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

[Radian]

I'm talking about an open vented (heat only boiler) system here. While adding some more loft insulation I noticed a little surge of water coming into the bottom of the F&E tank every time the circulating pump switches off (and a little draw down when it starts). Nothing is present at the mouth of the vent pipe (not a pump over problem). The water in the tank can sometimes be a little cloudy and warm as well.

The layout of the pipework is exactly as recommended in the Boiler installation manual: close coupled feed and vent in the sequence: feed, vent, pump all well within 150mm of each other. The pump is on the primary flow pipe just before the S plan valves.

The heating system otherwise works just fine - with the exception being the occasional bubbling sound and need for certain radiators to be bled a little now and then. I know the feed pipe has a clear path to the system as witnessed by the rapid filling after drain-down. The vent pipe should also be in good shape (I replaced the original 22mm pipe with 28mm after experiencing pump-over from day one of the house being built - another story!).

What I understand is that there is nowhere for the water displacement to go other than to compress/expand trapped air which may be present even though I tend to bleed it off when I suspect it's there - OR for the pressure change (pump start/stop) to shift water between the F&E tank and the open vent pipe? How I wish I could see the level of the head in the vent pipe! If this was going up when the level in the tank drops and vice-versa then that would fit the bill but why would this not always be a problem for all vented systems?

 

[John.g]

I expect that when the pump stops that there is a pressure surge which is higher at the mot. valve, pump and cold feed which causes the level to rise in the cold water leg which will have to be replaced by a fall in the vent pipe level.?
What I find more difficult to understand is when the system is operating normally where you might have 15 LPM flowing in a 22MM pipe with say 100mm distance between the vent and cold feed, the pipe friction loss in this 100MM will be ~ 0.0052M and as the F&E tank can have a surface area of 350 times that of the 22MM pipe then one might think that there could be up to 1.8M difference in head (height) between the vent pipe level and the F&E tank level or somewhere in between. A bit of clear plastic pipe on the vent would soon show what is going on.
Generally, a combined vent and cold feed stops all this.

 

[Radian]

Hi John.g, I agree with your friction loss figure but I'm not sure about multiplying by the area ratio of 350...? I would just expect to see ~5mm change in the vent pipe and 1/350th of that delta in the tank at most.

I thought about having a clear section in the vent when I had problems with over-pumping a long while ago. I fixed that by replacing the original 22mm vent pipe with 28mm. The house is on the large side with 19 radiators and the boiler downstairs is connected by a ~12m long pair of 28mm pipes to the upstairs airing cupboard. Despite the best efforts of the installation team (who even raised the F&E tank as high as it would go in the loft when they came to replace our original boiler) it continued to over-pump. The original pump was also replaced in the boiler swap and I think the new one was producing a greater head to overcome the greater restriction in the newer heat exchanger even at its lowest speed. Then when I thought about the velocity differential between the 28 & 22 pipework I could see how the vent might over-top. Sure enough restoring the F&E tank to its original height but replacing the 22mm vent with 28mm made the problem go away.

 

[Chuck]

I suspect that at the level of detail you are applying in your analysis you need to include some dynamic effects. In principle, the coupled levels in the tank and vent pipe form an oscillatory system. The kick that occurs when the pump starts (or stops) may be exciting damped oscillation, and the surge of water into the tank you see is the transient response. Changing the vent pipe diameter will change lots of parameters in this model.

I haven't thought this through in any detail so I may be wrong.

 

[John.g]

I always think of the F&E tank and the vent (U tube) as akin to a well type manometer where the slightest change in pressure on the SURFACE of the vent (the measuring point of the manometer) will cause a huge difference in the relative heights (proportional to that area difference) but I suppose not quite the same with pipe friction losses at that ~ 100mm distance.
After reading all the tales of grief on here I'm glad my house was built > 50 years ago with a combined cold feed & Vent.

 

[Chuck]

I always think of the F&E tank and the vent (U tube) as akin to a well type manometer where the slightest change in pressure on the SURFACE of the vent (the measuring point of the manometer) will cause a huge difference in the relative heights (proportional to that area difference)

A given change in pressure at one surface will cause the same change in height difference independent of the areas of the surfaces involved.

delta(p) = rho * g * delta(h)

 

[John.g]

The same change in height difference but not equal changes in each leg if different areas?.

 

[Chuck]

The same change in height difference but not equal changes in each leg if different areas?.

You wrote 'difference in relative heights', which is 'h' on your diagram. The changes in the absolute heights, i.e. measured with respect to a fixed point, differ in the way you describe.

