No hot water, rads fine, open vented system

[purpss]

Hi all, long time reader, first time poster.

I have a very simple gravity-fed system, open vent, no valves, and one pump for the C/H. Open vent is just a tee off near the header tank. Then there is a common pipe (for both supply and the vent) going to a T on the hot water cylinder supply.

We are no longer getting hot water. Problem was intermittent for a long time, but now it's permanent. The water is not freezing cold (but close enough) - there is SOME heating going on. The primary system feed to the cylinder is nice and hot (upper pipe) but cylinder "out" is not hot at all (the lower pipe).

My old man thinks it would be worth pressurising the system with mains pressure as there could potentially be an air-lock in the heat exchanger, but I don't see how that can be possible given that the supply from the header is exactly in this location. Also the problem used to be intermittent.

Any thoughts would be very welcome.

Cheers, Matt.

 

[75pete]

You could try to increase the flow by upping the pump speed for 10mins.
How old is the system. Sorry just reread and seen pump ch only.
Anyway, you must have flow as you say, pipe hot. So, air or pipe part blockage. How do you control the water temp?

 

[Burnerman]

If you suspect an air lock, how about cracking open the nuts at the heating coil flow / return pipes?
You may have a blockage in the coil within the cylinder, or the lower pipe returning to the boiler.
John

 

[purpss]

Thanks for the replies chaps.

@75pete
Great idea. I turned the pump up to max, turned the room 'stat up so it stayed on, turned all the rads off, and left for about 30 minutes. The upper pipe got REALLY hot.... but still nothing in the lower pipe. The pipe going up to the loft (the open vent/supply from header tank) was getting quite hot as well.

Yeah the pump is only on the CH, but still I thought it was worth a try. There is of course nothing to stop the water from flowing around the rad supply/return. The system is circa 1974. I realise I am polishing turds here, but it's only for a year - putting a whole new system in next summer. Just need to keep this one going a bit longer.

CAN we say for sure that I do have flow? The system is gravity fed - even if there was complete blockage, surely the water could still heat up? I think this is proved by the oven vent/supply from header pipe heating up.

Water temp is controlled by the boiler, just replaced the boiler 'stat and thermocouple, but as I say rads are fine. Not convinced it's relevant to this problem.

@Burnerman
My DAD suspects an airlock , having not seen the system or really knowing how to maintain old ones, definitely a case of the blind leading the blind.

Please check out the photo. You can see the vertical pipe coming off the tee at the heater exchange inlet - that is the supply-from-header-come-open-vent pipe. If it WAS an airlock, it would have to be inside the heat exchanger. My question, is that how could an airlock even be there given the design of the system? Would any air not bubble up the open vent, given that the tee is right there at the highest point? Or does the fact that it's a common open-vent/supply from header STOP air from escaping properly?

It is a manky old corroded system, and whilst I would love to go balls-deep and crack things open to see if any air escapes, my concern would be damaging it/putting it back together again.

My money is on blockage in the coil of the cylinder as you said, but I don't understand enough about it to say for sure.

Untitled

• purpss
• 14 Jun 2012
• 1

Cheers, Matt.

 

[Burnerman]

Matt, only personally speaking but so long as there is water in the small header tank, the prevention of hot water flow has got to be due to an air lock or blockage either in the copper cylinder coil or on the bottom pipe length returning to the boiler.
Before you do anything, find a magnet and hold it to the lower pipe. If the magnet attracts, then there is magnetite ( deoxygenated rust, if you like) in the system.
Are there no air vents near or at the boiler where you can bleed?
At the end of the day I reckon you'll need to disconnect the tank to see whats what, but I'd try cracking the nuts open first......be careful though, you dont want to tear the coil connection out of the tank.
John

 

[purpss]

There is water in the header tank, and there is water in the pipe between the header and that tee.

I will try the magnet thing.

There are no air vents near or at the boiler, or at the cylinder. Just the drain off at the bottom of the system, near the boiler.

If I drain the system down, would it be worth trying to refill it with mains pressure? Either at the boiler drain down point or at the header tank supply?

I would love it to just be an airlock, but I just don't see how it can be, given the potential location. If the blockage was further down, nearer the boiler, surely the exit pipe from the coil would still get hot?

I think I will install an immersion heater in any event - starting to get some serious earache.

