My feed and expansion cistern is overflowing through the overflow pipe. The problem is not the ball-valve. This has been changed and I have monitored the cistern. The water is not coming in from the rising main via the ball-valve but I cannot tell where it is coming from. Where else COULD it come from? A plumber I spoke to said something about a possible "pin hole in the coil". What does this mean and how could this cause the expansion tank to overfill? Any ideas very welcome.
Joe.
A standard vented heating system has two independent water circuits.
Each circuit has a header tank with a ball valve. When any water is lost from the circuit (such as when you turn on a tap) the water level in the header tank drops, the ball valve opens, the water level rises and the valve turns off again. No problem. If the ball valve develops a fault there is an overflow to prevent flooding.
Now, although the circuits are independent they do meet each other inside the hot water cylinder. There is a heating coil (a copper pipe bent into a spiral) containing water from the heating circuit inside the tank full of water from the tap water circuit. This is how the tap water (inside the cylinder) gets hot.
It looks like there is a leak in this pipe. In this case one of two things will happen - either "dirty" water from the heating circuit will leak into the tap water in the cylinder, or tap water from the cylinder will leak into the heating circuit.
The two circuits are no longer independent and in effect you have two header tanks linked by a pipe. What now happens is that gravity attempts to make the water level in each header tank the same (like the two sides of a U-bend) and as heating header tanks are smaller than tap water headers the level in the heating header will rise - until it hits the overflow...
The hot water heating circuit is a closed system with water circulating through the boiler and heating coil in the cylinder. Normally the only change in volume is through expansion and contraction which the expansion tank takes care of.
If the heating coil in the hot water cylinder develops a leak the domestic hot water in the cylinder is at a slightly higher pressure than the expansion tank as the water level is usually a foot or so higher than that in the expansion tank. Water therefore flows from the hot water tank through the hole in its heating coil and this causes the water level in the expansion tank to rise to the overflow level.
I had a similar problem in our last house, and had BOTH of the problems Matt describes - the expansion tank slowly overflowed, and baths were filled with slighly orange rusty water. Incidentally my HW cylinder didn't have a coil, but it was what I now understand to be called a 'primatic cylinder'.
Ben.
It does not.
That is the "cold water storage tank", not a header tank. The big tank in the loft. The CH system has a "feed & expansion" tank. The small tank in the loft.
They don't meet each other at all. They are totally separate as you said.
The fresh water in the hot water storage cylinder is supplied by the cold water storage tank
In the loft, the feed & expansion tank should always be at a lower level than the cold water storage tank. If the heat transfer coil inside the hot water storage cylinder is holed, the greater head (pressure) of the fresh water will push into the CH circuit and cause the water in the feed and expansion tank to rise, which will then flow out of the overflow pipe. Having the tanks at differing levels keeps the fresh water clear.
If it's the same as happened here, you'll find that the overflow drip continues even if you turn the water off at the mains. If so, it's almost certainly coming from the main cold water tank, which feeds your hot water cylinder.
The big main tank has a higher water level than the overflow of the CH expansion tank, right? That head of pressure is forcing water from the hot-water side of your cylinder, through a small pinhole leak in the heating coil, into the CH side of the system, and finally back into the expansion tank where verily it overfloweth.
If all that fits together, the plumber was almost certainly right and you'll need a new cylinder.
This is a particular problem in hard water areas, because limescale forms on the outside of the coil. When it becomes thick enough to split off due to thermal expansion, it also pulls off a very small amount of copper. (You may well find that the bottom of the cylinder is silted up with little curved shells of limescale, with their inside surface a delicate turquoise blue.) After several years this will wear right through the coil somewhere, and there's your pinhole leak.
BTW, a bouquet to Thames Water: back when I was trying to work out what was happening, they phoned to say, "We were reading your water meter at
1:30 this morning, and noticed it was slowly going round. Then we noticed your overflow dripping, so we thought you'd like to know."
Generally the cold water storage tank that feeds the hot water cylinder has a higher water level than the primary feed/expansion tank. If the coil in the HW cylinder develops a hole two tanks attempt to find their level meaning that the feed/expansion tank fills via the hole and overflows.
As the ball valve has been changed has the level been set correctly? Such that the primary expansion doesn't cause an overflow. When it contracts more water is let in to replace that lost and the cycle repeats. This will cause an overflow from the feed expansion tank as the system gets heats up.
A hole in the coil will (eventually) cause an overflow when the system is cold.
A standard problem with primatic cylinders, which have an air gap within the cylinder to separate the primary and secondary circulations. It was easy, particularly on solid fuel systems, to 'blow the bubble out'.
I have always regard them as an expensive waste of time. A lot of huff to save on the cost of an F&E tank.
So exactly what value have you added to Matt's very good explanation - other than a bit of pedantry regarding the offical names of the two tanks?
Hmm interesting. After a little more research I might be incorrect in thinking it was a primatic cylinder as the system did have a F&E tank. The cylinder did appear to have connection points where you would expect the coil to be (but nothing was connected to them), however the feed and return from the heating circuit were supplied via 15mm copper pipe through where I would have expected the immersion heater to go.
When I removed the cylinder (I replaced the system with a combi), I made a hole in the top and peered in, and could see two kind of 'bell' like things (I think, it was dark in there!) Any idea what that was all about then?
Cheers, Ben
Thanks SS
The OP didn't understand what his plumber meant when he suggested it was a leak in the coil, so I assumed that he did not have a working knowledge of heating systems and so rather than using the "correct" jargon I used names that would be easy for a layman to understand. I thought that it was best to show the similarities in the basic circuits and to avoid unnecessary details about whole system to allow the problem to be understood, rather than use correct names and end up with confusion about "why does the cold water storage tank connect to the hot water cylinder?" and such issues!
Indeed - and it was a perfectly good explanation without the need for anyone to pick holes in it to demonstrate how clever they are!
You had an immersion coil, which is a coil screwed into the immersion bosse. They didn't heat very quickly at all.
It wasn't a good explanation, that was why I clarified. He couldn't even get the terminology right.
Yup, second that...
In fact I think Matt's post was more accurate than Drivel's attempt at pedantry anyway ;-)
I didn't attempt anything, I just put him right. His clearly wasn't clear at all. Now you know, because beforehand you never.
As usual, you're in a minority of *one*!
Brilliant people usually are.
Yup.... true at your end of the bell curve as well.
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