There are a couple of significant heating ideas already featured on this site. One, the hybrid combined air/ground source heat pump, where a ground source pump has a connected tank warmed by the ambient air. And the other where big water tanks store cheap off-peak energy.
The first concept produces astonishing coefficients of performance and the second produces astonishingly low bills.
For the Grand Designs Heating System, we’ll combine these concepts and serve them up as a benchmark for what is possible. Here we go.
IVT E11 ground source heat pump 10kW
Running largely at night and avoiding all the cold air and defrosting malarky that would plague an ASHP. a ground source pump is the starting point. GSHPs produce slightly better seasonable COPs than ASHPs but we can raise the COP even more by putting extra energy back into the ground.
This chart compares the COP of a GSHP (black line) with the COP variation of an ASHP over the winter months. We can ignore that horrible blue line and focus on the divergence of the red line (warm air days) with the declining black line for the GSHP. Our mission is to grab some of the day time warmth and add it to the mix and thus flatten the black line to a COP of around 4. We do this with a tank and some air to water heat exchangers.
The outlet flow from the heat pump is typically around 0c or colder so the ambient air is nearly always warmer, especially during the daytime. The heat in the air is captured with three car type radiators and fans and stored in a large buffer tank. This water goes to the ground loops whenever the heat pump and its circulation pump runs. The design power of the radiator/fan combination is roughly equal to the heat pump to try to keep the ground temperature from depleting. There will be times when the tank is actually warm enough to feed energy back into the ground. Imagine a nice sunny day when the tank has been independently spooling up to 12c or so and the heat pump starts up and dumps 1,000 litres through the ground loops. This won’t happen often but the ground loops will rarely be fed temperatures below zero as is typical with most installations. The overall result is that we take a system that is intrinsically very good and make it much better. A higher COP gives much lower bills. That’s a good starting point but we can improve a lot more on that.
Concept number two:
Off-peak electricity can be had at night, for the car charging brigade, and when that is multiplied up by a heat pump the result is astonishingly cheap energy. A 10kW heat pump running for 7 hours nightly over the whole 200 days of winter delivers 14,000kW.hrs for about £260. Crazy but true! The maths says it all; 7.5p for1 kW.hr on a COP of 4 makes 4kW.hrs – divide 7.5 by 4 and you get 1.88p per kW hour. Compare that with electricity which costs around 24p/kW.hr. The difference is astonishing but that cheap energy has to be stored ready for the next day, hence some tanks. Two 1,000 litre tanks combine to make 2,000 litres for the heating storage. N.B To remove that energy from the tanks we need to get the tank temperature back down to 30c or less and that will require, at least some, underfloor heating.
The stored energy might be enough to get through most days without any additional heat at all. If there is a shortfall any day time running will be much more expensive but the solar panels can help to run the GSHP fairly cheaply (or free) and that high COP makes a big difference.
Naysayers will now be saying that buffer tanks are inefficient or that heat pumps should run 24/7 or that zoning does not sit well with heat pumps. They are right on all counts but efficiency is not the point; we are using one third priced electricity which easily trumps any minor gains elsewhere.
The tanks – all from OSO
The tech room will look rather impressive with three 1,000 litre tanks and another smaller one. One will be for the air side of the hybrid heat pump and the other two for heat storage. The smaller tank is for domestic hot water. (300 litres with a 3msq coil for heat pump compatibility).
EDDI solar diverter
You could argue that the GSHP can produce hot water cheaper than a direct electric heater – the immersion – but the EDDI picks off solar excesses in short bursts during the day and runs the tank up to much higher temperatures. The heat pump will be all the better for not firing up all the time, the legionella will get regularly fried, and the tank size is effectively bigger; wins all round.
Willis remote immersion heater
I like these because the circulation past the immersion element keeps the thermostat from shutting down under its own heat. Tank fitted immersion heaters can short cycle frequently and are harder to service too.
Transmission
Towel rails, fan coils in bedrooms, under-floor downstairs and in bathrooms. All independent of the heat pump and fully timed and zoned. The big tanks make this possible and simple too.
ESBE blender unit
This blends down the big tanks to suit the under-floor pipes and it does weather compensation too. The towel rails and fan-coil units are directly fed by pump(s) and timers.
Mini-split
On warm days an air-to-air heat pump will be more efficient than the GSHP and it will be useful for topping up especially if it’s lower powered and often running free off solar. The blown warm air makes a useful laundry drier and the cooling feature sorts out the need for air conditioning. Cold air pours across the floors making a single source surprisingly effective.
Solar panels + Enphase IQ8 micro-inverters
Each panel will have its own micro inverter for long term reliability, performance and also power if the grid fails – you know, in a Zombie Apocalypse scenario. At least 18 panels (about 7kWp) would often keep either heat pump running during the day. The GSHP draws just over 2kW and the mini-split just under 2kW so the panels should have that covered.
Electric car
It’s hard to get cheap off-peak electricity combined with a decent export rate so the car makes a good soak for any excess. You no longer buy petrol so that’s just as good as any export payments. N.B. There are no domestic batteries in our system – the money is better spent elsewhere. When the technology matures the car will be the battery anyway.
Solar tech room
All those radiators and the mini split would be neat and more efficient in a dedicated solar shed. If possible, on the roof would be good. Apart from being neat and tidy the solar side helps the air powered mini-split to avoid defrosting cycles.
Conclusion
If you want your Grand Designs house to stand out and be the best of the best, this could be the way to go. What do you think Kevin?
BTW If you own a castle or something and this looks a bit light on, then check out the meaty version on
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