Gravity Hot Water circulation

[Glanmor]

When the pump is running, the pressure at boiler return is higher than at flow, due to the boiler headloss. This tends to cause flow in the "wrong" direction through the coil. But on a big old boiler headloss won't be much, maybe only ~ 0.2m, clearly not enough to cure the problem.
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Fixitflav, you might just have explained another strange behaviour i noted...
During packup, after failing to move this issue, I flick switches on the control pannel to Central Heating. This brings on a strange effect. It begins to pull water out of Flow, such that the flow pipe becomes cold very quickly, and the Reurn pipe gets boiling hot instead, though it does not get hot all the way to the cylinder. Not sure if this is the culprit.

In my next attempt, im considering opening draincock near the Boiler and letting slow escape of water from there. This would soon kick-in a slow Feed supply from the F&E Tank given the 15mm Feed pipe is actually attached to this Return pipe near the Cylinder. I then start Boiler on WATER-ONLY setting. Im hoping this top-down stream of water in the Return encourages the desired circulation in the Flow.

Could the Cylinder be at fault?
I got no issues changing the Cylinder but i know i wouldnt be able to find identical replacement. Any new will want re-routing 28mm pipes which i want to avoid.

 

[fixitflav]

This brings on a strange effect. It begins to pull water out of Flow, such that the flow pipe becomes cold very quickly, and the Return pipe gets boiling hot instead, though it does not get hot all the way to the cylinder.

That's about what I'd expect, but interestingly, only if there is free flow through the cylinder coil. If there's an airlock in there in theory it should block it. I suppose a certain amount of flow must be occurring. Also I wouldn't expect the return to get hot until the rads return to the boiler has heated up. Not getting to the cylinder indicates no continuous flow, but you already know that only too well!

In my next attempt, im considering opening draincock near the Boiler and letting slow escape of water from there.

It might work, I wouldn't like to predict.

I still think you need to bleed the pipework near the cylinder, either install a bleed nipple or from a fitting. Your gummed-up one should free if you warm it up a bit. But use 2 spanners pulling against each other, so not putting a net force on it (if that's not telling you what you already know). You don't want to distort the pipework or cause a leak in the cylinder wall.

 

[Glanmor]

Sorry for no update/feedback. Couldnt get chance to have a another go.

I will report back here.

Cheers

 

[fixitflav]

Sorry for no update/feedback. Couldnt get chance to have a another go.

I will report back here.

Cheers

No prob, keep us posted.

 

[Glanmor]

Hi

Had another go at this last night.
I can now say that the WATER circuit looks like this pic below. I have omitted the CENTRAL HEATING circuit on the right hand side of the boiler.

With hose pipe on draincock A (incidently this is not the Bolier draincock, that is a seprate one further down at Boiler base level) I allow slow water escape.
Can feel residual hot water from boiler, and its immediate surrounding pipes, leaving.

After a short while I notice that the escaping water is now cold. Flow pipe C is cold but Return pipe B is still warm to touch.
Bingo.... this must mean Feed F must have a blockage - but F&E Tank ballcock is running merrily replacing escaping water at draincock A, how is that reaching the boiler and why hasnt return pipe B gone cold?
Looking at the diagram it must be going into the Cylinder via pipe D then out of pipe E and down into the boiler via C.
Now convinced I have blockage in pipe B.

In order to achieve sure flush of pipe B, I decided to cut pipe D (the horizotal bit from the Cylinder) but then noticed I do not have much play in this pipe as the Return goes into the floor quickly and its quite hard bedded. OK, for the pupose of flushing I cut pipe E instead, (i did that last week). Pushed mains water hose into the Cylinder direction and blasted it (had one eye on F&E Tank just inacse it went up). Fairly quickly this water came out again from the other end of the cut on E. It was quite colourful to begin with but few buckets later clean water flowing. Then repeated in the oppsite direction.
Now convinced I shifted any blockage in pipe B. Infact pipe E, the Coil, D, B, the Boiler itself and pipe C cannot be blcoked. Soldered back E successfuly.
Re-filled using mains water fed via draincock A (assuming mains fill will overcome any air pockets).
Was this enough to solve HOT water isssue - sadly not. Boiler upto old tricks again, kettles off and doesnt send hot water up Flow pipe C. Left it running like this overnight assuming it needs time but still no change.

I am really lost now.
There must be a trick to 'encourage' water flow down pipes D and B, short of adding a pump in the circuit. These pipes are pretty close to floors and walls and cutting into 28mm pipes without being able to turn a pipe-cutter is going to be quite hard.

Any thoughts on what is going on here please. Determined to fix this having come this far.

Cheers,
Glanmor

 

[Glanmor]

Hi All

just dug out something that might be causing circulation issues for me. Its some kind of valve attched to the veritcal Flow pipe (pipe C in previous pic) and its half way up the wall from where the Boiler is.

Does anyone identify what this valve is please:

It appears to be an in-line contraption in that it does not chage the flow/direction of water.
It was burried in plywood boxing the pipework along the wall.
I havent tried to open it yet.

If this is blocking, all previous mains water flushes would not have been successful.

.
.

