After draining heating system 2 rads stone cold

[TimF]

Drained the central heating system down today (long story why!). When I refilled, the boiler won't stay on for more than 5 minutes and there are 2 rads at the top of the house stone cold (and sound like they are empty when I tap them). And when I open their bleed valves nothing happens - no hiss of escaping air ... no nothing.

I've got a Potterton Netaheat (quite old, but working fine) and a gravity fed system. No problems with the header tank filling up and emptying. All the other rads in the house have bled fine. Top 2 rads don't seem to want to fill. I suspect an airlock and that is probably the cause of both my problems - central heating not staying on and 2 cold rads. What do you think?

I've tried re-draining the system and refilling it ... but still got the same problem. (I even tried reverse filling the system from the draincock on the kitchen rad with cold mains water, but all it did was p*ss water out of the draincock all over the kitchen!!).

Thanks to anyone who might be able to help/advise.

 

[captain_pugwash]

if the boiler is shutting down after 5 mins then it could be overheating and the overheat protection could be cutting to shut it down - usually there would be a lot of banging and hissing of steam from an air vent in the boiler if this is happening.

The reason for the overheat is often the pump not working - but usually this fails completely and so no rads would get hot. If its the usual grundfos pump you could take off the inspection cover (get ready with a cloth to catch dribbles) and you should see if the spindle rotating.

If the pump is fine then it could be a major air lock which is stopping the water getting to rads and stopping the boiler pumping out the hot water.

You could try to shift the air lock by shutting down the working rads to firce the water at the cold ones and then open the bleed valves to allow the air to escape - fizzing and gurgling should happen initially followed by water - you should hear them filling up

good luck,

 

[TimF]

Pugwash,

Thanks for your prompt reply. Pump is working fine. So I'll try your other remedy and shut down all other rads to force flow of hot water to the cold rads. At least you've given me something to go on tomorrow morning.

Thanks again, I'll let you know how I get on.

 

[jpm321]

Timf

I would be interested in knowing how you get on because i have the same problem, but only one cold rad.
I have turned all working rads off totry and force suspected airblock but this didnt work, i have mircobore on the rad in question and was told that this is common problem in hard water areas which mine is

 

[TimF]

Pugwash (and others),

Pleased to report I got system up and running this morning. In the end I didn't need to close off other rads. All I did was run the boiler on hot water only setting (i.e. with CH turned off). After 5 mins the boiler shut itself down (although hot water tank not up to temp). A few minutes later there was then loads of gurgling noises at the back of the boiler and whatever the problem was it seemed to have freed itself. Then the boiler fired itself up again and carried on heating the hot water.

I then went upstairs and found that when I opened the bleed valves on the 2 rads that hadn't filled they now bled out the air, and the header tank started refilling as the system topped itself up.

JPM ... don't think this helps you much. I did have a rad once which was fitted by a builder (not a plumber) and it was stone cold. The builder was baffled. I wasn't once I discovered he had put isolating connector valves on the bottom of the rads and he had fitted it without opening the isolators. Just a half turn with a screwdriver was all that was needed!! Doubt this helps you, but I mention it just in case ... good luck.

Thanks again,