Having now discovered that hybrid systems are possible (but not necessarily the correct solution) my next questions is why "we" seem to be focusing on air source heat pumps.
My limited knowledge suggests there are two ways of using ground source:
1) dig a series of relatively shallow trenches and run piping through them
2) drill one (or do you need more?) hole straight down and feed that to your heat pump. Almost everyone who talks about ground source talks about the shallow trench system but the single deep drill doesn't need as much space so why not use that?
Cost. They can go down 100 metres if needed, or whenever bedrock is hit. Clearly it wouldnt work in some areas of the UK, as the rock is quite close to the surface. Its around £10k to get a deep enough borehole. Getting 3 or 4 would improve efficiency, but the cost is a lot more to install. Low level trenches are not the best, as the winter goes on, the ground around the pipes gets chilled, the ground cannot replenish the heat lost, and the HP gets less efficient. ASHP wins, generally ,in the UK due to this, as, the air temprature is above 5 degree C for around 85%+ of the time, making ASHP give a COP of 3+ for that time, and a lot more in the summer, where the GSHP will be getting 10 degrees from the ground, even in Summer, the ASHP will be getting 15 deg C+ to heat the water. So GS is fine, if you can get vertical boreholes, and preferably more than one, or, a big horizontal area, that is more than 1 metre deep. AS is good for most areas of the UK, for those areas that receive the lowest temperatures in the winter,it would still work, but wouldnt be as efficient if the outside temperature was low for long periods, but, it would still be more efficient than standard electric heating.
Basically GS only makes sense if you have a lot of outdoor space, like a paddock, or a lot of heating load in a constrained site, like an office or school.
For the average suburban house we don't have enough land or enough heating demands to make the extra cost worthwhile.
Basically because the alternative is much more expensive and not even feasible at reasonable cost with the average residential house.
And that needs more open ground that is available with most houses.
And that doesn't come cheap and isnt even feasible with some houses, its not possible to get the drilling rig where it needs to be. They are big truck mounted things to get the depth you need.
Because of the cost and the difficulty of access for the truck.
Because heatpumps are all about meeting stupid government targets at minimum cost and maximum profit, not about industrial strenght solutoins. And most people dont have big gardens
Because its around ten times more expensive. The extra energy for a heatpump doesn't come from geothermal energy seeping up from below It comes from summer sunlight beating down at about 1KW per sq meter.
The less square meters, the less energy. Boreholes will work for a while, as the ground around them slowly cools, but in the end thy will suck all the energy out and it wont be replaced.
It's the same for many geothermal projects like e,g. down coal mines etc. After a decade or two they stop working.
There was a spate of grumbles in the farming press some years back when it became apparent that soil thermal conductivity is an issue with
*slinky* type arrays.
I'm no hydrologist but I would expect boreholes in a river valley to find the water moves albeit slowly. A bromate pollution issue near here has moved about 5 miles in 40 or so years. I was pleased to sell some land so an abstraction borehole could be installed *upstream* of the original source.
What about mixing it with those water heating panels on rooves? I have a friend with those, and apart from cleaning them every so often it has reduces the leccy bill a lot. Its been in some years now. Brian
Unless the flow is reversed during the summer perhaps. The alternative energy place in North Wales tried to make a seasonal heat store out of waste slate in a hole in the ground.
Also ground water flow may replenish some as IIRC from SEWTHA some
40W/m2 flows from the core
Cost makes ground source unattractive compared with air source and passive house looks better investment than either.
I think that depends very much on the house in question - including its location - unless you can DIY a lot of the work. Retrofitting a Victorian terrace to passive house standard costs an arm and a leg; and in London the full Nelson (and probably a bollock or tit too). A nice example in N London was £230,000 (£2,000 per sq m) in 2016.
I don't see that as an issue with heat recovery ventilation but it does seem impractical to apply to an existing building.
MHRV with a built in heat pump maybe??
There is no way I can approach the necessary reduction in heat loss in my tiny semidetached victorian house so heat pumps for winter heat are out but maybe feasible for DHW 7 months of the year.
If it's airtight then MVHR will do you. If it's not air sealed then you're getting fresh air exchange anyway.
They tend to be quite small in terms of kW - only really useful if it's sufficiently airtight and well insulated.
Do you know what your heat losses are? Domestic heatpumps go into the 20kW+ territory, and commercial ones are bigger. If your house is small I'd have thought a domestic HP would be fine, even if the house is poorly insulated.
(note the size of your boiler is not a good guide to your heat losses, you need to do the calculations. There are spreadsheets around that will do them if you input your wall dimensions and construction)
Yes that's what I thought as it will only be taking heat from the air that it vents from the house. In a well sealed house this will only be the required air changes.
As it would have to be ASHP it may be worth considering for the autumn and spring, I suppose a split unit could also utilise the coil in DHW tank to pre warm it to 40C, using the existing immersion to boost it to above 60C.
Its hard to quantify my heating requirement as I am woodfired apart from DHW. Utility bills less than £600/annum. A guestimate from the volume of wood I use is about 8MWh/annum.
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