Air lock? in hot water/heatiing system

[porkpie]

I have an oil-fired boiler (Camray 2, old but working well) which serves an indirect hot water system and central heating.

The heating is not at the moment just the hot water.The hot water system also serves two towel rails.One is turned off.

when the boiler is on the pipe going to the other towel rail gets scalding hot but the towel rail stays cold (no air when bleed key used). After about 30 mins there is a huge rushing sound for about a minute and the towel rail begins to heat up. During this bit the pipe going to the rail cools down briefly. In another five minutes the noise starts again and lasts two minutes and the towel rail gets hotter. After another five minutes the noise starts again. It's then time for the boiler to switch off.

I went into the loft this morning to see if anything was wrong there. It wasn't but when these noises were going on there was almighty coughing out of the vent pipe into the header tank, so much so that the (very hot)water splashed all over the place.

This problem started a little while ago and I hoped it would solve itself (we had the heating on for a while to see if the pump would clear it but it won't). It seems air is getting in somewhere but I can't see where - there don't appear to be any leaks and the radiators all get hot eventually.

One further thing. Even though the heating is off the upstairs radiators are warming up via the heating pipes unless individually switched off. There is no check valve to stop this happening but I don't remember it happening before the noise problem started.

If anyone can shed any light I'd be very grateful - sorry for going on a bit

Thanks

 

[chrishutt]

You need to supply a little more detail. Firstly is the circulation from boiler to cylinder gravity or pumped, and if pumped what motorised valves are there?

 

[porkpie]

Thanks The hot water cylinder works on gravity - there are no motorised valves.
There is a pump for the heating but that is a separate circuit from the same boiler (although hot water is convecting up this circuit to the upstairs radiators and sometimes when the noise is going on the rushing/vibrating noise is in these pipes too).

I hope this clarifies a bit.

Thanks again

 

[chrishutt]

I think the boiler may be actually "boiling", which it's not meant to do. The gas escaping would then be steam. The boiler thermostat may have failed. Can you test the operation of the thermostat?

 

[porkpie]

The Boiler thermostat seems ok because the boiler is cutting in and out.
Turned the system off yesterday, and today turned the boiler temperature down (from 70 to 60 - it won't go any lower), and also opened the valves fully on the towel rail - and the noise etc. didn't happen! Header tank water is cold again too.

So, possibly, it's solved.

Thanks again for help.

 

[porkpie]

I'm afraid the problem has recurred but I think I know what is happening. For some reason the water is not flowing freely and therefore the return (which determines whether the boiler cuts out) is not getting to temperature so the boiler keeps heating. When it eventually forces its way through and back to the boiler the temperature is nearly 80 when the boiler is set at 60.

Any ideas as to how I can get the water to flow round properly (like it used to)? Would draining down and flushing out help?

Many thanks