Rayburn hot water problem

[bower1]

I have a problem with the gravity fed hot water tank on my oil rayburn. It seems to stop circulating every few days, and seems to start working again when the central heating part is put on for a while (A bit vague I know!)

I've checked that there is water in the expansion tank, and its been drained down quite recently and didn't seem too dirty. When it stops working, the cooker is still hot, in fact hotter than usual, presumably because the water is not taking some of the heat away.

Any other ideas?

 

[NT]

airlock?

 

[MdB]

We had a similar problem in that the water circulation either slowed down or stopped causing the water in the rayburn to boil. The rising steam in the pipes was enough to start the circulation going again but this obviously was not ideal. We never solved the problem as the oil tank split so we just switched over to electric heating.

<guess mode on>
In our situation (and possibly yours) my feeling was that the problem was lack of insulation in the pipes from the Rayburn to the tank and a well insulated tank. When the tank is cool, the heated water quickly cools and is hence denser and a good circulation occurs. When the tank heats up, a bubble of hot water can end up in the tank stopping the circulation. The water in the Rayburn conducts up both the out and return pipes and there is not a large enough difference between the two to get the circulation going again. One possible solution is to insulate the out pipe from the Rayburn to the tank. This will keep this hotter than the return pipe so will aid in keeping the circulation going. Don't insulate the down/return pipe as you want this to cool on the way back. I didn't try this myself but it was one thing I was thinking of doing.

A bubble in the pipe as NT suggests it also worth checking.

 

[bower1]

Thanks for the suggestions.

The system has been installed in its current state for about a year and there were no problems at all last winter, so I'm not sure it will be because the pipes need lagging.

I was thinking it might be an airlock, but as the pipes rise all the way to the tank, the airlock must be in the tank?

It seems strange though that it starts working OK when the heating is on.

Any ideas for clearing an airlock?

 

[Penners]

Does the Rayburn also do the central heating? Is there a contact thermostat controlled motorised valve that diverts the primary hot water either to the hot water or the heating circuit?

If the answer to both questions is yes, then I would cast a critical eye over the contact stat and/or the motorised valve. It could be that one or the other is faulty.

 

[bower1]

Nothing so clever, just a gravity loop and a pumped central heating loop, joining at the boiler.

 

[Penners]

Do the two loops join before or after the pump? In other words, when the pump is working, will it also be exterting any effect on the hot water loop?

A sketch of the layout might help.

 

[bower1]

I didn't think it should have any effect, but when I turn the pump on (with the central heating burner off) the hot water from the cooker burner goes straight to the hot water tank. This seems a little odd to me.

It is installed as described here:
http://www.rayburn-web.co.uk/raytech/chtg3.htm

 

[Lime]

Almost certainly this problem is caused by an airlock as NT has already stated.
Turning on the central heating causes the air that is blocking circulation to move and vent presumably.
The hotwater tank will be heated by the boiler water indirectly through a copper coil within the tank and it is usual to have a vent pipe coming off the "up" pipe (flow) from the boiler where it connects to the tank.
The vent pipe goes upto and above the expansion tank.
You say that this problem did not happen when it was first put in and that it has been drained down quite recently.
If a corrosion inhibitor was not added to the expansion tank when it was filling up after being drained that is almost certainly the cause of the problem.
The recirculating water starts corroding the metal (copper and iron) in the system and produces a gas in large quantities.
I would drain and flush the system again and this time put an ihhibitor in the expansion tank prior to refilling.
After a few weeks of use no further venting of radiators will be neccessary.

Make sure you put an adequate amount of inhibitor in.
Many people do not (because it is expensive) and live to regret it.
Eventually radiators rust away and leak meanwhile a large amount of sludge builds up in the system.
Airlocks and corrosion should not happen at all in a suitably inhibited system.

 

[Penners]

The recirculating water starts corroding the metal (copper and iron) in the system and produces a gas in large quantities.

Lime - I wasn't aware that the effects of not putting inhibitor into the water were so instant. Can this really cause sufficient gassing to create airlocks from the moment the system is refilled and switched back on?

 

[bower1]

Thanks for all the ideas.

I have put inhibitor in, and it should be enough. It wasn't the most expensive brand!, but its the same one that has been in for the last year.

I also hadn't realised it would cause problems quickly, so thanks for the info, but I don't think that can be the problem.

