Another boiler Q again

A fully modulating boiler is one that can vary the heat output according to demand. A combination boiler is one that incorporates a hot water system to supply taps. Most combi boilers are of the instantaneous type.

It is essential for sane operation that such instantaneous combis are fully modulating. Otherwise, turning the tap down will cause the temperature to rise until the boiler decides it needs to cut out, turning the water cold. A fully modulating type will just turn down the gas to keep the temperature at the set point.

Christian.

Not quite. Some do. Some modulate down to maintain a flow temperature stetpoint, which may not be quite in synch with demand.

Thanks for the simple explanation, the concept is at least understood now.

I have finally received the installation ans servicing manual from Vaillant and in it, although there is nothing explicit which says that it is fully modulating, there is a section for fault finding of which 1 sheet is headed "check burner modulation" would this indicate that this boiler is fully modulated, or am I way off the mark

Thanks for the replies.

Regards Gaffar

I'm nigh on certain that all Vaillant combis are modulating. I think there may be some very early Vokeras which aren't.

-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at