Radiators - where and size?

Bendy_House

Deedoodee said:

So it’s a balance of ugly large rad sizes against higher output.

Well, more a balance of ugly large rads vs less energy efficiency.

You should get the 'same' heat out from the large rad running on cooler water, as a smaller rad supplied by hot water. But having the boiler delivering the cooler water will make it obtain more energy from the burnt gases, and not send it out the flue.

ComicGeek

Water temps for heating circuits are based on worst case design conditions, ie heating up a cold house at sub zero outside temps with no internal heat gains etc. Older heating systems may have been installed before new double glazing, loft insulation etc was installed, so based on higher than actual heat demand. Also may not have calculated heat demand accurately and just installed a much larger rad to fit nicely in an area, or bulk order across multiple rooms.

Existing rads therefore may already have significant design margin in them. I had 1 room undersized by 20%, but the others were 30-40% too big anyway. 

I also work on the basis that when I eventually fully switch from gas to heat pump, it will be cheaper to plug in an electric 3kW heater for a few hours a year than change all my radiators. Or raise the heat pump temp slightly on those colder days, knowing that it will reduce efficiency. But definitely not spend money on huge radiators that just get in the way.

Bendy_House

Good points, CG.

I would certainly only recommend going 'large' at this stage if the owner had to replace the rad(s) in any case. The potential slight gains from each rad will take a while to recoup from the ~£200 cost of each rad!

Deedoodee

On more careful inspection, the bedroom radiators old terrible and if the output estimator is correct, they’re producing 0.5-0.75 what they ‘should be’ so we’re looking at quotes to replace and upsize in line with downstairs.