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I wonder if somebody could please enlighten me as to how my central heating system actually works and so also be able to point me in the right direction with regards to a solving a problem that I am having with it.

I think that I have an open vented , pumped system, if that is the correct terminology!! There are 2 tanks in the loft, one small feed and expansion tank for the central hearing and another large tank to hold the cold water for the hot water cylinder feed.
The boiler is an old Glowworm space saver 30!! There is a single motorised valve (only 2 way) in the pipework and I think that this means that I can either have the radiators on or the hot water on, or both, but if I have the central heating on the water in the hot water cylinder is also heated. Finally there is the pump.

The problem that I am having is that every few days the feed and expansion tank for the boiler / radiators will start to overflow. Checking the contents of the small feed and expansion tank confirms that the water level is just over the overflow pipe level.

I have checked the ball vale and this seems to be operating correctly and shuts off ok. I have taken the surplus water out of the feed and expansion tank on a couple of occasions and all seems ok for a few days, until it fills up again of course!!

I thus assume that the water must be coming back into the F+E tank from the boiler / radiator pipe work? Is this the only possibility? (I have read elsewhere about coils in HWC cracking, but I think that this would cause the cold water tank to fill up, not the F+E tank for the central heating?).I have ran the system for a couple of hours or so and checked the expansion pipe going into the F+E tank in the loft but nothing seems to be coming out of it and the level in the F+E tank seems to be staying the same. It seems to just creep up over a few days, so maybe I need to keep a closer eye on the level on a more regular basis.

Any pointers would be much appreciated, as this is driving me crazy!!

Many Thanks

Andrew

andrew-webber,
What are the relative levels of the water surface in the cold water tank and the CH tank? If the water is higher in the cold water tank it is very likely that the coil in the hot water cylinder has perforated allowing water to pass between the two systems.
end

Hi and Thanks for the reply.

Apologies for sounding a bit dim but I am not clear on what you are really asking?

The Central heating F+E tank is obviously a lot smaller than the cold water tank that feeds the hot water cylinder and so it's water level is always going to be lower is it not?

If the problem were a cracked coil in the hot water cylinder would the water from the hot water cylinder always be forced back into the central heating pipwork as it is under more pressure from the larger cold water tank in the loft, as opossed to the opposite way (i.e. central heating water being forced into the hot water cylinder)?
The reason for asking is that I would not like the central heating water to contaminate the hot water cylinder water, which I wash / shower in, as I have added inhibitor into the cenral heating water.

Both the F+E and cold water tanks are sitting next to each other at the same physical level(each resting on a few bricks!!)

Apologies again if I have completely mis-understood you Stoneyboy

Thanks & Best Regards

Andrew

Unfortunately, you are right in your assumption. The CH F&E tank can only overfill with water from the DHW system (except ball cock of course). One other point worth mentioning, but I assume you would have noticed this, overflows from both tanks are occasionally connected together so if the higher DHW tank overflows, it may flow into the smaller tank instead of outside.

Other than the tank coil, there is no other point where water could enter the heating circuit.

Inhibitor poses no health hazard as cross contamination is always a possibility and the contents of the heating system are really no worse than the sludge and muck found in header tanks. Of course this fault needs fixing soon, but I wouldn't drink hot water from an open system anyway!!

andrew-webber,
If the plumbing had been done correctly the CH tank should have been put on a platform so that the levels of water in the two tanks would be near enough the same. This way there is no differential pressure between the two systems. That way if the heating coil does perforate then there should be minimum flow between the two systems.
If you can arrange it, turn off your mains water overnight, mark the levels of the water in the two tanks, make sure no water is drawn off anywhere in the house and next morning look to see what has happened to the water levels. If the cold tank level has gone down and the CH level has gone up I'm sorry to tell you that the heating coil has perforated.
You could leave things for a while and see if it gets worse.
end

R U sure the water is not coming from the vent pipes above the tank/s as steam, which is not always visible except after the system has been running for some time. My mother in law had a similar situation and she called out the gas board (under mtce contract) and it seemed that there was back pressure comming from the c/h pipes due to sludge and they said that the most likely cure would be to have it power flushed which she did (but not by them to costly) by a local c/h plumber and so far all seems ok. And when her contract exspires at the end of feb she will not be renewing it (£298 per year) I have just cancelled mine. :D

hi
how old is the hot water cylinder?
if its more than say 15-20 id go along the lines of the coil as perforated and would consider replacing it,
the cost of power flushing and the cost of replacing the tank may not be that far apart and considering the age of the tank?
bob

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Hi Andrew what was the final out come of your problem on this situation, my daughter is having a similar problem. I am sure that her problem is due to the fact that its a 2 pumped system i/e 1 for the DHW & 1 for the C/H and that the dhw pump is regulated wrong although i am yet to confirm this.

hi andrew what was the out-come of your Q.