Nightmare airlock in F&E system

[Jonny]

Hi all,

Got a nightmare situation with a central heating system I recently adopted. It's an open vent system with a combined F&E pipe, with a Worcester greenstar danesmoor 18/25 oil boiler. Covers 10 radiators mostly large double panels, some towel rails and smaller rads too, as well as an indirect gravity fed HW cylinder. 2 rads upstairs the other 8 downstairs. F&E tank is in upstairs attic above highest rad. Some 10mm microbore branches, the rest is 15mm branches from a central-ish point. It's never worked right since day 1, lots of gurgling in pipes and boiler would often cut out/lock out due to air inside it so clearly the system had air in it. Anyway the time has come to sort it out, long story short the system was recently drained down about halfway ie. Upstairs rads and underfloor piping only was drained plus one downstairs rad.

Upon trying to refill, no water came through from the F&E tank. Removed top plug in the towel rad and tried using a wet vac to suck water through - no luck. Tried using and pulsing the CH pump to suck water through, nothing. Eventually managed to refill the upstairs rads by shooting mains pressure water into the system through the drain off, however there's still airlocks in some piping as we can hear it and the boiler keeps cutting out due to air inside it. All the rads in the house bleed water not air, so they appear full. Pump (DAB evosta 3) does run but at only 4-8W when set to maximum speed (it self regulates) and it sometimes shuts off with error code 'running dry'. Spent literally all day playing with mains water pressure, sucking, pulsing the pump, draining more water through the system etc. Whatever air locks exist they will not go.

F&E combined pipe is definitely not blocked, it connects to the system just before the pump so the pump should be pulling water through, but isn't. CH and HW valves been on fill/vent/open setting throughout. We can also see and hear air bubbles going up through the pipe into the f&e tank so it appears to function like it should. Pump assumed to be operating correctly as it was replaced recently since the old one was knackered.

Would really appreciate any advice, I'm slightly losing my mind over this as I can't see any logical reason why the airlocks can't be removed or even found especially since all the rads are full of water!

Thanks, and sorry for the long post, but hope it explains the system reasonably well.

 

[Ricky]

they are often on a y plan system with no vent on top of the pipework, a picture of layout would help

 

[Jonny]

they are often on a y plan system with no vent on top of the pipework, a picture of layout would help

Haven't got a good pic unfortunately, but can get one tomorrow. However it is an S plan system, and there are no automatic airvents, there is a manual air vent however, which is at a level above all the downstairs rads and probably about halfway up the upstairs rads. Like the radiators, this bleeds water not air, which isn't really helpful.

 

[John.g]

As above, pic of combined vent and cold feed and see if the vent carries on up and over the F&E tank.
The pump is running dry at only 8 watts, it probably shows no flow as well, 0.0 m3/hr?. so shut it off or set it to the speed setting 2 or even 1 until all air is eliminated.

 

[Jonny]

As above, pic of combined vent and cold feed and see if the vent carries on up and over the F&E tank.
The pump is running dry at only 8 watts, it probably shows no flow as well, 0.0 m3/hr?. so shut it off or set it to the speed setting 2 or even 1 until all air is eliminated.

Yep will do tomorrow morning. Since the vent and feed are combined in one pipe its just connected to the bottom of the tank like a standard feed pipe. Correct, it shows 'Low' m3/hr, I was only using the pump on high as a tool to try to pull water through, appreciate that's not really the best thing to do! Will turn the speed down.

 

[ShaunCorbs]

Sounds like you have a blockage in the feed pipe time to get a magnet out

 

[Jonny]

Sounds like you have a blockage in the feed pipe time to get a magnet out

I thought the same initially, but I've seen the F&E tank empty rapidly when the main draincock is opened, plus air bubbles go up it so it can't be blocked. But good shout, I'll do the magnet check at some other points in the pipework to check for any major restrictions like at motorised valves etc. That could certainly explain why rads were impossible to fill without mains pressure!

