"David Hill" wrote in message
...

I seem to remember talk of having bottles of water with washing soda in
solution in them, the idea being that when the temp dropped to below 50
then
the soda would crystallize giving off heat, and when the water was over
about 60 then the soda would absorb heat as it went back into solution.
Now have this in tanks with some sort of heat exchanger taking heat from
the
top of the house in the day and heating the tanks.........

It's called solar heating with heat storage. Or if not using heat from the
sun heat pump systems can take it from the environment directly.

--

Chris Thomas
West Cork
Ireland