Radiator Sizing

[DursleyDonkey]

I've been recommended this little tool for calculating radiator sizing http://www.kermi.co.uk/EN/Information/Computer_Solutions/index.phtml

It looks like a fairly sharp tool - and you know what they say about sharp tools in the hands of idiots - but my question is this:
If I can work out what the requirements are for each room, how do I know the output of the old radiators I have? They're about 30 years old as far as I know - is there a formula to calculate it from the radiator dimensions? Or do TRVs make this all irrelevant?

 

[D_Hailsham]

I've been recommended this little tool for calculating radiator sizing http://www.kermi.co.uk/EN/Information/Computer_Solutions/index.phtml

I have tried the Kermi Calculator and found the Barlo Heat Loss Calculator easier to use.

If I can work out what the requirements are for each room, how do I know the output of the old radiators I have? They're about 30 years old as far as I know

Doe it really matter what the output of your old rads was? I doubt if they were sized in any scientific manner; a lot of sizing was done by rule of thumb in those days.

do TRVs make this all irrelevant?

NO!
The TRV is nothing more than a switch, which stops water flowing through the radiator when the room reaches the desired temperature. If the radiator is too small, the room will never reach temp; and, if it is too big, the TRV will be closed most of the time.

Which boiler do you have? Many condensing boilers work best when the temperature difference between flow and return is about 20C. Radiator calculators usually assume a difference of 10C. If you boiler says 20C, you will need to install rads which are 20% larger than suggested by the calculator.

 

[DursleyDonkey]

Thanks for the useful information Mr Hailsham. The reason I want to work out the size of the old radiators is that I don't want to replace them yet unless they are way off - which I suspect some of them are.

The boiler is (will be when we get to the front of the installer's queue) a Vaillant 937.

On the subject of TRVs, I had always supposed that they gradually restrict the flow of water as the room warms up - effectively reducing the capacity of the radiator. I didn't know they were just on or off. Thank you for clarifying.

 

[D_Hailsham]

The reason I want to work out the size of the old radiators is that I don't want to replace them yet unless they are way off.

If they are flat panel steel rads, not cast iron type, the info in the Stelrad Elite Catalogue will give you a very good idea of their outputs.

The boiler ... will be ... a Vaillant 937.

This works best with a 20C differential, so you need to oversize rads by 20%

TRVs, I had always supposed that they gradually restrict the flow of water as the room warms up - effectively reducing the capacity of the radiator. I didn't know they were just on or off.

There is an element of restriction, due to the time it takes to go from fully open to closed; but they are, effectively, on/off devices.

You will still need to balance your rads.