How do Combi-Boilers deal with variable water flow rates?

[LoungeLizard]

Suppose that someone is running a hot tap that is supplied with its hot water from a Combi-Boiler.

The tap is only about a third open; and water pours out at the pre-set temperature on the boiler.

The tap is then opened fully: and a lot more water pours out at the pre-set temperature on the boiler.

My question is: How does the the Combi-Boiler vary its heat-output to suit the different flow rates?

Does it, for example, have an infinitely variable gas control valve? Or does it have a sequence of on/off gas control valves? Or does it have a very big loop of pipe inside so that there's a lot of residence time to adjust the temperature with a single on/off gas control valve (switching on and off rapidly to suit)?

[pilotlight]

LoungeLizard, You are not too far off the mark. Combi boilers " modulate". This means the gas valve varies the volume of gas it allows to feed the burner. i.e. if the hot water tap is fully open ,the gas valve will allow the correct amount of gas to the burner to provide the temperature selected,should the water tap the be restricted from being fully open, the gas valve is " informed " by various components in the boiler that the volume of gas required is less, and restricts the gas flow, so -- modulating.

Hope this helps.