I have a Gas Central heating Boiler that heats the central heating, and also a separate flow for the hot water...if that is the correct term. When my Gas Boiler is fired up and the Central Heating and heating the Hot Water are both on (ie neither Hot Water or central Heating thermostat has turned them off), does it use more gas for both than just if it was heating the central heating ONLY?
When you have the heating switched on the boiler cycles, it fires and cuts out and keeps the heating temperature to that set on the thermostat. If you turn a hot tap the boiler fires regardless of whether it is firing for heating or not and diverts the heat to supply the hot water. When supplying the hot water is over the boiler has to keep fired to bring the heating back up to to temperature effectively playing catch up. Some boilers have a three port system where they can do both at the same time.
It follows that if you are asking about gas consumption at a particular moment in time when heating the hot water then the boiler will be using exactly the same if the boiler was fired at the moment you turned the hot tap but would be more if the boiler happened to be at rest. Either way, the boiler has to heat the additional hot water supply so it will be playing catch up on the heating and so the gas consumed over a period will obviously be more. Overall, more heat equals more gas.
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