GCH/Water problem

Firstly, apologies if this has been asked before and/or I'm asking in the wrong group.
Anyway, I'd be grateful if there's anyone who could tell me why my hot water temperature is lower when I have my central heating on than if not. I recently had some rewiring and as far as I can remember before the wiring it was the same temperature whether I had the heating on or not. However, the thermostat, boiler, pump etc all appear do the right things at the right time (ie pump comes on when thermostat tells it to, boiler comes on when requested, etc). Maybe its to be expected if both are on - the boiler can only do so much ?
Any help gratefully received.
Charlie Mustard
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Charlie Mustard wrote:

Here are a couple of possibilities. 1) The HW cylinder is not getting as much heat from the boiler when the CH is on and so is not heating up as quickly. - This is a balancing issue between the radiators and the cylinder.
2) The boiler is so under powered it is not gettting enough to its own internal target temperature when the CH is on.
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Ed Sirett - Property maintainer and registered gas fitter.
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You'll need to tell us what boiler you have. They are very different in the way they modulate their flow temperatures.
Christian.
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