Why does boiler need minimal demand before heating water?

Question

If I turn my kitchen tap on hot to a gentle stream, the boiler doesn't turn on to heat the water at all. It will stay cold forever. It's only when I unleash a jet wash torrent of gushing water out of the tap that the boiler finally turns on and begins making the water hot. If I turn the tap pressure down below a certain threshold after it's heated up, it will turn cold again even though it's still running. Why is this?

Answer 1

Some instant or on-demand water heaters have the ability to vary their firing rate according to the amount of water flowing through. In other words, they can modulate the amount of gas they burn or the amount of electricity they pass to a heating element. This allows the heater to approximately maintain a set discharge temperature across a range of flow rates (gallons or liters per minute). Others do not have this ability; they're either full-off or full-on. This latter kind gives hotter water when the flow is low and cooler water when the flow is high.

The variable-firing heaters aren't perfect, though. They switch on with some minimum amount of gas burning (or electricity flowing). While in theory we could engineer a heater to be continuously variable all the way down to zero, in practice that would definitely increase cost and would likely reduce reliability.

Both the variable and the fixed-firing rate heaters have a "minimum flow" threshold: they require a certain volume of water to be flowing before they'll turn on any heat at all. This is a protection for both the heater itself and for the people who will use the hot water. The minimum flow guarantees that the heater will not activate when there is zero flow, potentially resulting in a steam explosion. It also guarantees that the heater will not activate when the flow is too low for the minimum firing rate, which would release excessively hot water with the potential to scald or burn the user.