"Andrew Gabriel" wrote in message
...
In article et,
"Dave Liquorice" writes:
On Wed, 11 Feb 2009 16:57:46 +0000, Richard A Downing wrote:

As you probably know it gets quite cold here and I continually plot
ideas to save heating oil - I used 1000l since the beginning of
November.

Only 1000? We've got through nearly 2000 since the end of November. Will
be due to order another 2000 in about 10 days time...

But is there a simple way of using the waste heat in the grey-water for
heating (aside from letting it go cold in the bath tub before pulling
the plug)?

Even if you can effectively recover it what are you going to do with such
"low grade" heat? Any medium you recover the heat into is only going to
be
around the upper 30's C at best. Too cool even for under floor heating.

How much actual energy is there in a bath full of water? Is it worth
recovering given that it is such low grade?

You could just leave it to cool down before pulling the plug.
That way, the heat has passed into the house. The moisture it
generates by evaporation might not be welcome though. Could
cover the top to prevent evaporation, but then it will take
much longer to cool.

If you're into DIY refrigeration, another option would be to
pump the heat from the bath back into the hot water cylinder.
You could affix evaporater pipework to the back of a (metal)
bath, and have a condenser coil in the bottom of your hot water
cylinder. Then you cool or even chill the bath water, dumping
the heat back into the hot water cylinder. Almost certainly
not worth it, but might be good fun. ;-)

I've thought about recovering the heat from a shower. That's
easier because you can just construct a reverse flow heat
exchanger to transfer the waste heat to the hot water cylinder
inlet flow, as they'd both be flowing at the same time.

See my post on the gfx. If can be DIYed by having a pipe in pipe and reverse
flow.