A picture is indeed worth a thousand words!

 

[John.g]

Do you think that the flow rate and resulting friction loss as in post #2 can result in any significant change in absolute heights and in which direction? ie + or - change in respect to vent height level.

 

[John.g]

I'm talking about an open vented (heat only boiler) system here. While adding some more loft insulation I noticed a little surge of water coming into the bottom of the F&E tank every time the circulating pump switches off (and a little draw down when it starts). Nothing is present at the mouth of the vent pipe (not a pump over problem). The water in the tank can sometimes be a little cloudy and warm as well.

The layout of the pipework is exactly as recommended in the Boiler installation manual: close coupled feed and vent in the sequence: feed, vent, pump all well within 150mm of each other. The pump is on the primary flow pipe just before the S plan valves.

The heating system otherwise works just fine - with the exception being the occasional bubbling sound and need for certain radiators to be bled a little now and then. I know the feed pipe has a clear path to the system as witnessed by the rapid filling after drain-down. The vent pipe should also be in good shape (I replaced the original 22mm pipe with 28mm after experiencing pump-over from day one of the house being built - another story!).

What I understand is that there is nowhere for the water displacement to go other than to compress/expand trapped air which may be present even though I tend to bleed it off when I suspect it's there - OR for the pressure change (pump start/stop) to shift water between the F&E tank and the open vent pipe? How I wish I could see the level of the head in the vent pipe! If this was going up when the level in the tank drops and vice-versa then that would fit the bill but why would this not always be a problem for all vented systems?

"The layout of the pipework is exactly as recommended in the Boiler installation manual: close coupled feed and vent in the sequence: feed, vent, pump all well within 150mm of each other. The pump is on the primary flow pipe just before the S plan valves."

VCP..... (boiler), Vent then Cold Feed then Pump?

 

[Radian]

Well spotted. I did indeed type that sequence wrong! It is actually Vent, feed then pump. The 15mm feed is reduced down to 22mm behind the spirovent as per photo.

 

[Radian]

So I just recorded a 5mm fall in the F&E tank when the CH came on this afternoon. You should be able to see a steady drop in level in less than 10 seconds. Given the tank is roughly 0.45m x 0.3m this represents 675ml going somewhere... if it was all transferring into the 28mm vent (the only place it can really go other than compressing trapped air) then the extra volume would raise the level in that pipe by around 1.4m
675ml is also about 4m worth of 15mm pipe so if not all transferring to/from the vent, it would explain why it's 'reaching' into the hot water.

 

[John.g]

How far above the water level is your vent gooseneck, I often see figures of 450mm minimum but could never figure out why, or maybe this only applies to HW cylinder vents.

 

[Radian]

How far above the water level is your vent gooseneck, I often see figures of 450mm minimum but could never figure out why, or maybe this only applies to HW cylinder vents.

The old BS6700 (in force when my property was built) just said:

the vent pipe shall run from the highest point of the primary circuit to a point above the primary feed and expansion cistern at a height that will prevent a discharge of water from vent pipe and/or air entrainment into the system under normal working conditions. Due allowance shall be made for the head induced by any circulating pump used

I atually have 1575mm as the vent is provided by two connected 3m lengths of 28mm Pex Barrier Pipe forming a broad arch which I can walk under. This is because the rise from the airing cupboard lands on one side of a walkway in the attic with the F&E tank on a platform the other side. The original 22m copper vent pipe ran most of the way horizontally before rising for the gooseneck which seems to go against the standard:

No valves shall be fitted to any vent pipe and the pipe shall rise continuously from its point of connection to the hot water system to its end except where it is permitted to be bent so as to terminate downwards (see figures 5 and 6). Vent pipes shall not be less than 19 mm bore.

Anyway, even if the full 675ml displacement is all going up the vent pipe, then I've still got some headroom before it spills over. I never verified that this was the reason my 'fix' worked. I think I'm more mindful of the energy loss than I was then - what with the prices going through the roof (literally).

Also from the old BS6700:

One pipe shall not serve as both open vent pipe and cold feed pipe, unless the associated system or circuit has: 1) the energy supply to each heater under thermostatic control; 2) the energy supply to each heater fitted with a temperature-operated manually reset energy cut-out independent of the thermostatic control; and 3) a temperature relief valve in accordance with BS 6283 : Part 2, or a combined temperature and pressure relief valve in accordance with BS 6283 : Part 3, e.g. as required by BS 7206 and BS 3456 : Part 102 : Section 102.21 and BS EN 60335-2-21.

