Trying to work out existing KW output from radiators

[Simon Hawthorne]

Hi all I'm having a new CH system fitted next week - and the engineer has specced up some rads, based on heatloss etc. I've been trying to calculate the current KW output from the existing rads - but have been unable to find any similar rads anywhere. The closest are Myson roundtop - but all the current ones have convector fins. I worked out they seem to give off 0.15 watts per 1cm2 - so I've used this to guess the output of mine. I am sure this isn't right - so my questions is, what % are current roundtop convector rads better than old style non convector rads... Make sense...? I just want to be sure we have the right sizes for the new ones - engineer is happy - but I just want to be sure - and I know someone on here will know the answer.....! Thanks in advance Simon

[Andy Hall]

If you look at the various datasheets, you can see that those with fins have increased output. Most of the heat ouput from "radiators" is really convection anyway. Adding the fins increases the surface area with air able to flow past it, hence more output.

Take a look at other manufacturers like Barlo and Stelrad and you should find some non-fin radiators. You can make a reasonable estimate from these by scaling from the height width and number of panels.

If you measure the existing radiators and get the sizes for the new ones and scale them together, then you can compare with the equivalent ratios taken from datasheets.

[Simon Hawthorne]

Thanks Andy. I actually found a really old post on uk.d-i-y and figured out a non convector single rad outputs approx 0.12 watts per

1cm2 compared to about 0.15 watts for a convector. I just wanted to be sure that the new rads were more powerful than the existing - even though I've done the 'heatloss' calcs. The room in question will increase from about 3.5kw to 4.5kw - and I reckon I need 4.1kw - so all looks good (so far!)

Thanks again

Simon

[Set Square]

Maybe!

Don't forget that radiator heat outputs are quoted for a specified Delta-T (difference between mean rad and room temperature) which is often higher than your actual value - and thus need to be downrated somewhat.

For example, suppose you have flow and return temperatures of 80 and 70 degC (mean 75) and a room temperature of 20 degC. Delta-T is thus 55. If the output of your rads is quoted at a Delta-T of 60 (which many are), you'll only achieve about 90% of that - so your "4.5kW" rad is marginal against a requirement of 4.1kW.

[Simon Hawthorne]

Now you've got me worried. It is a big room that takes an age to heat, but once warm, stays warm. I wanted 'bigger' rads to enable a quicker walm-up...

Might need to speak to the installer....

Thanks for your help!

Si

[Andy Hall]

If that's the situation, then it's worth oversizing the radiator, or even better having more than one if you don't mind the wall space issue. The TRVs will back them off when the room is warm.

This is a way of dealing with a situation of wanting some rooms warm more quickly than others if you want to achieve that.