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In article ,
(Mark Begbie) writes:
I am now leaning toward overrating the rads so that the total is
closer to the boiler output (18kW) than the "required" value of 6kW
that the online radiator calculators come out with. Probably put in
about 12kW worth of radiator area.

Not clear if this includes downrating the rads for lower temperature
rating. In my case, I had to downrate the rads to 60% of their data
sheet ratings for condensing boiler operation down to -3C outside.
Some manufacturers overrate even worse and their rads will need
downrating by 50%. That gives you your 6kW output from 12kW of rads.
On the other hand, there no necessity to have the boiler still in
the condensing region of operation when outside temperature are
down to -3C.

Something else this enables you to do is to avoid predictive heating
using a timeswitch, where in many cases for me at least, the heating
would end up coming on ages before I arrived home. By being able to
dump far more heat into the house than it needs to compensate for
losses, you can do a damn fast bringing up to temperature. I now
turn my heating on when I get in, and the warmup time is so fast
I just don't notice it. (If it's really cold, I can ring up the
home automation system and have it switch on the heating and/or
hot water before I get home, or I can do it as I leave the office
via the Internet;-)

--
Andrew Gabriel