2 cold radiators in the lounge

[verabc]

Hi, i have 2 radiators that stay cold when the central heating is on. They don't need bleeding (done that). They work fine when all other radiators are off, but when i turn all of the rads on, these 2 stay cold.
What can it be? The pump is on the maximum.
Thanks

[pilotlight]

Your system probably needs 'balancing'. You could possibly do this yourself, by adjusting the lockshield valves ( the valves on the other side of the radiator to the ones you normally use ).
Turn on the radiators that don't get warm. Pull the plastic caps off the lockshield valves on the radiators that do get warm and turn the spindles down, clockwise until you can feel that it has closed. then open it back up again, but only a couple of full turns ( you will probably need a small spanner ). After you have done this you should find the other two radiators warming up.
If you find that any of the other radiators don't now warm up, open the lockshield valve a little more.
If you don't want to carry this out, or it doesn't work, You will need a heating engineer to do this for you. He should make sure this is done correctly i.e. so that there is a 10 degree difference between the flow and return pipes.
Hope this helps.

[chas2008]

could just be sludge in them, when the other rads are off it just gives it that extra pressure to force the water around. Assuming the house has been plumbed properly the rads are not connected so one runs into the other they are all independant so if one clogs up the others carry on working it only needs a small lump of sludge/crud to do this.Plumbers love power flushing systems but it can be expensive, the other alternative is to take the rad off and flush them through with a hose pipe.

http://www.lowepricegas.com/installation.html

[The Builder]

Have they all worked OK together in the past Vera?
Also, can you say what the (approx) size of the pipework to the rads is (diameter)?

[verabc]

thank you for the suggestions.
"The Builder", we have moved into this house only in summer so can't tell whether they worked properly before or not :(

[verabc]

oh and the pipes are about half an inch in diameter

[The Builder]

OK, so I would try Pilot's suggestion first....... balancing. Failing that, maybe de-sludging, especially if the pipework is small in diameter (minibore/microbore).

De-sludging is usually the last resort. Since you're new to the house, it may be the layout of the pipework. If the right pipe sizes and layout haven't been set up properly, you may have a "dead leg" i.e. a run that is so badly positioned, that the general flow tends to bypass it.... even under pump pressure. Again, unless it's a diabolical DIY installation, balancing should overcome it.

[The Builder]

Oh...... crossposted ...... 15mm pipework. Not usually bunged, unless.... it's an old system.

[verabc]

thanks so much! Will try to balance them.