Hi, I've got a problem with a hot water expansion tank overflowing at
a property I let out. Its a 5 yr old GCH gravity system i.e. the type
that must heat the hot water when the CH is on. The tank that is
overflowing is the large hot water expansion tank in the loft not the
small C/H tank.

Water is not entering the expansion tank via the pipe that hooks over
the top (tested by putting paper over it), it looks like it is coming
back up from the hot water tank itself. The problem often happens
overnight so I have changed the ball-cock in case the increase in
water pressure was making it leak. My plumber suggested that the
thermostat on the boiler was probably overheating the water in the
tank causing it to overheat and expand too much. I have changed the
boiler thermostat and set the boiler to the lowest setting, still no
change.

The cylinder also has a thermostat strapped around the outside, I have
tested this by turning it down and checking that it cuts out the
boiler - it does.

My other thought was that the coil in the hot water tank had a hole in
it but I don't think it does. I have tested this by cutting off the
water supply to the C/H overflow tank (holding the ball cock up) and
the level does not drop over a couple of days.

I have set the thermostat strapped to the tank down to about 55
degrees and the hot water does not seem to get very hot, but still
when I look in the expansion tank I can see hot water coming up from
the hot tank. I am convinced that the hot water tank is not
over-heating.

My only theory left is of someone elses thought (an old posting on
this newsgroup) that it could be down to a mixer tap problem ... I
installed a thermostatic mixer tap in the bathroom a couple of years
ago & are wondering if that might be causing the problem. There is
also a mixer tap on the kitchen sink, just a basic one. But as the
problem often manifests itself at night i.e. when nobody is using the
shower, I am not certain this is the problem.

I'm not a trained plumber, just somebody using my own bits of limited
knowledge and logic. Have I missed something, before I start ripping
out expensive thermostatic mixer taps? If you do suspect the
thermostatic mixer tap, has anyone any idea how I can test it easily
(not easy to isolate it for 24 hours as the tenants use the shower
often).

Many thanks for reading this,

Peter

"Peter Cherry" wrote in message
om...
Hi, I've got a problem with a hot water expansion tank overflowing at
a property I let out. Its a 5 yr old GCH gravity system i.e. the type
that must heat the hot water when the CH is on. The tank that is
overflowing is the large hot water expansion tank in the loft not the
small C/H tank.

Water is not entering the expansion tank via the pipe that hooks over
the top (tested by putting paper over it), it looks like it is coming
back up from the hot water tank itself. The problem often happens
overnight so I have changed the ball-cock in case the increase in
water pressure was making it leak. My plumber suggested that the
thermostat on the boiler was probably overheating the water in the
tank causing it to overheat and expand too much. I have changed the
boiler thermostat and set the boiler to the lowest setting, still no
change.

The cylinder also has a thermostat strapped around the outside, I have
tested this by turning it down and checking that it cuts out the
boiler - it does.

My other thought was that the coil in the hot water tank had a hole in
it but I don't think it does. I have tested this by cutting off the
water supply to the C/H overflow tank (holding the ball cock up) and
the level does not drop over a couple of days.

I have set the thermostat strapped to the tank down to about 55
degrees and the hot water does not seem to get very hot, but still
when I look in the expansion tank I can see hot water coming up from
the hot tank. I am convinced that the hot water tank is not
over-heating.

My only theory left is of someone elses thought (an old posting on
this newsgroup) that it could be down to a mixer tap problem ... I
installed a thermostatic mixer tap in the bathroom a couple of years
ago & are wondering if that might be causing the problem. There is
also a mixer tap on the kitchen sink, just a basic one. But as the
problem often manifests itself at night i.e. when nobody is using the
shower, I am not certain this is the problem.

I'm not a trained plumber, just somebody using my own bits of limited
knowledge and logic. Have I missed something, before I start ripping
out expensive thermostatic mixer taps? If you do suspect the
thermostatic mixer tap, has anyone any idea how I can test it easily

If you can see water entering the tank, does it stop when you turn off the
water mains stopcock ? If not then the water must be due to expansion
(i.e. it's getting hotter) but if so then a mixer valve is possible. Are
there service valves near these you can isolate them with ?


(not easy to isolate it for 24 hours as the tenants use the shower often).

Not students then :-)

On 17 Nov 2004 15:42:00 -0800, Peter Cherry wrote:

Hi, I've got a problem with a hot water expansion tank overflowing
at a property I let out. Its a 5 yr old GCH gravity system i.e. the
type that must heat the hot water when the CH is on. The tank that
is overflowing is the large hot water expansion tank in the loft not
the small C/H tank.

snip rest of excellent posting describing tests and results already
done

You've already gone through the common causes:

Hole in cylinder coil (though that normally causes the CH header tank
to overflow as it normally has the lower water level being a smaller
tank).

Expansion would only happen as the system heated up from cold, how far
below the overflow is the water when the ball valve shuts off? If it's
more than a couple of inches on a 50 gallon tank thats a heck of a lot
of expansion.

Does the overflow happen in the middle of the night a few hours after
the last ho****er is drawn or when the boiler fires up in the morning.

Is the flow constant? Expansion would stop once everything is hot.

