Water exchange in F&E tank via feed pipe to system

[Radian]

The remodelling work I did a few weeks ago involved a complete drain-down and from calculating the amount of system water (using the different expansion level heights at two given temperatures) I see that I now have around 200l of which I understand up to 17% by volume could be entrained air. If all that was released then, eventually, up to 30l of air might find its way into the tops of rads and awkward legs of pipework requiring a significant amount of bleeding off. A rough calc. suggests that a 3m head from the pump might be enough to displace the kind of amounts of water seen in the F&E tank even if only a few litres of air was spread around the system.

Also, I've been able to get a much better idea of what's going on in the vent pipe after getting my old FLIR camera working on my phone again (a new phone with USB C which needed an adaptor - the first one I bought didn't work but I've now managed to find one that's compatible).

So on the left it's showing the level in the vent pipe with the pump on, then on the right, maybe a 1cm rise when not pumping. This is similar to what it looks like in the F&E tank so the displacement isn't into the vent but the general level in the system goes down when under pump pressure.

 

[John.g]

In post #12 you say 5mm change in F&E tank = ~ 675ml so 1cm must = 1350ml or 1.35Litres, its not appearing in the vent so does this indicate that its compressing the entrained air in the system water to explain this?.

 

[Radian]

In post #12 you say 5mm change in F&E tank = ~ 675ml so 1cm must = 1350ml or 1.35Litres, its not appearing in the vent so does this indicate that its compressing the entrained air in the system water to explain this?.

I think so. The tests I did in post #20 were done with the pump at maximum so yes there's over a litre being taken up by air somewhere under these conditions - and the fact that it varies with pump speed might be seen as a confirmation. I think the important thing is that it is not (the very common) issue with vent pipe layout so I'm not rushing to re-pipe this to combine feed and vent. As I mentioned, this was tried by the firm that did my boiler replacement way back in 2008:


Granted, that was not done as you would probably recommend (the 15mm pipe to the T in the vent snakes about 0.5m to the tank outlet rather than being as close as possible) but it was absolutely not preventing copious amounts of pump-over.

I'd be curious to know what you make of my solution - changing to 22mm vent pipe to 28mm? I got the idea when thinking about the change in velocity of the water coming into the pump inlet as it transitioned from 10 or more meters of 28mm copper from the boiler down to the short 22mm run (including the two 22mm T's for vent and feed) leading into the pump. I wish I had some hydronic simulation software equivalent to the kind I use for electronics! The latter are really good - and free! The plumbing equivalents all seem to ask for £££.

 

[John.g]

Difficult to say if your cure has actually worked as you had large quantities of air this AM introduced IMO by running the pump at very high heads during tests, the only logical place where I can see being introduced is via the vent even though no pump over or excessive see sawing.
Of course combining the cold feed & vent in the manner like mine may not cure the problem either, it certainly has a better chance of doing so than in the original set up. My circ pump is mounted on the return, very close to the boiler on the ground floor.

 

[Radian]

Difficult to say if your cure has actually worked as you had large quantities of air this AM introduced IMO by running the pump at very high heads during tests, the only logical place where I can see being introduced is via the vent even though no pump over or excessive see sawing.

One neat thing about the thermal camera is that it 'sees' a record of the temperature over a period of time - if you just touch a surface and remove your finger you can still see the 'fingerprint' for a surprising amount of time (watch out for people with odd looking cameras loitering around cashpoints!).

In the case of the pex vent pipe the thermal 'imprint' lasts several minutes given the contrast between the cold environment and the hot system water. Because of this I'm totally convinced that the level in the vent is stable within approx. 1cm - despite my best efforts to 'shock' the system by rapid starting the pump/shutting valves etc.

Given that this level in the vent pipe is always a couple of meters above the connection point to the system I really can't see how air could get down there - but my intuition isn't always infallible ☺️ I guess we're just spinning our wheels until either all the air is given up or I find a leak. I copied your trick of turning off the supply tap to the F&E tank to see if fresh water is being drawn in. I even stationed a Raspberry Pi camera looking at the tank level (where my video above came from). I haven't seen any losses yet.

 

[John.g]

I only open the F&E tank filling valve every few months (to check for any leaks) and I have never ever heard a drop of water making up which I find extraordinary in a OV system, unfortunately my tank is so high up that I have no head room to look into it, I must rig up a periscope to have a look in., the water level is ~ a meter above the top of the attic converted bedroom rad top which runs with a very slight negative pressure but still gets perfectly hot and has never caused any problems as > 35 years old.