My system has just started to heat downstairs radiators also when I want hot water only.The red light for the downstairs heating is not illuminated,only the hot water one is!!
that is a motorised valve failing, most likely a three port valve, its most often in the airing cupboard above the pump, if you can buy an identical replacement it may save you a bit of money if you call an independant tradesman in
john
January 2010
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