Which is why I shied away from combining them (the installers also initially tried this but it didn't work so restored separate feed/vent when they raised the F&E tank onto a higher platform.

 

[Chuck]

Do you think that the flow rate and resulting friction loss as in post #2 can result in any significant change in absolute heights and in which direction? ie + or - change in respect to vent height level.

If I understand what you are saying in #2, you are predicting a pressure difference of 5mm(H2O) between the bottom of the vent pipe and the bottom of the feed pipe. This would be matched by the change of height difference at the top. So, I'd expect the absolute height of the free surfaces in the vent to rise by ca 5mm and the absolute height of the water in the f/e tank to fall by 1/350*5mm.

Now that I know we're talking about 650ml of water being sucked out of the the f/e tank, the effects I commented on in #4 are clearly not the explanation. At the moment one possibility seems likely to me:

1. A largish volume of trapped air somewhere, but there would need to be several litres hiding, which is a bit high but it is a biggish system so it might have found somewhere to hide.

and I also have a long-shot, if the the DHW is also vented and the tank heights happened to to be close enough for the levels to match:

2. A leak in the DHW heat exchanger coil. This would mean your 5mm(H2O) pressure difference would pump 0.65 litre of water from the F/E tank into the DHW system so the F/E level would fail by about 5mm and the larger area CW feed tank level would rise by, say, 1mm or so.

 

[Radian]

and I also have a long-shot, if the the DHW is also vented and the tank heights happened to to be close enough for the levels to match:

2. A leak in the DHW heat exchanger coil. This would mean your 5mm(H2O) pressure difference would pump 0.65 litre of water from the F/E tank into the DHW system so the F/E level would fail by about 5mm and the larger area CW feed tank level would rise by, say, 1mm or so.

Good thought Chuck, but I replaced the DHW cylinder with shiny new SS one earlier this year so I doubt that's the cause. But trapped air has always been a big problem in this system. I dread drain-downs because we have to spend several days bleeding rads. I even rebuilt the pipework to make manifold style rails for the S plan valves with a deliberate rise towards an automatic air bleed valve at the highest end (the common distribution rail for the pump output side)

 

[Chuck]

I dread drain-downs because we have to spend several days bleeding rads.

If you have been using top-down filling via the float valve in the F/E tank, try bottom-up filling by putting a hose onto the drain-down plug. Don't take things too fast.

 

[Deleted member 123817]

Is there any compression fcoupling on the vent that's say 0.5 M or so higher than the water level that you could break and check for water?

 

[Radian]

If you have been using top-down filling via the float valve in the F/E tank, try bottom-up filling by putting a hose onto the drain-down plug. Don't take things too fast.

I have tried that before but it wasn't much help. Thinking about it now, there's a bleed point on top of the boiler conveniently situated by the back door. At least this might purge the boiler drop which is always where the first big air lock is to be found.

Is there any compression fcoupling on the vent that's say 0.5 M or so higher than the water level that you could break and check for water?

I think that's going to be my next move. The two 3m lengths of PEX have a coupler between them. I can wrangle the first one around a bit to keep it above the F&E level - but will get the missus ready on the system main switch just in case it starts to overtop. Will have to wait for the weekend to try it.

 

[Radian]

The level in the vent pipe isn't changing significantly from what I could see.

I have hot-wired the main circulation pump so I can run it at will. This made it easier to look at the drop in the F&E tank when opening each of the three zone valves in turn. With all three closed, the circulation is just through the auto bypass and the level in the F&E stays the same whether the pump is running at maximum or switched off. With the hot water cylinder zone valve open the level changes by up to 3mm. The single room underfloor heating zone shifts it 4mm to 5mm and the radiators 7mm to 8mm. At the usual pump speed the change from the radiator zone is 5mm as I reported in my OP.

So the change in level seems to be directly relate to the size of the hydraulic circuit and the pump speed. Logically the only place for the displaced water to go is into an air pocket but it has to be outside the main loop: boiler->pump->bypass->boiler as circulation here fails to affect the F&E level (I think it also rules out the T-off for the vent as this is under more pressure when the smaller circuits are in use due to their lower frictional losses - the exact opposite of what is happening).



Valves left to right: Bypass, radiators, (pump), UFH, HW. The return for the bypass and radiators T's into the return leg back to the boiler under the floor.

As it came on early this morning the boiler threw a fault (F25 usually meaning air in system) and there was indeed a huge amount of air circulating around. This is the first time it has done this since I sorted the over-pumping (the vent is still not discharging) so now I'm looking for air leaks.

 

[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.