Air pumped into F+E

[PSweeney]

Hi Guys, firstly apologies for visiting just for help but I'm £90 down to British Gas and still not able to sort my central heating. The problem is a huge rumbling and expulsion of steam / air and water into the F+E tank. This noise is super loud and I can only liken it to an overfilled kettle boiling but magnified 100 times. When it does this the F+E tank fills and overlows. Before I get into this, some history;

The DHW tank and F+E are in an airing cupboard upstairs. There is about 50cm height between the F+E tanks and the highest point (towel rail). We had an issue with the system pumping over and the last engineer to visit advised raising the height of the vent and turning the pump speed down. I did this and it's been OK for approx 4 years, but the system get loads of air. I drain maybe half a rads worth from a towel rail once a week (in one go)

Recently, this huge noise appeared in the airing cupboard and it transpired it's the cold feed puking water, steam and air up the submerged cold feed pipe into the bottom of the F+E tank. Bolier cuts out but can restart via thermal switch once cooled down. British gas came to look at it and and could not diagnose the fault as it was operating fine when they called. The BG engineer tested the temp of the flow and return and differential was 11 degrees which apparently rules out blockage and sludge. This cost me £90 for 30mins consultation....the guy said he had to leave as staying longer would bump it to £280....not happy.

The problem remains and occurs maybe every other day, no discernable pattern of HW/CH etc and having researched online I now see that the open vent is on the positive side of the pump tee'd off the flow at the point it enters the motorised valve. The cold feed is 15mm pipe and connects to the flow on the negative side of the pump approx 150mm from the pump. So, reading up, it appears the vent is on the wrong side, something I may address but it does not explain why the system is now puking air / steam up the cold feed.

When this occures, the pump seems laboured and the rumbling builds up and the F+E tank height rises and overflows. Initially I switched the boiler / pump off at the wall spur and this seems to curtail the bubbling but it would only cease fully once the system has cooled.

A few nights ago, I drained the system down, water was reasonably clean. The F+E tank water was like oil, I drained the F+E and cleaned maybe 6mm of sludge from the bottom. I refilled, bleeding away with X800 (squirted down the cold feed) and it's still sat in the system hopefully doing something although I'm not convinced this is sludge / scale. One oddity when filling was the upstairs rads filled up first from the cold feed ?

I've read about combining the cold feed and vent but I want to be sure this is going to help and not be dangerous. The point at which I could combine thiem is maybe 3-4 meters from where the cold feed joins the flow on neg side of the pump. My theory on this is that the open vent position is allowing see-sawing and pulling in air, goes through the boiler and gets stuck in the pump, which is either stalled by an air lock or siezing momentarilly. Air / Steam pressure buillds as boiler overheats, then finds it's way out the system via the cold feed (as vent is on other side of the pump).

I'm due to drain the stsyem tonigth but could really do with some proper advice. I'm loathed to call BG back out and I'm struggling to find an independant engineer to take this on.

Thanks

Phil

 

[Simon F]

hi, is it air that you are venting?
it could be hydrogen caused by a corroded system.
is the vent pipe blocked?
sounds like the pipework needs reconfiguring or is it an option to make it a sealed system?
what boiler, what options?

post your details in the looking for a plumber section,
someone local to you might respond

 

[PSweeney]

not yet tested whether it's air or hydrogen

vent defo open.

It's a potterton netaheat, not sure on model

tried to refill it tonight and the F+E tank struggled to fill the system even with every rad open. F+E ballcock almost shut off yet system not full, thats not right. Could cold feed be blocked ??

 

[PhoenixServices]

Could be cold feed partially blocked or an airlock. Backfill with mains on a drain off or shunt the pump.

 

[kay-jay]

Best to repipe it. Backfilling system or flicking the pump on are not gonna cure rhe cause.

If you have sludge in the f and e tank and the cold feed is blocked then your other symptoms are likely to be related to this.

Your banging and steam hissing will likely be system operating on low water content after pumping over and being unable to feed back in and overheating.

My advice
Repipe cold feed
Add cleanser to system
Run system for two weeks
Drain and refill
Observe system operation
If ok add inhibitor

 

[PSweeney]

OK so quick update, cold feed is not blocked, magnet does not stick to the tee and I managed to fill the system in under 30 mins. It wouldn't fill until I opened the drain cock so presumably it was again air-locked. System filled up and boiler switched on, the problem returned almost immediately.

