Airlock in CH system?

[mcb7]

My CH (open-vented) system had undoubtedly got very clogged up with sludge - all the rads on the ground floor had very black water when I bleed them, though the water was clear from the upstairs rads, plus the system was very very slow to get warm.

So rather than just flush the system I thought I'd run it with some cleanser first for a few days. I went for Sentinel X400. The system has 18 rads, some single, some double so I went for a double dose (2 x 1 Ltr).

I started by tying up the FT ballcock and bailing out the FT which was very sludgy (brown stuff like wet mud). I then opened a drain value on one of the ground floor rads (there's no system drain point), and let about 3 litres of water out. Then using a hose, I poured the X400 straight into the pipe where the water would fill the system from the FT. I then untied the ballcock and let the FT and system fill.

I then went around all the rads and tried to bleed them. The ones on the ground floor were fine and no air was present but when it came the upstairs rads, I could hear air escaping for a short while but then no air or water was coming out. This was the same for all the upstairs rads. So I fired up the boiler but it only runs for a minute or so and none of the rads get hot. I've been around all the rads again several times but there's either no air/water or only a few seconds of air coming out of the upstairs ones.

So I'm guessing there must be an airlock but I don't know how I can find it / cure it. I can't see a bleed valve on the HW tank and the pump is a Grunfos Alpha 2L which I don't think has a bleed valve either.

All the rads are open on the manual side but I didn't open them up on the lock shield side - is that likely to help? Is there anything else I can do?
Thanks

 

[Crofton Gas]

Is three port valve in mid or heating position?

 

[Terrywookfit]

Was the air you heard escaping or sucking in.?
The lock shields should already be open, Unless you have closed them.

 

[Mottie]

I once had a similar airlock/filling problem when I replaced a radiator on my old vented system. The header tank would fill up but it wasn't going into the system. A plumber friend came round, cut about a metre of pipe off of my garden hose, stuck one end into the pipe that goes from the filler tank to the heating system, gave one mighty blow through the other end of the pipe, pulled the pipe out and whoosh - the system started filling up. Tried it a few times myself afterwards whenever I drained the system and needed to fill it and it worked every time.

 

[mcb7]

@Crofton Gas, I'm not sure where the valve is, all I can see on the side of it is a lever that has Auto and Manual positions - it's currently in the Auto position.

@Terrywookfit I'm pretty sure the air was coming out, I put some spit on the bleed hole to see if air was coming out silently and it momentarily created a bubble. Yes I haven't touched the lock shields so they are in their balanced system setting but from what I've read on here you should open them up fully if you were draining and refilling the system.

@Motman thanks I might try that. I was using a piece of hose to get the X400 into the system via the same method and I have to say the liquid didn't flow as freely as I would have expected.

 

[DaveHerns]

Maybe you have a blockage where the pipe from the F&E tank joins the system? I did and found it when trying to put inhibitor in the system the way you are doing.

 

[JohnD]

you say the system is open, old, sludgy, so a blockage is more likely than an airlock

Have you got a magnet?

see FAQ 8
https://www.diynot.com/diy/threads/faqs.37170/

 

[mcb7]

Thanks @JohnD, I'll give that a try. The F&E tank was very sludgy, probably about 2cm of brown sludge at the bottom.

 

[mcb7]

Thanks for all the replies. I left the system as I was away for a couple of days and the blockage appears to have shifted as I was able to bleed all the rads easily. The system is still very slow to heat the rads up though but that was one of the reasons for adding the cleanser. I'm hoping by leaving it in the system for a couple of weeks I'll have more success when I flush it.

 

[75pete]

I would be very tempted to remove each rad, one at a time, and flush them out. Now would be the time to have a go, before the weather really changes.

 

[JohnD]

yes, a bad one may benefit from a couple of cleans. If you can afford £100 or so, fit a system filter. You will be amazed and gratified at the amount of sludge it captures even after your clean and drain.

After you have loosened it with the X400, it will carry on washing around for months.

https://www.screwfix.com/p/adey-cp1-03-00022-01-2-magnetic-filtration-22mm/49961
the cheaper one is an old model, prone to leaks.