Condensing boilers ought to be run at an average temperature of 55°C to get the full benefit of condensing efficiency (bold simplification). However BS EN422-1997 rates radiators at an average temperature of 70°C. So, instead of a 50°C mean difference above 20°C room temperature, we only have 35°C. That implies that radiators could be 50% bigger for a condensing boiler than a 'conventional' boiler. Radiators cost about £60/kW at 70°C, so this rises to £90/kW at 55°C. For a fully loaded 25kW boiler, this could cost an extra £750 on radiators to save 8% at most on fuel bills.
Yes, OK, this is extreme. There will be some savings from a condensing boiler without increasing radiator sizes. However, this makes the efficiency savings from larger radiators even less effective.
I think these sums imply that it might not be worth increasing radiator sizes just to improve condensing efficiency. Some savings are possible with good boiler controls (compared to running the boiler at 80°C all the time).
Yes, OK, this is extreme. There will be some savings from a condensing boiler without increasing radiator sizes. However, this makes the efficiency savings from larger radiators even less effective.
I think these sums imply that it might not be worth increasing radiator sizes just to improve condensing efficiency. Some savings are possible with good boiler controls (compared to running the boiler at 80°C all the time).
