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...

Matt Beard wrote:

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.


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.

"Matt Beard" wrote in message
oups.com...

A standard vented heating system has two independent water circuits.

Each circuit has a header tank with a ball valve.

It does not.

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.

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.

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.

They don't meet each other at all. They are totally separate as you said.

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.

The fresh water in the hot water storage cylinder is supplied by the cold
water storage tank

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.

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.

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...

In an earlier contribution to this discussion,
Doctor Drivel wrote:

"Matt Beard" wrote in message
oups.com...

A standard vented heating system has two independent water circuits.

Each circuit has a header tank with a ball valve.

It does not.

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.

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.

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.

They don't meet each other at all. They are totally separate as you
said.

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.

The fresh water in the hot water storage cylinder is supplied by the
cold water storage tank

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.

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.

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...

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?
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

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!

In an earlier contribution to this discussion,
Matt Beard wrote:

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!
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

"Set Square" wrote in message
...
In an earlier contribution to this discussion,
Doctor Drivel wrote:

"Matt Beard" wrote in message
oups.com...

A standard vented heating system has two independent water circuits.

Each circuit has a header tank with a ball valve.

It does not.

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.

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.

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.

They don't meet each other at all. They are totally separate as you
said.

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.

The fresh water in the hot water storage cylinder is supplied by the
cold water storage tank

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.

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.

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...

So exactly what value have you added to Matt's very good explanation

It wasn't a good explanation, that was why I clarified. He couldn't even
get the terminology right.

Doctor Drivel wrote:


It wasn't a good explanation, that was why I clarified. He couldn't even
get the terminology right.

What, like using "cold water storage tank" rather than "cistern" as one
should?



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 11 Aug 2005 02:05:30 -0700, "Joe" wrote:

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.

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.

--
Peter Parry.
http://www.wpp.ltd.uk/

Joe wrote:
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.

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."



--
Ian White

On 11 Aug 2005 02:05:30 -0700, Joe wrote:

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.

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.

--
Cheers
Dave. pam is missing e-mail