Open Vent - fully pumped....slow refill, possible airlock?

[dishman]

Hello all.

I drained my system down a while ago. I moved a radiator, and added a boiler filter too as it seemed sensible.

I followed a strict checklist for the refill.

All valves open....bleeders closed. Three way valve to manual....etc...

Started filling the header tank.

Then began bleeding the last/lowest radiator on the system.

Initially I had good air...but then it just withered and died.....

Tried a few other ground floor rads….but...similar story...

I then went under the floor to the drain cock, so the lowest point in the system.....opened that....and heard bubbling in the system and even with a hosepipe attached I got dripping from the connection. Although this was enough to cause dripping at the other end of the hosepipe too (it had slight upwards travel) it did not become a regular flow.

I cleaned out the header tank before filling with a vacuum. So I don't think any crap got flushed down it.

Any ideas? I am leaning to more of an airlock rather than a blockage.....but...unsure.

I did not want to start randomly draining other parts of the system.

Thanks in advance.

 

[ianmcd]

a gravity HW system and an open vent system are two completely different things, if you have what you call a three way valve you do not have a Gravity system you have a fully pumped open vent system, not the same thing. you probably have a blocked cold feed

 

[dishman]

Yes, sorry....fully pumped open vent.

What is the diagnosis method for cold feed blockage? Magnet to that area?

If some water is getting through...and the three way valve is on manual....when I open the drain/bleed valve for the cylinder coil....should I expect at least some escaping air or a drip?

 

[Mottie]

When I had an open vent system, I had a bugger of a job filling and bleeding it until a plumber I had in showed me what to do. He got a 2 ft length of hose pipe, put one end into the fill pipe at the bottom of the header tank and gave it a good long blow at the other end and then whipped it out quick. There was a gurgle and then the water would rush into the system. I used that method several times successfully after that.

 

[dishman]

I was thinking of doing something like that....but did not want to go about it without checking.

So basically small section of pipe in the central heating header the, and just stick in in the fill pipe opening....and blow?

 

[Mottie]

Yep. One big lungful and then pull the pipe out.

 

[ianmcd]

yaes magnet around the Tee piece where it connects, if attracted cut it out and re-pipe

 

[dishman]

I traced the cold feed and checked the first cold feed junction (which seems to be the common place for a physical blockage to occur) with a magnet, and felt no sort of magnetic attraction. I may be able to exclude a physical sludge blockage at this point.

I may try the hose method next.

I assume this should be done with at least one bleed valve open?

 

[polesapart]

Or connect a hose from the mains to an open draincock and force water back up.

Position a helper at the expansion tank to shout/phone when to stop.

 

[dishman]

That is also doable....although sounds less fun...I'll try the blow method first

 

[dishman]

Hmm, no good on the blow method....

The pipe is the same size as the outlet so can't make a great seal...it seems to bubble back into the tank.

I have on a few tries got a good seal and I seem to be able to blow into the cold feed pipe well enough but nothing seems to happen.

Is there a chance the 3 way valve is buggered?

 

[dishman]

So, current update....

I went to the drain cock under the floors again and just had a bucket (rather than a connected hose pipe) so I could see the flow directly.

It was a pretty constant dribble....then every so often would slow to nothing.

I have tried the blowing with a hose in the header tank and it did not seem to shift anything. Or have any impact.

Any other trouble shooting ideas?

I'm going to test the pipes with a stronger magnet. See if that detects anything.

Could this be an issue with the 3 way valve?

I opened the drain cock on the hot water tank coil and again, this slowly bubbled up with a slow but consistent dribble.

There is a gate valve after the three way valve and before the cylinder (if I have got the direction of flow correct). I hesitate to mess with these as I know they can be used to balance a system. But....this one is less than half way open. It is about 2 turns open from a total 5.5. I am pretty sure I did not adjust this in the interim.

Any other troubleshooting ideas? Anything obvious I have missed? Airlock around the pump? Could the boiler have an airlock?

 

[Mottie]

Connect a hose pipe up to the drain and force some water in?

 

[dishman]

I'd rather do that as a last resort....I can see it going wrong and me getting very wet

 

[dishman]

Thought: Could the filter I added be causing a large airlock before the boiler?