"Sue Begg" wrote in message
...

What we had been initially thinking of
was a geothermal system with the
windmill to help drive the pump running
the geothermal. The windmill
that was recommended plugs into the
household mains and feeds any excess
back into the grid. (I can't see there being
much excess !! )
We had solar panels on the old house
for 25 years but the amount of heat
this house will need. Geothermal seemed
the way to go.

A whole full solar roof is far different from a panel or two and may cost
less than the 8k for a geothermal heat pump, which you have to pay to run. A
solar roof is just the cost of running a pump. A windmill to give 3kW
output to run the heat pump will be not be cheap either. At best a heat
pump can compete with a natural gas boiler in running costs. You may, if
lucky, drop the running cost by 1/3 by using the windmill. Still overall an
expensive setup using a windmill/heat pump.

A heat pump may have a COP of say 3. That is for every kW you use it outputs
3kW. But in winter you will freeze, or make the ground very cold, so less
heat is available to pump into the house. Then the COP drops. When the
ground gets very cold around the bore pipe, the heat pump may not be capable
of raising the water to DHW temperatures. For example, a heat pump with a
COP of 4 takes 1 unit of electrical energy and pulls 3 units of heat energy
from the ground, water, etc, to supply 4 units of heat energy where needed.
If the heat pump cannot pull the heat energy from somewhere, the COP drops
back to 1.

Then expensive to run electric immersions have to be used, as well aslso
with a COP of 1.

This thread has thrown up alternatives and appear to be more economical
options:

1. A full solar roof heating a thermal store that provides UFH and DHW. You
may need a backup immersion in the thermal store for cloudy days.

2. A directly connected windmill constantly heating a large thermal store.
There again some backup heating on windless days, but the thermal store
sized large enough may hold enough energy to keep the house warm enough
until the wind picks up. An immersion may be required for the hotter DHW.

If I was you I would reassess your proposed heating system.

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