Trapped air in CH (probably)

Grenage

We reinstated the kitchen radiator after removal for decorating ...about a year ago; I noticed a knocking noise a couple of days later from from the pump.  The pump bounces for a second when it first spins up, but only if the CH is engaged - the DHW is fine.  It's an old open-vented setup.

I assume it's trapped air, but I've bled the radiators and then the pump several times.  Does anyone have any suggestions?

greatcrested

Drain and refill the sytem?

Powerflush?

But you say it does it " for a second when it first spins up". Never recurs if CH stays on (ie air bubble circulating round and round)? Maybe it's a pump issue.......

gamston

i used to have probelms with air in my system when i changed/moved radiator, I fitted a spirovent air separator near the pump ( not cheap but works) 
if you do a full system fill up you could fill from the bottom via a drain valve and garden hose, ( fit a drain valve in a conveniant position, I have 8 drain valves on my system because of the way its laid out ) by filling from the bottom you should be pushing the air upwards use the rad bleed valves as normal to release air
if its just 1 radiator to fill   open bleed screw, open 1 valve and fill about half way then close that valve, open the other valve fill till water comes out of bleed valve

Grenage

Cheers chaps.

I've ruled out the pump because it doesn't occur if only DHW is active.  I would also have expected the air to move up the vent rather than to keep dancing round the system and end up at the pump every time.  All I could think of is a bit of an air lock further down the heating run and the pump has to overcome the initial resistance in the pipework.  None of the radiators are cold.

I was hoping to avoid draining down the system as I'll have to pull up a tiled floor to install a drain on the 22m run, but I suspect it may come to that anyway.

bris

You should have an air vent or bleed screw at the highest point in your system. See if you have one in your loft, I have solved this a few times in lofts. In really old systems you need to be careful though, they don't open easily, if you find them are finding it difficult use a penetratimg oil and let it soak for a few hours.

FreeBear

Grenage said: I've ruled out the pump because it doesn't occur if only DHW is active.  I would also have expected the air to move up the vent rather than to keep dancing round the system and end up at the pump every time.

If it is an old gravity system, then the pump won't be running when the controller calls for just DHW. S & Y plan system would have the pump running continuously and there would be a zone valve or two in the system somewhere.

Air will get trapped in the top of the radiators, so bleeding them should be the first port of call. A pump that "clunks" on start up is more likely to be a worn pump on it's last legs. They are cheap enough, so it wouldn't hurt to get a spare to stick on the shelf for when the old one finally fails.

Grenage

Thanks Bris, that sounds worth investigating.  I'll have a root around for anything that might be a vent.

Grenage

FreeBear said:

Grenage said: I've ruled out the pump because it doesn't occur if only DHW is active.  I would also have expected the air to move up the vent rather than to keep dancing round the system and end up at the pump every time.

If it is an old gravity system, then the pump won't be running when the controller calls for just DHW. S & Y plan system would have the pump running continuously and there would be a zone valve or two in the system somewhere.

Air will get trapped in the top of the radiators, so bleeding them should be the first port of call. A pump that "clunks" on start up is more likely to be a worn pump on it's last legs. They are cheap enough, so it wouldn't hurt to get a spare to stick on the shelf for when the old one finally fails.

Hi Freebear,

What I mean is if the hot water has been off all day and I turn on only the hot water, the boiler fires up and the pump kicks in to heat the tank without any problem.  It's only when the valve switches to CH that the problem occurs.  The pump itself is only about a year old as the bearings went.

FreeBear

Grenage said: What I mean is if the hot water has been off all day and I turn on only the hot water, the boiler fires up and the pump kicks in to heat the tank without any problem.  It's only when the valve switches to CH that the problem occurs.  The pump itself is only about a year old as the bearings went.

OK. So you have a valve or two in the system. Have you checked to see if the valves are operating correctly and not sticking momentarily ?

Grenage

Hi Freebear,

Yup I have a mid position valve which was replaced at the start of the year (motor died). It moves correctly between all three positions when in use.