Cold radiators - air lock?

[JoeJoe]

Woke up this morning to find out that two of our 17 radiators are stone cold. They are the two furthest away from the (combi) boiler - upstairs, boiler downstairs.

The inlet and outlet pipes into both of then are also cold.

I noticed that the last two radiators before the two offending ones are hot near the top, but much colder near the bottom.

I bled all the radiators in the house, however, there was no sign (sound) or air in any. Radiators are still cold after that.

Any idea what do to next?

PS: Not sure if this is related, but you may recall from my messages of several weeks ago that I had a problem with my expansion vessel causing the pressure to drop very rapidly. This has now been re-pressurised and everything seemed to be working OK since.

[JoeJoe]

Why would you think that? The pressure has remained stable ever since the vessel has been re-pressurised.

[Dave Liquorice]

Even after you bled the radiators? I'd expect some small drop. Jammed in the off state thermostatic valves is another possibilty.

[JoeJoe]

It is -14c outside at the moment, so can't really leave the system turned off for long enough to check... But it seems as if the pressure is stilll stable.

Is that possible for all 3 (just noticed that it was in fact the last 3 and not two radiators that were cold) valves were jammed at the same time?

I tried to "play" with te valves of the offending radiators, but no luck...

[Ronald Raygun]

It does seem too much of a coincidence for 2-3 TRVs to stick as if by conspiracy. But it's still worth checking. "Playing" with them is not very likely to free them up (I take it you mean just turning the knob back and forth several times). You need to take the valve head off.

This is a simple job. They're generally held on either with a knurled ring nut or with a collar clamp secured with a single screw. Undo this and the head just pulls off, exposing the pin which actuates the valve. Grab hold of the pin with pliers and give it a bit of a tug, then you should hear the water rushing through.

However, the symptom you described, of nearby radiators being hot on top and cold on the bottom, suggests a different problem. If there is air in them you would expect the reverse, i.e. hot below and cold on top, and bleeding would fix this. I suspect there is a build up of sludge causing a blockage.

You might be able to shift the blockage by turning off all the good radiators in the hope that this might give bit of extra push through the bad ones. Otherwise you're probably looking at having to power flush.

[Dave Liquorice]

Everything is possible.

But some things are not very probable. B-)

Jammed valves are normally found straight after the summer after they have been forced off for long periods. Depends on room use and exterior temp how long it is before one is noticed. I only found one last week when digging out the Christmas stuff from an unused room....

Try turning off all your radiators but the cold one nearest the boiler put the pump up to max and fire up the heating. That might shift an airlock.

[Andrew Gabriel]

Two possibilities. Flow too low - inlet pipe will be hot, outlet pipe cold. Radiator silted up - both inlet and outlet pipes will be hot.

Try turning off all the ones which are working, and see what happens with the others. If it's an air-lock, that might force it out, but be careful of it forcing the airlock just to the pump and then the circulation stopping and boiler overheating - be ready to turn off the boiler if it suddenly starts sounding like a boiling kettle.

Could be related depending on exactly what happened.

Another thought - do the pipes run somewhere where they might have frozen?

[JoeJoe]

Just had a look at it again:

Both pipes on the last warm radiator are hot. From it the pipes run for around 5m under the floor boards and along the external wall to the first of the cold radiators (and then to the other two cold ones). I cannot remember

100%, but I have a feeling that these pipes are copper and uninsulated, and that there is a very large vent to the eaves - i.e. very exposed to the outside.

Although I am certain that the radiators were working last night, is it possible the pipes leading to them simply froze overnight, causing a blockage?

Sludge accumulation is a possibility. However I have had to top the system on a daily basis for weeks before it was finally fixed a few weeks ago, and also the water here is as good as mineral water (i.e. soft - we've been using the same kettle for the last ten years...). So I assume sludge it less likely?

[JoeJoe]

You took the words out of my mouth! See my reply above.

Assuming that:

- the pipes leading to the cold rads froze, and that;

- access to them is not easy (+ school closed so kid around), and;

- temperature tomorrow is supposed to be ~10c warmer than the last few days,

Can I simply ignore it (we have "enough" useable rooms) and hope that everything will be back to normal once pipes defrost?

[Mark]

Do these normally work? Have they gradually or suddenly stopped heating up?

Radiators which are cold at the bottom and hot at the top are often this way because they are partly full of sludge. It may be worth removing them and flushing them out.

[newshound]

No, repeated top up is a recipe for sludge. Each time you top up, you add air so the steel radiators can rust a bit more, especially as you are diluting your inhibitor (if you still have any left). The sludge is iron oxide, not the calcium and other limescale products which come from hard water.

[The Natural Philosopher]

airlock is always possible after any work on CH that requires even partial drain down.

shutdown, on the isolators if necessary, all hot rads, and see if that pumps the air around. Then bleed any rads with cold tops and hot bottoms. Use max pump speed if its adjustable.

Be paitent, It took me several hours to get partial clearance and several more MONTHS of bleeding to get all the gas out, especially if its fresh pipework with a bit of flux left in.

Re pressurise as required, and remember the new water will also have dissolved gases in it that will need to be bled..

[The Natural Philosopher]

It is, but I hope not.

I had a porch once. With rad in it. I never used it in winter. I turned the rad off. It froze. I burst the rad. I patched it with CAR BODY FILLER enough to be able to run the CH till the landlord could get a plumber in.

scale yes, not sure about sludge.

>

[The Natural Philosopher]

You can hope. See my earlier reply.. I had water gushing out of a split rad..instead.

>

[Mike Lane]

JoeJoe wrote on Dec 8, 2010:

I live in a very soft-water area (3 degrees Clarke according to the local water company). My microbore CH pipes clogged up last year so that nothing would clear them; I had to get most of the piping replaced.

[Mark]

Beware -- An empty house a few doors from me have had their ceilings down and lots of problems from their pipes freezing. I'd turn the water off if it's not a sealed system and in anycase check all the pipes.

[JoeJoe]

I am pretty clueless about these things - just looking for an explanation that sounds rational and makes sense before I look into it. So please bear with me a little longer...

What I cannot understand is how it is possible for all 3 radiators to suddenly turned cold (worked at 11pm, didn't come to life at 6am the next morning), and also, why only the three rads that are the furthest from the boiler?

As the pipes leading to them could potentially freeze overnight this is why that possibility seemed most likely to my ignorant mind...

BTW, apart from the problem with the expansion vessel that we have had for the last year+ and required us to top the system up regularly, we have never had any problem otherwise (i.e. air in the system, radiators not uniformly warm, etc). I just cannot understand why suddenly three rads will die at once...

Weather predicted to warm up a little in the next few days - it has not been above -8c for the last week, so maybe that will eliminate the freezing option?

[Dave Liquorice]

Simplistically radiators are connected in paralle if you get a blockage in the flow or return any raditors after that point won't work:

Flow >---+-----+-----+--X--+-----+ | | | | | R1 R2 R3 R4 R5 | | | | | Return

[JoeJoe]

Thanks Dave - that makes sense!

I was, for some reason, under the impressoion that air locks only occur inside a radiator and not in the pipework.

[BartC]

I've only had frozen pipes when I've been away with the heating off, and the temperature well below freezing for days on end. Or when the pipe runs outside.

For a CH pipe (which had been hot a few hours earlier) to freeze overnight and stay frozen (with heat from the rest of the house permeating through) sounds unlikely. Do the pipes actually run through an area where the temperature is currently below freezing?