Just to clarify, it's not pumping over, it's pumping boiling system water up the cold feed and the F+E tank level is rising and falling. The boiler cuts out as it should.

Although the open vent is in the wrong place, i'm not sure it's pulling in air as the level of hot water in the vent pipe is consistantly level or higher than the F+E tank.

Really stumped on this one.

 

[kay-jay]

If water is boiling you have fault with boiler or you have a circulation problem

 

[PSweeney]

OK just bringing this one back to the top for an update

Drained the system, ran X800 in it for 48hrs, drained it down and refilled, problem came back immediatley. Eventually restarting the boiler a few time all rads hot (all over) system working well, small amount of kettling at boiler, no noise in the pipes, then without warning huge rumbling, it violently pumps water out the open vent and water /air up the cold feed at the same time. Boiler cuts out, pump goes noisey and you can hear air in pipe and rads being circulated. If you leave the system to cool for 15 mins it will restart but likely pump over / up cold feed again. Leave it to cool few hours and works OK but we've noticed a pattern whereby it occurs when the boilers been on for 3-4 hours from cold.

I had a plumber round to look at it, he spent 2 hours looking at it, and he's stumped.

I'm not convinced it's blocked piping. The F+E tank has been cleaned out, water runs clear, rads all get hot, only 11degree drop over flow/return. It's been chemically flushed and magnets are not sticking to joints etc

Having looked at the system, I'm focusing now on the boiler. Can a heat exchanger be removed to check for blockages ? I think due to the sudden and violent nature of it, something is causing a blockage in the boiler and causing the water to boil producing steam and pressure between boiler and 3 way valve which is venting up the cold feed (first tee on flow) and going through the pump then up the open vent (which we know is technically in the wrong place)

Thanks for reading, I welcome any suggestions

 

[jam1979]

Is the pump on the right way round

 

[Aquazone]

To be honest, I think you had your answer in post 7.

Post in the 'Looking for plumber' section and get a professional to look and give you options.

Because a magnet doesn't stick doesn't mean it's not blocked. Had it loads of times and when pipe is cut out its about 5/6mm ID.

 

[PSweeney]

Is the pump on the right way round

it is but I suspect the pump maybe part of the problem. When this happens I think the pump is getting stalled or jamming, stopping circulation. The boiler carries on heating until it boils, creates steam and cuts out, steam has to exit somewhere, goes up the cold feed as its the first port. I think I'll be pulling the pump apart this weekend to check it out

 

[PSweeney]

To be honest, I think you had your answer in post 7.

Post in the 'Looking for plumber' section and get a professional to look and give you options.

Because a magnet doesn't stick doesn't mean it's not blocked. Had it loads of times and when pipe is cut out its about 5/6mm ID.

thanks, what could be wrong with the boiler to cause this expulsion of air / steam intermitantly and so violently ?

I've pretty much ruled out there being a blockage as the system fills quickly via the cold feed, gets hot all over the house (and across all rads) and the temp differential across flow and return is only 11 degrees. The systems has been flushed with X800 and operates fine until the boiler chucks out a load of steam / air which finds it's way out the system via the shortest route, which in this system is the cold feed and open vent after the pump.

If it was blocked, surely I'd have a consistent problem with circulation, manifesting itself as cold rads and a large temp differential across flow and return ?

 

[kay-jay]

thanks, what could be wrong with the boiler to cause this expulsion of air / steam intermitantly and so violently ?

I've pretty much ruled out there being a blockage as the system fills quickly via the cold feed, gets hot all over the house (and across all rads) and the temp differential across flow and return is only 11 degrees. The systems has been flushed with X800 and operates fine until the boiler chucks out a load of steam / air which finds it's way out the system via the shortest route, which in this system is the cold feed and open vent after the pump.

If it was blocked, surely I'd have a consistent problem with circulation, manifesting itself as cold rads and a large temp differential across flow and return ?

Im afraid we cant advise you on potential issues to do with the boiler itself other than to tell you to call a local professional to look at it.

 

[Dotty]

Job 38.11