Is this air in rads or just an inefficient pump?

Hi

We have a dual (back boiler + OFCH) system. Had a bit of a problem over the last few weeks with apparent air in the system - one radiator stone cold. Then noticed that sometimes it was alright so TBH just didn't really do anything about it - the room in question isn't often used.

Then, noticed it again but also noticed a pattern - I think.

It seems that when we use the OFCH all the rads heat fine but when we use the back boiler/pump is when we have problems - could this be right and does it mean e.g. that the pump has become weaker and is maybe on it's way out? It wasn't always like this.

I'm certainly no plumbing expert, but could it be that the rad in question is the last in line and the back boiler doesn't produce enough heat to warm it?

Thanks Sean - yes, that had crossed my mind but it doesn't quite make sense if the back boiler had heated it before. But again, I suppose there could be sufficient variation in the heat potential of individual fires to make a difference.

Is there any rule of thumb or general way of knowing if this particualr rad is the last in line though?

Caveat,

The radiator which heats occasionally may do so only when thermostat valves on other radiators are closing. See if it heats when you close one or two of the valves on the other radiators. If this works, then try turning the circulating pump speed up - assuming there is an adjustment switch on the circulating pump. Check then that all radiators heat.

Ok thanks I'll have a look.

Did you bleed problem rad? did air come out?

You say it heats when the OFCH is on?

Is the rad the furthest away from pump on stove?

When the stove pump is running, can you hear it?

Bled rad - no air. Ok, had fire + back boiler last night and rad was cold. Just had OFCH on today and it was hot, so I guess the pump/heat of fire isn't strong enough. Although I'm convinced it did heat before via back boiler which leads me to think that maybe pump is on it's way out.

When the stove pump is running, can you hear it?

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Sorry not sure what you mean - hear the rad or hear the pump?

No gurgling etc from the rad AFAIK - if that's what you mean.

Yes, I can hear the pump sometimes - a rhythmic pulsing/thumping noise - don't always hear it though.

Difficult to know in which order the rads heat. The rad in question is one of the furthest away but others are just as far and they heat fine - bungalow, pump is roughly half way along the length of the house. However, the rad is the furthest from the actual fire - if that's what you mean.

Is the oil boiler close to problem rad? When I say hear, i mean the pump. By and large, if the pump is making noise it is either pumping water with air in it or on the way out.

As already suggested, when back boiler is on, turn off a few rads that heat to see if problem rad heats.

You have a complex enough case, you put in a stronger pump and it may cause pitching (Water being pumped into small tank in attic). Dual systems can be a pain when there are problems.

Hard to advise without seeing how your system is set up.

DavyJones said:

Is the oil boiler close to problem rad?

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No probably the furthest away.

When I say hear, i mean the pump. By and large, if the pump is making noise it is either pumping water with air in it or on the way out.

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Hmmm. Well I've noticed that it tends to make noise when I open up the draught 'flap' in the fire, increasing the draw - if that means anything. Also, when the fire has gotten very low it tends to happen too - so at both extremes.

As already suggested, when back boiler is on, turn off a few rads that heat to see if problem rad heats.

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I'll try that then and see what happens.

You have a complex enough case, you put in a stronger pump and it may cause pitching (Water being pumped into small tank in attic). Dual systems can be a pain when there are problems.

Hard to advise without seeing how your system is set up.

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Ok, thanks for all your input Davy.

Quite so, Davy Jones.

We had similar symptoms here - one (or two) radiators heating occasionally - others OK. Real problem proved to be air pockets in the piping in the roofspace. The main piping circuit is in the roofspace - it feeds rads on the ground floor.
The pump speed/pressure proved inadequate to maintain a good flow to all rads when pump was at its lowest setting and when there was a very small amount of air in the pipes. There are air bleed valves in the roofspace, but they are very difficult to get to.
Problems solved by:
1. Turn pump speed up.
2. Close all rads except one. Repeat with a different one open. During this, you may hear air getting pushed into the open radiator.
3. With pump off, bleed any air from the rads.

Iderown said:

2. Close all rads except one. Repeat with a different one open. During this, you may hear air getting pushed into the open radiator.

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Thanks again - how would this manifest itself? Would I hear 'whooshing' or what? Reason I ask is occasionally (and excuse the silly analogy) I do hear a stuttering creaking noise for a few seconds from some of the rads, one in particular (not the problem one) and it sounds like a 19th century ship's mast during stormy weather - well as depicted in movies anyway.

Do you know what I mean?

Caveat: Copper pipes can "creak" a bit while they are heating - thermal expansion.

The sound of air bubbles getting pushed into rads is pretty much as you would imagine it to be. It would occur even when the water is cold - immediately after starting the system boiler/pump.

OK - thanks.

Will experiment with the rads over the weekend probably.