Cheers, Matt.

 

[Burnerman]

The lower pipework will only get hot if there is water actually circulating - and if there was then you wouldn't have this problem. The upper pipe can get hot simply due to heat transfer up there.
If you can blast mains water into that coil circuit then it may help, but thats not the easiest thing to do.
An immersion back up is a great idea, and dead simple - so long as the cylinder has a blanking plug.
Its a bit of a job, no mistake, but why not empty the cylinder completely, close off the header tanks too and remove the cylinder for a decent look?
Its not in its first flush of youth, and you may be surprised at what lurks within!
Best of luck with it.
John

 

[purpss]

Thanks for the advice. One more question before I get off my butt and do it (at least the immersion heater...) - can an air lock develop over time, or will it have been there since I last refilled the system? I drained it down about 2 months ago to fix a radiator, and the hot water was fine for a good while after I filled it back up. I know corrosion can introduce air into a system, but let's just assume that hasn't happened in the last few months!

I also must reiterate that the airlock would surely have escaped up the open event given its potential location.

Oh and I tried the magnet thing. It stuck to the pipes connecting to the coil. Maybe not surprising given the age, but as you say, I won't know the extent of the problem until I get it all out i.e. is it a thin coating, or a big fat lump.

Cheers, Matt.

 

[Boilerman2]

Surely the answer is blindingly obvious?? THE COLD FEED IS BLOCKED
Simples!!

 

[Burnerman]

Matt, the fact that the magnet attracted tells us volumes.....all of the other tank parts are non ferrous, and therefore the magnet doesn't stick.
There is magnetite in there - (ferrous oxide) and a slow build up diminishes flow from the boiler very slowly until there's hardly any circulation. At the tank, disconnect the flow / return pipes from the boiler and I'll bet there will be a gungy mess in there....
Airlocks can occur after work has been done, and they don't always disappear of their own accord - even during bleeding they don't always shift. Fresh water into any circulating system isn't good - always get in there with the inhibitor.
I don't think air locks are your problem.
John

 

[purpss]

@Boilerman2

Thanks for the reply. It may appear blindingly obvious to some, but for a young chap who has only just bought his first house, it is not always so obvious.

Once I have done my job for 37 years, I imagine I might find certain aspects of it blindingly obvious as well, and then I also will be able to roll my eyes at people who ask silly questions. I can't wait.

Obviously I am going to investigate all the possibilities when I get to it, but I am interested in the theory behind it all and other possible issues etc for when things crop up in the future. If that's OK with you.

@Burnerman

Thanks for your help. My money has been on a blocked cold return the whole time, but it's all too easy to make assumptions when you don't really understand CH systems! I like the magnet check, great suggestion.

I'm going to put an immersion heater in this weekend as well, with a timer.

Cheers, Matt.

 

[Burnerman]

Just a wee point regarding the immersion heater Matt - choose the length that suits you best, you'll get a fibre washer with the element but also use some non setting sealant and a few turns of ptfe tape on the threads.....so often they tend to leak after a day or two so its best to get it right first time.
John

 

[mysteryman]

It is blindingly obvious that there is a blockage or obstruction - somewhere.

It could be in the cold feed; it is certainly somewhere in the gravity primaries. It may be worth trying a drain down and refill, and the use of a chemical cleaner. Removing connections from a 38 year old cylinder is not for the faint hearted. If in doubt, don't. This is especially so if you are in a hard water area.

 

[purpss]

@Burnerman

Good shout, I'll get some tape and LS-X on it.

@mysteryman

As you say, the blockage could be anywhere in the gravity primaries.

I've drained down and refilled recently to replace a radiator valve (maybe that caused the problem!) but I did not use a cleaner/flush nor did I put inhibitor in afterwards. I was concerned that using a flush might dislodge bits of crap and make things worst...

And yes I quite agree that I might struggle with this, the cylinder is at least that old, and yes it is a hard water area. The blanking plug on the top looks bad enough.

I just need to polish this turd for another year before it all gets replaced. As long as I can use the CH during winter (although I suspect that probably isn't recommended if you have a blockage in the gravity part?), then I hope I can get away with just fitting the immersion and timer.

Cheers, Matt.

 

[purpss]

Should I bang some Sentinel X400 in there, or is that not going to help me because the problem is in the gravity part?