Rads filling with air & F & E tank keeps over-flowin

[Harvo]

Can anybody figure out what’s happening here? I have an open vented system with a gravity circuit from the boiler for the hot water and central heating on a separate pumped circuit from the boiler. It serves 13 radiators on a ‘one-pipe’ arrangement and has worked well for years.

In the summer I needed to flush the coil of the HW cylinder since that had stopped working, but while that’s now fine, the central heating has started to play up. The radiators keep filling with air (not hydrogen – it doesn’t burn), and then the F& E tank backfills with hot water and starts to overflow. Turning the central heating pump off stops it overflowing. Bleeding all the radiators makes everything work fine for a day or two, but then they begin filling with air again and eventually the overflow starts to run.

The cold water tank is higher than the F & E tank, but since the F& E tank isn’t overflowing all the time I think the HW cylinder coil is OK.

I’ve already tried sludge cleaner after the HW cylinder problem and have carried out a mains flush over the weekend as described in the FAQ. All the waste water was clear and it’s made no difference now the system is filled and running again.

So where’s all this air coming from and why does the F & E keep overflowing? This is driving me mad! Any ideas?

Thanks

 

[seco services]

is the vent pipe drawing in air ?

 

[Goldspoon]

What speed is the pump running on?

 

[Harvo]

The outlet from the bottom of the F & E tank is connected by a pipe to a tee joint to the feed to the HW cylinder coil. The vent is an extension of this pipe and simply rises above the tank and is bent over 180 deg. I've never seen or heard it drawing air. Wouldn't the tank have to be empty for this to happen? Is there a way to check?

The pump is on no. 3 setting and has always been set like this. I did try slowing it to no. 2, but the central heating just stopped altogether.

Nothings been done to the system recently apart from me fitting an anti-gravity / one-way valve to the main central heating flow pipe from the boiler. This shouldn't cause any problems should it?

 

[seco services]

what do you mean the heating just stops when the pump turned to speed 1,2 ?

 

[Harvo]

I only tried it once. The rads all cooled and I presumed the pump had stopped. Turned it back to 3 and it started pumping again. Its an old pump and it may just have dirty connections, but for years its been set at 3 and everythings been fine.

 

[seco services]

have you never flushed the heating/rads out ?

 

[Harvo]

I put sludge cleaner in the system back in the summer after the problem I had with the blockage in the HW cylinder coil and drained it out some time later.

I suspected there may still be some sort of blockage or restriction so last weekend I also did a mains flush, but the waste water was almost completely clean. I could feel the cold temperature of the mains water flowing though the distribution pipework and the rads when I was flushing so it must have been getting through. Also once refilled the rads do get warm, but then after a day or two the air starts getting in and the overflow begins running again.

 

[seco services]

try putting the vent pipe in a glass of water see if it draws the water in.

 

[JohnD]

...Nothings been done to the system recently apart from me fitting an anti-gravity / one-way valve to the main central heating flow pipe from the boiler. This shouldn't cause any problems should it?

I don't know, but when a system starts going wrong after you've made a change, it's always worth changing it back to how it was before to see if the problem goes away.

Also make sure the joints are very tight (especially on the suction side of the pump) and cannot draw air in.

good idea about putting the vent pipe in a glass of water to see if it's sucking.

I am just a householder

 

[jameshayes]

raise expansion pipe over expansion tank to as high as you can get it,then add internal leak sealer.
think about an upgrade when you can afford it.

 

[Harvo]

Thanks for all the advice. Here’s a progress report - hopefully this thread hasn’t gone cold!

On Monday might I partially drained the system sufficient to remove the one way valve and replaced it with a full-bore 22mm isolation valve as JohnD suggested, so no possible restriction there. I refilled the system, put in some new Fernox sludge remover for good measure and bled the rads as normal. Bled the rads again on Tuesday night, but only a bit of air in a couple of them upstairs and generally all seemed well.

On Wednesday night I put a jug under the open end of the vent pipe over the F & E tank and half filled the jug with water so the pipe was submerged. By 8.30 the following morning (today), all the water had been sucked from the jug (about 600ml), the system is making gurgling noises and some of the rads are cold. The F & E tank is also full of some fairly murky, dark red/brown water.

Any suggestions what to do next please?