 

[Nige F]

It's just a spring safety valve - leave alone, it doesn't interfere with the pipe diameter.

 

[algas]

Almost certainly an air lock in the coil you will have to slacken the top union to the cylinder, someone else told you how.

 

[Hot&Cold]

Glanmor is there a thermostatic cyltrol valve or similar on the cylinder return tapping ?

could be a stubborn air lock,often shoving a section of hose pipe up the open vent then blow & suck will move some air. Other is the boil method but be careful doing that.

could also be partially restricted in the gravity return into the boiler heat exchanger,maybe get a gas technician that understands ancient systems to visit and rectify.

 

[chrisw99]

I had a similar set up to this and once had a problem similar to you describe, and it turned out there was a little jumper on the aquastat that could be set to GRAVITY HW or PUMPED HW. We had the aquastat replaced and the engineer who did that must have left it at the default of PUMPED HW.

Changed it to GRAVITY HW and all started working again.

 

[chrisw99]

That's from the manual. When it was set to FULLY PUMPED, the hot water wasn't coming out of the gravity hot water flow pipe and instead the return pipes were getting hot for a few inches.

But unless you've actually changed a part, then it wouldn't just happen out of the blue I guess..

 

[Glanmor]

update...
still stuck, no change.
Thanks to all the guys who responded since last post. Chrisw99, thanks for extra effort of printing out the manual page.

I have since cut Cylinder return pipe (pipe D) and now 100% sure there is no blockage in the Coil.
Tried loosening top union as suggested by ALGAS but just pure water comes out.
As a desprate attempt I decided to empty Sentinel X800 down Vent pipe and down feed pipe. Flushed it out several times after 1 hour. Didnt make any difference.
Fortunately Central Heating still works.

I cannot bring myself to a belief that the Boiler is gone. It can only be the Flow and Rturn pipes - be it blocked, airlocked, narrowed or leaking.
-Chances of a 28MM pipe blocking are small though possible but I can circulate mains hose water thru them.
-Airlock is only possible in the Cylinder given the way configuration pipes are arranged, like letter "H" kind of, so if vertical pipes fill quickly the cylinder can trap air. I've tried filling verly slowly to prevent this and also tried loosning top union trick.
-Narrowed pipes is a possibility hence the Sentinel X800 attempt.
-Any leaks will have surely shown up by now.

If it wasnt for the brickwork/fireplace around the Boiler I would have had a go at pipes around there too.

Any advise/pointers as to what I can do next please.
The only time I will give up is when an un-repairable fault can be seen infront of me.

Thanks for your time.

 

[Glanmor]

I promised to update on this.

Some good news and some bad…

Good news is that its working now.
Bad news is that I don’t know why.

I repeated all of the suggestions you gave several times since the last post but nothing changing.
Isolated the Cylinder coil by cutting pipes (cylinder flow and return) to test (again) the Boiler Flow and Return circuit.
Good volume of water going in and good volume of water coming out so there was no sign of blockage. Same for the Cylinder coil.
Soldered everything back.
To save un-doing unions again, included a bleed valve in the Cylinder flow horizontal, to facilitate easy bleeding.

Lifted floor boards to check on horizontal run of pipes across to the bathroom, all good there. Good uphill inclination run and good bend angles.

Lost count on how many times I drained/refill but nothing changed the status quo. Even tried re-fill from different points in the circuit and using main hose pipe not just replying on F&E Feed.

Check this…. I even rodded the pipes using a flexy 4m net curtain hanging rope, which I think was a brilliant tip from my wife.
Unfortunately no results.

Did nothing for two weeks. Instead started buying various 28mm fittings as next plan was to cut pipes near the Boiler on ground floor.
The pipes are too close to the wall and too close to each other to even incorporate a compression straight that a spanner can turn without hitting the adjacent pipe.

As if by magic, over last two nights the hot water circuit started flowing again.

Delighted and angry at the same time. What has ‘encouraged’ this flow. I have done nothing for two weeks to this. As a consumer I am happy but as an Engineer I haven’t been able to sleep on this.


There is 10% chance that this was helped by a gallon of Sentinel x800 I left in the circuit but I never believed this stuff anyway as I had tried this already earlier on. May be the last x800 was only left in the circuit for 1 hour as it says on the tin.

Recent one has been in for 2 weeks and its still in there. Haven’t got an ounce of brevity left in me to flush it out – scared that issue might come back again.
90% of me says Sentinel x800 did not fix this problem. Why, because for me its hard to believe a 28mm pipe becoming blocked that bad. And I know for sure these were not blocked. I even rodded these buggers!


So, it must have had to be airlock.
Given the pipe layout, pipe runs, pipe angles and pipe sizes, bleeds, refill speeds, refill places and I am stumped that it was airlock.
Mains water refills must have been able to shift any airlocks.

Thank you to all of you who helped out over the last few weeks. Sometimes even a simple idea gave me huge confidence boost.

Just before we close up on this, could leaving in the x800 until Summer be a bad idea? Could I mitigate it buy adding a dose of inhibitor ontop may be?



Cheers.

Glanmor

 

[ianmcd]

Do not leave x800 in your system,