System venting water

[fixitflav]

Hope somebody can help with this!

I have a conventional system, heat-only boiler, F/E tank, HW cylinder, diverter valve. Upgraded an earlier system in 1998, plumbed boiler - open vent (22mm) – cold fill (15mm) - pump. Open vent and cold fill are the recommended 150mm apart. Cold fill up-and-under. Both pipes go straight up to the F/E tank.

When I retired in 2010 I did some improvements including fitting a Magnaclean and changing the galvanised F/E tank for plastic. There was some sign of water coming out of the open vent, so I lengthened it to 750mm above the water in the F/E tank.

I’m fairly sure it was OK after that (no more water coming out of the vent).

Earlier this year the pump (Grundfos 15-50) was getting noisy, so I changed it for a Grundfos UPS 2. Everything seemed fine, but the other day I happened to feel the pipes and the open vent and cold fill were hot. There was a steady trickle of water from the open vent.

The vent outlet is brought back down to just below the cover of the F/E tank, but the trickle is nowhere near enough to form a siphon (and if I thought it was I would drill a hole in the top of the inverted U). The system has Sentinel inhibitor and 2 – 3 weeks ago I took a sample when emptying the Magnaclean for a bright wire nail test, and they’re still OK.

I’ve turned the pump down to minimum, maybe a little improvement, but it hasn’t stopped. The trickle starts immediately HW is called, doesn’t have to get hot first.

I’m baffled, as according to my calcs the headloss in 150mm pipe length is < 0.2m, even at 5m3/h, which is something like the pump output at maximum setting and zero head. And at say 2m3/h at minimum setting I get headloss 0.03m. So I would expect my 0.75 m to have stacks in hand even in worst case. Two boiler manuals I have both say minimum height of open vent loop 450mm above F/E tank water level.

Some pics attached. I don’t think anything outside the airing cupboard has any bearing on the case, but I can give more details – boiler model etc if necessary.

Has anybody had anything similar or can make any suggestions?

 

[stem]

Sounds like the system is 'pumping over' it's been discussed on here at great length, causes include incorrect pipework, or blocked feed pipe. If you search this forum under 'pumping over' you will find the threads.

There is also the FAQ's in the Wiki.

 

[dilalio]

Replace the “h” close-coupled feed and vent with a combined feed and vent or seal the system. Resistance is causing water to take the easiest route.

 

[Gasguru]

A combined feed and vent or a sealed system can only be considered if the boiler instructions permit it.

I would check the pipe work around and between the feed and vent pipes for partial blockage.

Sort out that dodgy wiring...that's a proper diy lash up the pipework's rough.

 

[dilalio]

Compression fittings everywhere

 

[picasso]

The gate valve you have on the flow to the rads is it fully open ? and why do you have a gate valve on the flow to the rads ?

 

[fixitflav]

I would check the pipe work around and between the feed and vent pipes for partial blockage.

I believe I'm right thinking the only place a partial blockage would cause these symptoms is in the 150mm between the open vent and cold fill connections on the pipe. I suppose that can't be ruled out, but it seems pretty unlikely. Anywhere else and blockage would reduce the flow, making the symptoms less likely.

But I can't think of a more plausible explanation, and it shouldn't be too hard to take a section of pipe out and rod it, so I'll do that sometime.

Sort out that dodgy wiring...that's a proper diy lash up the pipework's rough.

I know it's traditional to criticise the previous guy's work, but what's dodgy about the wiring? How would you or a sparky have done it? What's wrong with a strip connector? It's been OK for 20-odd years.
Maybe a bit more solder showing than a real expert would leave, but I can't see much else wrong with the plumbing.

 

[fixitflav]

The gate valve you have on the flow to the rads is it fully open ? and why do you have a gate valve on the flow to the rads ?

Yes, it's wide open. I added it as an aid for possible future maintenance, and it's been useful when I've had to work on the 3-port valve. Also I don't entirely trust the pump iso valves to give perfect shut-off.

There's a bit of a development - the symptoms I described are when it's HW only calling (either/or CH/HW, not a mid-position valve). Now the weather has cooled off I've tried it on CH and it was OK, at pump minimum. Switched pump to max and it started again. Went to pump mid and it continued. Back to min and it stopped.

So it does seem to be flow-related. The HW circuit must be quite low resistance, 28mm pipes except the last bit in the airing cupboard, as originally it was gravity HW, but there's still the boiler. But it's still way outside what I would expect, and obviously it's undesirable.

 

[flameport]

but what's dodgy about the wiring? How would you or a sparky have done it? What's wrong with a strip connector? It's been OK for 20-odd years.

Nothing wrong with strip connectors, but they must be installed in an enclosure and the cables / flex need to be fixed in position.
Both normally achieved by using a junction box designed for the purpose.

Installing it somewhere that is easily accessible is also recommended, and flex should not be left resting against hot pipes, even if it is heat resistant.

 

[fixitflav]

Nothing wrong with strip connectors, but they must be installed in an enclosure and the cables / flex need to be fixed in position.
Both normally achieved by using a junction box designed for the purpose.

View attachment 142352

Installing it somewhere that is easily accessible is also recommended, and flex should not be left resting against hot pipes, even if it is heat resistant.

OK thanks, point taken!

 

[JohnD]

start by giving it a chemical clean with Sentinel X400 or similar. It's a cheap and easy DIY job and may do the trick. Even if you end up needing additional work, cleaning out sludge and sediment will be a good start.

Meanwhile, purchase a strong neodynium magnet and test all the copper pipes for attraction, especially T-joints near the feed pipe. Some magnetite deposits, especially if combined with limescale, can be very hard and need to be cut or poked out.

These extending probes are very good for reaching into awkward corners

 

[fixitflav]

Sounds like the system is 'pumping over' it's been discussed on here at great length, causes include incorrect pipework, or blocked feed pipe. If you search this forum under 'pumping over' you will find the threads.

There is also the FAQ's in the Wiki.

OK thanks for that. I've done it, there's plenty to read on the subject!

 

[Hot&Cold]

Its all about circulation which will take an easy route will take the easy route to the nearest vent,seems like a poorly installed system.

see 100's of them

 

[JohnD]

But it's an old system, and surely hasn't been pumping over for the last thirty years. All the rads would have rusted away by now.

 

[Gasguru]

I believe I'm right thinking the only place a partial blockage would cause these symptoms is in the 150mm between the open vent and cold fill connections on the pipe. I suppose that can't be ruled out, but it seems pretty unlikely. Anywhere else and blockage would reduce the flow, making the symptoms less likely.

But I can't think of a more plausible explanation, and it shouldn't be too hard to take a section of pipe out and rod it, so I'll do that sometime.


I know it's traditional to criticise the previous guy's work, but what's dodgy about the wiring? How would you or a sparky have done it? What's wrong with a strip connector?

Are you for real...exposed mains terminals and that's acceptable?

Pipework supported on bits of floorboard, nasty ball pump valves (that always leak and seize up), etc.
We come across plenty of systems that have run for decades then begin to play up...often because the original design was marginal. Blocked pipework is not unusual...as mentioned get yourself a magnet. It won't pickup the non ferrous scale but it often gives a clue as to the state of the pipework. I wouldn't bother with chemical cleaning at this stage...just cut out feed, vent etc and check.