Anyway, I'm still not sure how there can be an air lock in the gravity feed system as the pipes both rise all the way to the tank, and the vent pipe is at the top of that part of the system.

 

[Lime]

The recirculating water starts corroding the metal (copper and iron) in the system and produces a gas in large quantities.

Lime - I wasn't aware that the effects of not putting inhibitor into the water were so instant. Can this really cause sufficient gassing to create airlocks from the moment the system is refilled and switched back on?

Initially the fresh water will have some dissolved gas (air) within it and when heated this comes out of solution.
However, galvanic corrosion can be very rapid indeed if the right circumstances are in place which will produce a huge amount of gas (usually hydrogen).
A lot depends on the water quality to start with too.
Never mix different inhibitors.
Out of all the inhibitors I have used the best IMO is fernox.
It also has a most strange characteristic of producing grey curly jelly-like deposits where there is a slight leak in the pipework.

 

[NT]

I have put inhibitor in, and it should be enough. It wasn't the most expensive brand!

That could be a problem.
http://www.wiki.diyfaq.org.uk/index.php?title=Inhibitor


NT

 

[Lime]

Your link NT supports my experience.
A system fully protected by Fernox does not produce any corrosion IME.
I would emphasise it is far better to have too much inhibitor in the system than too little.
Thorough flushing of the system prior to final fill is important too.


IME airlocks can sometimes appear to defy logic in as much that the gas blocking a pipe doesn't rise as you would expect it to.
Gas often congregates at joints or bends adhering to the pipe wall and is prone to remain stationary eventually blocking the flow.
It is made much worse by the practice of many plumbers to fit a vent pipe smaller than the pipes it is venting.

 

[bower1]

Interesting results. Yes I've used the one that isn't any good! I'll steer clear of it next time.

But I still find it hard to believe that will cause the problem after 1 month as any gas should be able to rise up to the vent pipe anyway.

 

[NT]

Interesting results. Yes I've used the one that isn't any good! I'll steer clear of it next time.

But I still find it hard to believe that will cause the problem after 1 month as any gas should be able to rise up to the vent pipe anyway.

There is that, admittedly


NT

 

[Penners]

A system fully protected by Fernox does not produce any corrosion IME.

Purely out of interest, do you know the science behind an inhibitor? Presumably the corrosion is caused by galvanic action between the different metals in the system (copper, steel, etc.) so the water must act as an electrolyte. How does an inhibitor prevent this?

Or have I got the principle wrong?

 

[Penners]

any gas should be able to rise up to the vent pipe anyway.

Provided the system has been well designed and installed, so that all runs of pipe allow gas to climb upwards. But too often careless work results in runs of pipe that have "peaks" in them where gas can gather.

 

[bower1]

But too often careless work results in runs of pipe that have "peaks" in them where gas can gather.

Unfortunately I can only really blame myself as I did most of the pipework! The only place I can see where the air can be trapped is in the hot water cylinder itself, as it is very slightly higher than the vent pipe due to the last rise in the pipe.

I'll try leaving the heating on for a bit longer, and with the pump turned up full, as I can't think of anything else to try.

 

[Penners]

Conincidentally, I also had an airlock problem in my central heating system yesterday, having drained it down to modify a radiator position, then refilled it.

This is the technique that I used the shift the stubborn airlock:

I connected a hosepipe to a tap at mains pressure (a tap fed from the header tank won't do). I then took the hose up into the attic, and stuffed it underwater and well into the downpipe from the header tank that feeds the system. I packed around it with cloths to create the best seal I could.

I then yelled down to Mrs P to turn on the tap.

There was a load of belching, coughing and spluttering from the vent pipe, then clear water ran steadily out of it, into the header tank. The airlock was cleared!

It is, of course, vital that your header tank operative is in clear communication with your tap operative, so he can order the tap to be turned off well before the header tank overflows!

One other technique:

Instead of taking your mains pressure hose up to the header tank, attach it to the drain cock that should be located at the lowest point of your system. Then follow the same procedure - operative 1 watches the header tank to guard against overflows, while operative 2 controls the tap. The only complication with this method is that there are two points to be turned on/off - the tap and the drain cock. And of course you need to put a jubilee clip tightly around the hose where it pushes on to the drain cock, as there will be considerable pressure here.

Anyway, for what they're worth, there are a couple of ideas to try (at your own risk). Good luck!