Air lock in central heating return?

[Slough]

I am trying to be a friendly neighbour by trouble-shooting someone's vented heating system. This system has operated fine for many years.

The problem: The boiler cuts-out after a few minutes after trying to heat t he radiators.

The problem started when trying to get the heating going after the summer. During the summer, the boiler had heated the hot water cylinder without a p roblem. The hot water is still working fine (the boiler flow is hot and the boiler return is warm when the hot water is 'on').

The boiler feeds two Honeywell valves (one for radiators, one for the cylin der) via a heating pump. The pump is gently vibrating when 'on' and the val ves have hot water on their outlet sides when operating. I have also tried manually opening the heating valve to eliminate it as the source of the pro blem.

I have bled all the radiators. I checked the small central heating 'top-up' tank in the roofspace (the ball valve was fine although the water was a bi t dirty).

When the central heating starts up, the boiler flow gets hot quickly but th e return remains cold. This happens even when I close off every radiator va lve. Where is the hot water flow going in this instance?

When the radiator valves are open, the corner of the radiator near the valv e warms up but the water doesn't seem to flow through the radiator.

Any diagnostic tests that I could perform, please?

[Roger Mills]

When you say that the boiler "cuts out", what exactly do you mean? Does something trip, and have to be re-set before it will fire again, or does it simply stop firing when the water temperature reaches the boiler thermostat setting?

In any event, it sounds as if there's no flow in the CH circuit. My hunch is that the pump is duff. The fact that it's vibrating doesn't necessarily mean that it's actually *pumping*. It could be seized, or its impeller could have sheared off.

It's possible that there's enough gravity circulation in the HW circuit to heat the hot water, even with a failed or partially failed pump - but heating the radiators needs a bit more urge.

The pump should have a circular cover with a large screwdriver slot in the middle, at the end furthest from the water connections. If you unscrew that and remove it [put an old towel underneath to catch the small of water which comes out] you can then see the end of the pump shaft and determine whether it's actually rotating. That, too, will have a screwdriver slot in the end. If the pump is seized, you may be able to free it by rotating the shaft back and forth.

Have a go at that, and come back to tell us what you find.

[Incidentally, if you turn off all the radiators you *guarantee* there's no flow in the CH circuit - which would pretty soon cause the boiler to turn itself off when the hot water has nowhere to go!]

[charles]

It seems as though there's a plug of sludge somewhere in the return pipework.

[Graham.]

[Slough]

It stops firing when the water temperature reaches the boiler thermostat limit.

I assumed you meant do this when the pump is powered off.

I followed your instruction. The shaft of the pump turns easily when I turn it with a screwdriver so I don't think it has seized. I can't tell if the impeller is damaged without removing the pump, which is possible if all else fails.

You diagnosis looks logical though. The only thing I can't quite square is why the hot water heats up. It seems hard to believe that convection currents alone would be sufficient to get the water through the cylinder coil and back to the boiler so quickly.

[Fredxxx]

I've never known that, more likely a duff pump. The OP hasn't determined if it runs, though it spins freely.

[Slough]

Any way that I can confirm that it is the pump before I remove it?

[charles]

I've certainly met a sludge plug, but not in a domestic system - it was in our village hall. 3 different plumbers had diagnosed an airlock, added lots of venting points but never solved the problem.

[Fredxxx]

Usually you can feel the vibration, which changes with speed setting.

If there is no vibration, nor change in tone when the speed is changed, I would be 95% it's the pump. However I would check that volts were going to the pump before taking it all apart.

Sometimes it's the capacitor in the pump body that's gone, but checking that might not be so easy.

[Slough]

Thanks to all for advice. I will try changing the pump to see if that fixes it.

[Dave Plowman (News)]

+1

[alan_m]

Try turning the pump to full speed and then turn off every radiator valve but one to force the water down one route. Repeat with another radiator as the only path.

[Michael Chare]

I have often had pumps get stuck when not used in the summer. I would turn the pump on when the shaft end is exposed so that you can see whether the shaft rotates.

[Slough]

Tried this but no change to previous symptoms unfortunately. Thanks for suggestion.

[Roger Mills]

If the pump shaft *is* rotating (and I'm not sure whether you've powered it up to check that with the cover removed) there's one more thing to try before changing the pump.

Let's assume that the pump is ok and that there's an airlock in the CH circuit. With the system not running, go to each radiator in turn and turn off one of the valves. Open the bleed screw and bleed off a decent amount - maybe a litre - of water. Then close that valve and open the other valve, and repeat the bleeding exercise. That ensures that water is reaching both sides of the radiator. [If you don't close each of the valves in turn, it can appear to bleed ok - but there can still be an airlock on one side.]

NOTES:

1. You may need to devise a means of keeping the CH zone vale open with the manual lever while you do this
2. In case the system has been balanced, when you close each lockshield valve, count the number (and fraction) of turns needed to close it so that you can put it back to the same position.

[Slough]

After removing the silver screw from the centre of the Grundfos pump, I set the heating going and observed that the pump shaft was spinning well.

Of course, it could be that the impeller has fallen off. The pump is slightly hotter than I would expect it to be (almost too hot to touch). I am guessing that a pump with broken impeller would run cool(ish)?

So I will investigate the possibility of an airlock again. I am going to get a plumber to help as the pipes in the loft are hidden under an impenetrable sea of mineral wool (the house is a bungalow).

Thanks again, Roger.

[Martin Brown]

OK so the shaft hasn't seized - a common fault after a summer unused.

Certainly cooler than one facing a pipe blockage and trying desperately to pump water against an impossible load.

I can't see how an airlock could cause these symptoms unless you have air inside the pump. Compacted sludge in the lowest point of the return would be my guess. How far round does the hot water get?

[Tricky Dicky]

Ah! A drop feed system notorious for air locking particularly after a drain down. On these systems you need a drain off c*ck on each radiator if not c onsiderable amounts of water remain in the system and refilling tends to tr ap air in the pipe work. A bit unusual for every radiator to not heat up it is usually one or two that get affected.

Richard

[Jonathan]

I had this problem many years ago and was advised by the plumber's wife to hit it with a hammer a few times, to save the call-out charge. It worked.

Jonathan

[charles]

Preferably NOT a metal hammer - A rubber one is ideal or else wooden mallet, otheriseyou could damage then pipe.