In article ,
David Hansen writes:

Running a 15kW boiler to heat some pipework, floorboards, a hot
water cylinder and a towel rail is not particularly efficient,
compared to 100% efficient immersion heaters.

If you're going to do that sort of comparison, you need to take
into account that the electricity as delivered to you is already
barely 40% efficient, so an immersion heater can't be any better
than that.

However, a lot depends
on the duty cycle and electricity tariff. Modulating boilers also
add to the difficulties of calculating what is best.

However, just to run a towel rail, a 120W - 240W immersion heater in
an oil filled towel rail is probably going to be a better bet than a
boiler that can modulate down to 5kW driving a water filled towel
rail.

If you happen to have a thermal store, that would be ideal to
drive a towel rail from. Personally, I think heated towel rails
are very much over rated -- I find an unheated towel rail a few
feet above the radiator more effective.

--
Andrew Gabriel