Lunny wrote:
Supposing that the BTU calc for the rads comes to about 50,000
(calculated at -3 degrees outside and with an extra 20% to be on the
safe side) and domestic HW is required, would the boiler below be a
good choice?


http://www.worcesterbosch.co.uk/inde...&con_id=125930

88,000 BTU max output, fully modulating etc, etc...

Cheers,
Matt


Sizing boilers and radiators on heat loss is not always such a good idea.

The peak output determines how FAST the place will heat up..

Then, providing you have proper thermostats, the heat loss will only
determine the duty cycle of the boiler.

And provided that you have a bit of hysteresis on the stats, that will
prevent short cycling.

So efficiency of heating is not really a function of the boiler or
radiator SIZE.

They just need to be big enough to heat the place up quickly enough.

If you are at home all day in the winter..you may run the heating more,
and need less peak output..OTOH if you leave the house empty in the day,
you should go for a bit more radiator size and a bit more of a boiler.

By the sound of it, something around 25KW for the boiler, and about 4KW
in the larger rooms and less in the smaller sounds fairly sound.

Pay more attention to thermostats and controls than to rad and boiler
sizing.

I think that if I were to do a rad installation again, I'd go for an
electrical stat in every room, with a motorized valve for every room,
and connect the switches all in series to control the boiler - no master
stat.. :-)