radiator heating is controlled from the (room-stat) if this set at say 16*none of the rooms are not going to heat up MORE THAN this even if the TRV are FULLY wide open.With the TRV wide open you looking for about 23+ on the room stat ALONG WITH WEARING SHORTS till the bill comes in
As for what the fitter said Combi's will GIVE priority to hot water, over central heating.
Warm water upstairs if boiler downstair could be the result of a long dead leg in the water pipes, this has to run off/drawn off, (before hot water)arrives at the tap if only washing hands insufficient water will have been drawn at the wash basin.
wirecutterman
December 2009