A few things can cause delays in getting hot water to the taps.
Distance from boiler to taps is the most common. The response time between turning the tap on and the boiler firing up is important. It should only take a few seconds. If it's taking longer the flow sensor (whatever type is fitted) may need attention.
When the heating isn't on the boiler has to respond to the call, fire-up, circulate the heat around the boiler and heat the water to the taps.
Has it just started to do this ? Does it do it every summer?
T.h.s, May 2007
