Relative costs of mains Gas/LPG/Oil

[IMM]

If you having ground loop then it is best to ditch the air sourced heat pump and go for a full ground source loop, rather than a half cocked affair. Then to warm the ground loop, using purchased energy can only be inefficient. You then would be heating the earth around the loop. Much of this heat, depending on earth temp, may just convect away, and may never be re-extracted when needed.

You could have a "large" water/glycol filled pipe loop, using a pump to circulate. This, when buried deep enough will extract useful heat that may be used to re-heat, or heat, a cylinder, or thermal store. Only using a pump. The size of the loop would have to be calculated, depending the temperature at certain depths and the local ground conditions.

[N. Thornton]

In principle one could use air temp to warm the loop at times, or add a solar panel to the loop. Adding solar like this, after the water comes out the ground and before the heat pump, would presumably help the heat pump efficiency by raising the source temp some of the time. Also I would imagine that the extra heat input might enable a slightly smaller ground loop to be used.

Bear in mind solar panels can be made very cheaply with a metal frame, twinwall plastic front (or just glass if you have a divertor valve & thermostat) and microbore pipe, and a bit of rockwool round the edges.

Or for the more daring one can use nothing more than a painted house radiator wrapped in polythene - not very efficient but a very cheap and simple way to capture a bit of free heat. Bare rads have been used before as preheat panels with low cost and low temp rise.

Finally the ground loop can provide a bit of cooling in summer too.

Regards, NT

[Pete C]

Hi IMM,

I never meant that the ground should be heated using purchased energy, only that is should be heated from the air when the air temperature allows.

I think adding a solar water heater would increase and complicate things quite a bit!

This is how it could work:

1) Above 0°C use heat from the air. 2) Below 0°C use heat from the ground loop. 3) When the air temperature is above the ground loop, heat the ground loop from the air.

Since a standard outdoor unit already has a finned coil for air heat exchange, this could also be used for ground loop heat exchange by stopping the fan and pumping water from the ground loop over it. Also the outdoor unit could be used for step 3) when it is running normally.

I would expect that a standard outdoor unit and a small ground is far cheaper than having a specialised system with a full ground loop.

cheers, Pete.

[IMM]

Air to air are inefficient near to 0C, they cannot provide enough heat for DHW, only acrting as a pre-heat.

You will be heating the earth around the loop, and much will escape. This cots energy in heat pump and.or water pump.

For the cost involved, you may as well convert a complete near south facing roof to a solar panel. Then store the water in a large thermal store and use low temp under floor heating (temps will be low in winter, but enough for UFH).

[Pete C]

Hi,

The high efficiency ones would be about 15% less efficient at 0°C. With a COP of 6 to 6.5 that's still pretty good

They are not really intended for DHW, for that electricity could still be used. Though heat pumps with UFH capability are coming available.

There is no need to use a heat pump for this, and a water pump would only be about 20-50W. The main purpose would be to top up the ground source after it had been used during a cold spell.

How much would this cost and how would it cope when the sky is overcast?

cheers, Pete.

[IMM]

In future homes DHW will consume more energy than CH. Having DHW run off peak electricty which is 3 to 4 times more expensive than gas is not the way.

If you had a cold spell and heat was extracted from the ground cooling the ground around the loop, then when the air warms, you are saying use a pump to run water though a radiator, extract heat from the air and into the ground. If the air is less than the ambient ground temp this is useless. It is best to leave the ground and allow the ground to warm up by convection from the warmer surrounding earth.

In the USA it is common to have foam insulation under the ground, in an umbrella formation around the house. The highest point would be the house. The upper part of the ground is the part which is affected most by the ambient air temp above. Then the convected heat is gained from the warmer earth beneath.

It is best to have two ground loops. One operational and one recovering, and switch between the two.

Probably not as much as an expesove to run heat pump that is high in capital cost.

That is what the large thermal store is for. Store enough heat to last 3 or

4 days. Or have a battery of them. A very large collector can produce heat from even a cloudy day. Low temp heat, but probably enough to run a UFH system in very well insulated house. Very cheap to run.

[Pete C]

I did say:

cheers, Pete.

[G&M]

Still struggling to see how these reach that COP figure as the laws of thermodynamics tend to get in the way. Talked to a friend who's more into physics and he's asking Hitachi for more information.

Though his first reply was along the lines of audio amplifiers in that it was "COP music power"

[Pete C]

Hi,

AFAIK the theoretical limit is still some way off, give it 5 years or so and a COP of 8 might be possible.

Still it's a better than only getting quoted only BTUs, or possibly even watts as well if your lucky.

cheers, Pete.