 

[John.g]

A combined vent and cold feed system must be filled very slowly by either tying up the ballcock to only allow a trickle of water to flow or installing a isol valve on the ballcock makeup, and the F&E tank should be the last to start filling, also on any system, the motorized valves should be latched open while filling

 

[Jonny]

A combined vent and cold feed system must be filled very slowly by either tying up the ballcock to only allow a trickle of water to flow or installing a isol valve on the ballcock makeup, and the F&E tank should be the last to start filling, also on any system, the motorized valves should be latched open while filling

That's the strange thing, I'd heard this advice so that's how I tried to fill it, but the water was literally going nowhere, so thought maybe theres an airlock in the feed pipe and more head pressure would help, so filled the tank all the way. I did this a couple of times after draining down a bit to get things moving, similar result each time. Zone valves were in the fill/vent position, assuming they work correctly of course but I have no reason to think they don't.

 

[Robooo]

Maybe the position of the pump is sucking air into the system through the F&E pipes (cold feed or vent pipe).
Make sure that the positioning of the pump is correct. Take into account the head hight of pump.

 

[John.g]

The whole system may be sludged up, the condition of the F&E Tank is a good indication or/and the magnetic filter, if installed.
Might be hydrogen gas?

 

[Jonny]

Maybe the position of the pump is sucking air into the system through the F&E pipes (cold feed or vent pipe).
Make sure that the positioning of the pump is correct. Take into account the head hight of pump.

The combined cold feed and vent is connected ~50cm before the pump, sucking air in seems unlikely since the pipe is full of water but possible? F&E tank approximately 3-4m above pump

 

[Jonny]

The whole system may be sludged up, the condition of the F&E Tank is a good indication or/and the magnetic filter, if installed.
Might be hydrogen gas?

Having cut open and cut out a number of pipes and replacing the pump etc. they don't seem overly sludged up, F&E tank had typical sludge in the bottom but this was cleaned before draining down initially so sludge shouldn't have been sucked in. Obviously can't speak for what may have happened previously but from what I've seen pipes are clear enough aside from black residue on the inner walls

 

[John.g]

The combined cold feed and vent is connected ~50cm before the pump, sucking air in seems unlikely since the pipe is full of water but possible? F&E tank approximately 3-4m above pump

Is this what you have with the vent and cold feed< 150mm apart? and is the pump pumping away from them? If its like the one in Rev1 then should be OK.

 

[Jonny]

Is this what you have with the vent and cold feed< 150mm apart? and is the pump pumping away from them? If its like the one in Rev1 then should be OK.

It's like this, distance between pump and F&E pipe connection around 50cm. I've seen a number of systems done like that, is that still a correct way of doing things?

 

[John.g]

Mine, like rev2 above still has the vent carried on up and over the tank so this may make a difference, it certainly does IMO while filling very slowly as the the air can pass straight up and out the vent (thats my theory anyhow), also during normal operation it may make some difference as well, some of my rads are over 40 years old with minimum inhibitor (if any) for years).

 

[Jonny]

Just an update, managed to finally get it working today, installed 2 AAVs on pipework where it looked like air could collect, there is also a manual bleed valve as well nearby. Still no luck with filling the system conventionally via the F&E tank, so again I back fed mains water through the draincock and managed to fill it up this time. Some purging with the pump seemed to help push all the air through and out of the AAVs, heard plenty of hissing! There is still a bit of air in the system by the sounds of it occasional gurgling etc. but hoping it will find its way out of the AAVs.

Based on this does it sound sensible to convert the system to a sealed system at some point in the future? The boiler is actually suitable already...

 

[Jonny]

Red circles show my new AAVs and the manual air vent. The higher AAV is on a high point return leg that covers all the rads but not the cylinder. The lower AAV is on a high point flow on a flow leg covering 4 rads, was hoping there would be enough flow through these to look after any stuck air. As you can see the plumbing generally is an absolute pile of **** some day it will hopefully all be redone. Combined F&E pipe is just out of shot to the left in the flow pipe leading to the pump.