Updating central heating system

[Colin Richmond]

I am going to replace the existing radiator valves with TRVs. Should those on the ground floor have a drain off facility? Also, can TRVs be fitted to all radiators or should one be left without a TRV?
I am also going to fit one additional small radiator. I am told that there should be no more than three radiators from a 22mm feed. Is this correct or can I add the new radiator by extending the 15mm pipework?

 

[Best]

You haven't given enough information, so it is just guesswork to answer.
You might be better to get a heating engineer to check it all out.
Some systems require one radiator to be on. There are also open vented systems that are designed badly and will pump over if a lot of radiators shut off.
Whoever said about 22mm pipes can only do 3 rads is wrong. It depends on output of the supplied rads, but even 15mm pipes on a 2 pipe system generally will be okay for 2 large radiators max.
If your rads are on drops, then they are better to have draincocks on tee offs or fit lockshields which incorporate draincocks

 

[leesparkykent]

The area that has the room thermostat (if you have one) shouldn't really have a TRV fitted to that particular Rad.