Depends on system.
If combi no they will not work at the same time, this is nature of the beast m'afraid.
If this is a system boiler or heat-only boiler, you may have an S or a Y plan system. If you have an S plan ( 2x 2-port zone valves) you can only do heating OR hot water, with a Y-plan (1x 3-port valve that will go mid-position) then you can have both at once.
BunSlinger
November 2008
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