vent overflowing after re-filling system

[cymro]

Hi Guys,
I,m sure this has been covered a multitude of times, but some additional advice would be most appreciated please.

I have been using the CH system for many years with no problems, it's about 30 years old, the boiler was replaced about 6 years ago (baxi) and the pump replaced about 4 years back.

Last Autumn, the 2 port valve ( there's only one in the system ) failed to close properly, so I couldn't have "hot water only" without the radiators getting hot, this wasn't a problem in the winter, but needed changing.

I replaced it last week, and took the opportunity whilst the system was drained to replace all the radiator valves, some of which were leaking.

I re-filled the system, adding some cleaner, and fired the boiler, all seemed well, but it kept cutting out, I suspected airlocks, but it seemed to work ok later.

I ran it for a couple of days, then drained out the inky water, re-filled it ran it for a bit then drained it again, with the water coming out quite clear.

Re filled the system with inhibitor added.

Had the heating on for a few hours last night , all ok but this morning the boiler cut out again, re-set it and it came on ok, but I noticed that the vent was overflowing into the header tank, (there's only one tank)

The pump was set to 2, which it's always been run at, I put it on 3, which increased the flow from the vent. Reducing it to 1 stopped the vent overflowing.

Incidentally, the level in the tank was not increasing, indicating that the system was re-filling itself so presumably no blockages in the feed from the tank

I'm pretty sure there's no blockages in the system, and there's no gurgling sounds, I checked all rads and none needed bleeding.

As nothing has effectively changed, could it just be an airlock somewhere? or maybe, as the system is now clean, perhaps less effort from the pump is required to circulate the water?

The vent pipe's only a few cm above the header tank, but there's never been a problem before.

Any comments would be most appreciated

thanks a lot...Pete

 

[JCplumb]

Get a neodymium magnet and check the tees that take the vent and feed pipes up and any horizontal runs around those tees, if there's any magnetic pull at all on those pipes they need cutting out and replacing.
It could be an air lock, if one or more of your rads don't get hot then an air lock would be probable.
Your vent may be too low, but if this is a new problem then it's probably fine.

 

[Unregistered]

Thanks for your very speedy reply, just checked with a magnet which does stick to the 22mm pipe which feeds the pump, and below it which connects to the 2 port valve, don't think there's a blockage though, as the gate valve from the feed in from the tank in the loft feeds directly into that. All the rads get hot, I've not done any balancing, as they were all wide open before I put the new ones on and seemed to work OK.

When i replaced the valve, the original one had been "built in" to the system, the lower 22mm pipe had an elbow resting on the joist, and the upper one connected into the copper cylinder and pump.
I had to cut the pipe above and below the valve, ( the piece i cut out was the same piece that the magnet sticks to, and was black on the inside, but no sign of a reduction in bore ) I put in two compression elbows, with short lengths of pipe to two more capillary elbows fitted directly to the valve, enabeling it to be pushed in sideways, I suppose having 4 elbows could possibly restrict the water flow? I've put a picture of it here www.petejones.co.uk/valve/valve.jpg

thanks again for the reply

cheers....Pete

 

[Cardiff Plumber]

I know it sounds weird but could you post a picture of the element flange on the cylinder, just a hunch I have.

 

[Cardiff Plumber]

I do suspect you have a Primatic cylinder.

If you do you will have to drain your cylinder and refill it, this will re-introduce the air gap and stop the overflow back into your tank.

And if it is a Primatic, DO NOT USE an inhibitor.