Continuous header tank overflow: corrosion in CH system?

[Clive Page]

I help look after an man in his 90s who lives in a small bungalow. In recent months the overflow pipe from the header tank of his old central heating system has been dripping continuously. I checked the tank and it appeared that the ball-valve (probably dating from 1960s) was leaking a bit, so I replaced it with a new one and removed a bit of water from the tank. A week later the dripping resumed.

An important point is that he now relies entirely on electric night-storage heaters, so this heating system is redundant and has been unused for several years (it was fitted around 1965 it had a solid-fuel boiler with small-bore copper pipes, pump, and radiators). Knowing this, I put a stop-valve in the feed to this header tank and turned it off, and took out some more water from the header tank so it was well below the overflow level. A week later the dripping resumed. This initially baffled me, as no water can now enter the system.

I now suspect there is corrosion in the system generating gas which is slowly pushing water back into the header tank. I thought I might fix it by putting inhibitor in the tank and circulating it around the system, but the pump seems to be seized up, as it probably hasn't been used for years (pump gets slightly warm when left on so I think the motor is probably trying to turn the shaft). There is a screw in the top of the pump which I think may give access to the shaft, but the screw is corroded and I couldn't shift it with my largest screwdriver.

Bleeding the radiators released a small amount of gas, but not very much, and some just released water. While I'm not entirely sure whether corrosion is responsible for the continually rising level in the header tank, I can't think of anything else.

If it is caused by corrosion, the options seem to be:

1. Ignore the problem: there is only a finite volume of water in the CH system and eventually most of it will get expelled by the gas.

1. Try to free up the pump and circulate corrosion inhibitor.

2. Try to find a drain point for the CH system and drain off as much water as I can. But this will no doubt still leave some water in the pipes and radiators, so corrosion may continue for some time.

If anyone has any suggestions...

[Martin Brown]

I'd be very surprised if corrosion was able to add that much new water in a week. Are you sure it isn't rain entering the header tank in the loft? It has been very wet this last week with 2" of rain in places.

My money would be find the drain point and drain the whole lot down. If it is corroding as rapidly as you imply here it is only a matter of time before something fails and you get a deluge of black muddy copper oxide sludge over everything.

[Toby]

If there is a hot water cylinder, and the water in the main cold tank for supplying it is higher than the overflow on the CH header tank, then it could be the coil in the cylinder that has a hole in it, so water is getting that way, if this is the case, then draining it will mean it will refill its self again.

There may be isolation valves on the coil if you are lucky, but probably gate valves that leak, so you may need to drain the CH system down and fit new foll more isolation valves if this is the problem...

Draining it all and seeing if it refills is probably the best bet, but make sure everything is closed off, so water cant escape through open bleed valves in the house.

[Brian Gaff]

Well I'd try to drain it in case a pipe leaks somewhere else and makes a mess, but then it sounds like this should have been done years ago. Brian

[Ian Jackson]

I had exactly these symptoms - and, yes, it was the tank coil (well, replacing the tank fixed it).

[Andy Champ]

I finally worked it out after I had physically disconnected the feed from the header tank, thus convincing myself there was no other water source.

Andy