I guess that many of us have come across primary vented systems which pump *over* - where there is a constant flow of water out of the vent pipe back into the F&E tank.
But is the opposite also possible - where air is sucked in through the vent pipe, and water forced back up the fill pipe into the F&E tank?
I have recently been tearing my hair out - what's left of it(!) - trying to sort out a friend's heating and hot water system.
The system has a floor-standing conventional boiler (Potterton Kingfisher Mf - c.2003) in the garage, with a vented primary circuit which has a fill & expansion tank in the attic. The hot water system is unvented, employing a Heatrae Sadia Megaflow indirect cylinder (c. 1995) in the first floor airing cupboard.
The CH and HW primary circuits each have their own pump and are entirely separate - only meeting at the boiler connections. [There is some relay logic which runs the boiler when one or other or both circuits are calling for heat]. The only motorised valve is part of the Megaflow installation. This opens in response to the tank stat, and its auxiliary contacts turn the pump on.
The basic system was designed and installed by my friend several decades ok, but has had a new boiler and pumps, plus replacing the vented HW cylinder by an unvented one in the meantime. The current configuration has remained unchanged since about 2003 - and has worked ok until recently.
Problem is that my friend is now in a care home, and his wife sends for me when things go wrong!
The initial problem reported to me was lots of gurgling and banging in the pipes when the HW was being heated. When I investigated, I found that the F&E tank in the attic was completely empty - the ball valve had stuck shut, and all the water had evaporated. That would explain the symptoms, I thought - with air entering the system because of the lack of top-up water. So I freed up the ball valve, allowed the system to re-fill and bled air out from all available places.
When I turned the system on, the boiler would run for a couple of minutes and then stop. The HW pump was running, but there didn't seem to be any flow - in that the flow pipe from the boiler wasn't getting hot. At this point, the pump speed was on its minimum setting - which is how I had found it. When I turned it up to max, the boiler fired for much longer, and the pipes got hot. I could still hear air going through the system though.
There is obviously still a problem. My feeling is that there is still air in the system, which I haven't succeeded in removing. With the pump on its lowest setting, there is no circulation - even though there used to be - and the water obviously doesn't get hot. So presumably, there is an air lock.
If I turn the pump speed up enough to get the water to flow, we're back to the dreadful gurgling and banging - all the way up to the F&E tank. On one occasion, the water in the F&E tank became very hot, and the level rose to the point where it started to overflow. [I began to wonder whether there was an internal leak in the coil in the HW cylinder, but the level went down again fairly quickly once I had turned everything off - so I'm pretty sure that's not the case].
My feeling is that, when the pump is run fast enough to make the water circulate despite an air lock, air is actually being drawn into the system through the vent pipe - what might be described as pumping
*under*, as per the title. Anyone come across this? In case it helps to visualise the system, I've drawn a somewhat crude diagram of just the HW circuit, which can be seen atAny suggestions as to how I can get all the air out, and get the system working as it did before the F&E tank dried up?
I'm almost tempted to suggest converting the primary circuit to unvented, which should make it easier to eliminate the air. But it's worked ok as a vented system for decades - so shouldn't be necessary.
Am I missing anything obvious?