Do I need an air vent in the CH system?

[Paul]

I have a conventional open vent CH/HW system, seperate pumps for CH and HW with their own 2-port motor valves. Since I installed a Megaflow HW cylinder a few years ago, it's been really difficult to refill after draining.

Today, although the CH is working OK, the HW isn't, and the pump seized as it got that hot and was noisy (a surfing sound). I *think* the problem is that the primary coil in the Megaflow is air locked at the top, and I *think* you can get various venting devices that I can fit to the Megaflow's primary return (the higher of the two pipes - Heatre Sadias recommendation suprisingly).

But I don't know what's available or what to ask for. And any 'tricks' I can do meantime please?

Can anyone help please?

Thanks,

Paul.

[Ed Sirett]

I'm not fully convince the problem is an air lock. However assuming it is then you have two choices manual and automatic. In particular I would check both the zone valve and the pump first.

For a manual vent you would add a Tee into the pipe a length (say 300mm) of pipe going vertically and finish with a manual vent like (screwfix

19155). You can use the 22mm version if that helps, or reduce to 15mm and then the 15mm version.

For an auto vent you would ask at a shop for an 'auto air vent'. These invariably have 1/4" or 3/8" BSP connectors on their inlet. So you will need a 3/8-1/2 bush and maybe a 1/4"-3/8" bush. The other fitting you need is 1/2" to 15mm (such as screwfix 12341).

[Paul]

Thanks Ed. In fact, I fitted an auto vent this morning. I assumed the pump was OK, 'cos I could see the impeller spinning, and I had set the motor valve manually open. Of course, they could still have been faulty, but as they were both working fine immediately before I drained the system, it would have been a bit of a coincidence if they'd have stopped in the meantime.

Anyway, I fitted the vent with about 300mm of vertical pipe, refilled, and expected to hear a massive amount of air coming out of the vent. Well, it did for a moment, and then nothing. I couldn't believe it, so I loosened the vent and water p***ed out, so it was functioing properly.

Anyway, eventually, I got the HW to circulate (I dunno how), and it's working fine now, but I still don't know what the problem is, so I'll have it again next time :-(.

I might put two or three vents in strategic spots. I think part of the problem is that the header feeds the HW primary flow (which, unconventionally is lower than the return), and somehow things are getting locked up as that water tries to get down into boiler.

BTW, the auto vent took a 15mm pipe, so that was easy.

Any further suggestions appreciated.

Thanks.

Paul.

[Ed Sirett]

I take it that there is only one pipe between the header cistern and the primary circuit? In which case the main cuase of the air-lock is in the combined feed and expansion tank, this causes the primary circuit to be empty with all the problems that ensue.

[Paul]

Indeed.

But if the cistern feeds into the flow, and the air vent I fitted is on the return, I don't see where or how the lock is occurring. But it is!

Regards,

Paul.

[Ed Sirett]

With only one feed/expansion pipe there is a tendency for an air lock to form in the feed/expansion pipe. How likely this is depends on the layout of that pipe. Once the heating circuit warms up there is a good chance that an air lock will get pushed back into the tank and escape.