I'm waiting for a quote. First one only deals with only ASHP and the second does ASHP and GSHP. No detailed surveys or heat loss calculations yet though I have given them house plans with measurements.

Few questions:

1. With respect to GDHP, there are a couple of options in the way it is pipe is laid e.g. 1) slinky style loops, 2) many deep trenches or 3) vertical bore holes. Can anyone comment on the costs of the three options and whether option 3 would be most suitable if the pipes that go down vertically into the ground hit the water table and get better heat transfers?
2. With ASHP, it's clearly more energy efficient to keep the unit as close to the property as possible so there is less pipe run to the heating system and cylinder but if the unit is e.g. 6m outside from the entrance to the building, even when insulated and running e.g. 400mm under the ground / soil, is that still acceptable from an efficiency perspective?

3. Does the RHI payback cover the whole cost of GSHP over the 7-years? What if the property was not fully occupied or heated to its full potential (we have a large home with not all rooms being used), does that mean we can actually profit from the payback?
I can't comment on the ground source but as far as air source goes, if you consider a split system (so only refrigerant passes through the pipes connecting the units, not water) you can go up to around 75 metres from the house to the outdoor unit, in fact the minimum pipe length on my system is 5 metres. With regards RHI it is just about possible to get a full payback for the air source boiler and install, I get £287 per quarter which I will get for a total of 7 years. The amount you get does depend on a number of factors though so I cant be sure what you will get. The government will ask for a questionnaire/declaration to be completed each year just in case you decide not to use air source or fit a gas boiler so it will have to remain your main source of heating and the house must be permanently occupied for any period that you receive the grant for. Normal holidays are allowed but not living at a 2nd property or overseas etc.
I know of two GSHP systems using well water as the heat source, both are very successful. One pumps water up from the source well to the heat exchange and the other has the heat changer below the water level in the well.

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