On 09/03/16 18:56, www.GymRatZ.co.uk wrote:
On 09/03/2016 17:47, Tim Watts wrote:

Wouldn't an over-sized radiator system make the boiler cycle more than a
system where the radiators are perfectly sized for all but the rare week
or 2 every 10 years or more when temperatures go into minus
double-digits?

I wouldn't have thought so - in that the boiler will at least not be
cycling on the water stat, only the room stat(s) - and the latter will
have quite a big hysteresis.

Don't all boilers modulate both pump and burner these days?


Within limits, yes. I only really know about Viessmann and WB as those
were the two I was reading teh spec sheets on.

My old boiler uses "V.C.O." (variable controlled output)
I'd have thought it was commonplace these days as it's got to be closing
in on 10 years old now.

Nominal Output To Heating 50/30C 30.7kW
Minimum Output To Heating 50/30C 5.1kW

80/60 F/R changes to anything between 28.4kW and 4.5kW


That is really low - but that will still require some well sized rads if
only a couple of them are demanding heat.

Hot water is variable between 4.8-36.0kW

Nice top limit