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Mogweed wrote:
I have a Baxi Bermuda 552 back boiler which gives pumped central
heating and
(what I believe is termed) gravity hot water, ie, when system is set
to hot
water only, the upstairs rads get warm as well.

no, thats called a plumbing ---up. Your upstairs rads are on the hot
water circuit.

More likely that the upstairs rads are getting gravity circulation when
the pump is off.

--
Roger