Peter Smithson wrote in
:

One pair of pipes gets hot and heads off to the hot
water system. The other pair don't get hot and are linked to the CH
system. So I thought they were seperate. Is that possible?


I am no expert, but in my recent studies I came across this

http://www.gasman.fsbusiness.co.uk/g...ed_heating.htm

which looks like what you are describing.

If it is, then you need, like everybody has said, to look at the pump.

HTH

mike r