I think the mixer points are worth a very close look, probably not the
kitchen one as that should keep the hot and cold seperate right to the
nozzle and any leakage past the valves would come out the spout and
not up the HW pipework... A worn out O ring or housing in the shower
seems favourite at the moment. As another poster has suggested does
flow of water into the tank stop if you turn off the rising main?

--
Cheers
Dave. pam is missing e-mail

If you can see water entering the tank, does it stop when you turn off the
water mains stopcock ? If not then the water must be due to expansion
(i.e. it's getting hotter) but if so then a mixer valve is possible. Are
there service valves near these you can isolate them with ?


(not easy to isolate it for 24 hours as the tenants use the shower often).

Not students then :-)


Great idea G&M. I will try and get over to the property this afternoon
and check. Yes, you're right - they are aren't students ! ;-)

Many thanks for reading this and replying - much appreciated.

Peter

"Peter Cherry" wrote in message
om...
Hi, I've got a problem with a hot water expansion tank overflowing at
a property I let out. Its a 5 yr old GCH gravity system i.e. the type
that must heat the hot water when the CH is on. The tank that is
overflowing is the large hot water expansion tank in the loft not the
small C/H tank.

Water is not entering the expansion tank via the pipe that hooks over
the top (tested by putting paper over it), it looks like it is coming
back up from the hot water tank itself. The problem often happens
overnight so I have changed the ball-cock in case the increase in
water pressure was making it leak. My plumber suggested that the
thermostat on the boiler was probably overheating the water in the
tank causing it to overheat and expand too much. I have changed the
boiler thermostat and set the boiler to the lowest setting, still no
change.

The cylinder also has a thermostat strapped around the outside, I have
tested this by turning it down and checking that it cuts out the
boiler - it does.

How does this work? Gravity fed CH systems have no stat on the tank. There
is nothing for them to connect to. Are you sure there are no valves?

Adam

On Thu, 18 Nov 2004 18:37:09 GMT, ARWadsworth wrote:

How does this work? Gravity fed CH systems have no stat on the tank.

I guess there are such things as gravity fed CH systems but they must
be close to hens teeth in rarity value... Gravity fed HW however is
very common.

You have a tank stat because you don't really want your HW at the 80C
that you really need for the CH circulation, scalding risk. You fit a
tank stat to a) stop the boiler cycling to keep the gravity loop hot
once the water is hot enough b) control the temp of the HW when there
is no CH demand. Of course when the CH calls for heat the HW temp will
rise above that of the stat, that is a failing of the gravity system.

--
Cheers
Dave. pam is missing e-mail

I think the mixer points are worth a very close look, probably not the
kitchen one as that should keep the hot and cold seperate right to the
nozzle and any leakage past the valves would come out the spout and
not up the HW pipework... A worn out O ring or housing in the shower
seems favourite at the moment. As another poster has suggested does
flow of water into the tank stop if you turn off the rising main?

Yes, it was the mixer. Its made by Cascade, now called Bristan (or
that's who answers the calls now). You can hear the water going up the
hot pipe when you put your ear next to the thermastatic mixer tap.
Despite changing the thermostatic valve (which Bristan supplied FOC)
the damn thing still leaks into the hot. I can only presume it is the
shower/bath knob that is causing a problem as well. These taps cost
about £120 when I bought them under 3 years ago, not great service.
Tried descaling and cleaning the parts but they still didn't work.
Will put in a separate non-return valve on the hot pipe tobe on the
safe-side when I finally sort it out. Good job the taps came with a 5
year warranty.

Thanks everyone for your help.

Peter

On 20 Nov 2004 11:06:48 -0800, Peter Cherry wrote:

Despite changing the thermostatic valve (which Bristan supplied FOC)
the damn thing still leaks into the hot.

Housing worn? Get back in touch with Bristan and say the problem
persists after the swap. It appears you have company that may actually
care.

These taps cost about =A3120 when I bought them under 3 years ago, not=

great service.

A free replacement thermostic element not good service? Seems OK to
me.

--
Cheers
Dave. pam is missing e-mail

On Sat, 20 Nov 2004 11:06:48 -0800, Peter Cherry wrote:


I think the mixer points are worth a very close look, probably not the
kitchen one as that should keep the hot and cold seperate right to the
nozzle and any leakage past the valves would come out the spout and
not up the HW pipework... A worn out O ring or housing in the shower
seems favourite at the moment. As another poster has suggested does
flow of water into the tank stop if you turn off the rising main?

Yes, it was the mixer. Its made by Cascade, now called Bristan (or
that's who answers the calls now). You can hear the water going up the
hot pipe when you put your ear next to the thermastatic mixer tap.
Despite changing the thermostatic valve (which Bristan supplied FOC)
the damn thing still leaks into the hot. I can only presume it is the
shower/bath knob that is causing a problem as well. These taps cost
about £120 when I bought them under 3 years ago, not great service.
Tried descaling and cleaning the parts but they still didn't work.
Will put in a separate non-return valve on the hot pipe tobe on the
safe-side when I finally sort it out. Good job the taps came with a 5
year warranty.


I have one of these in my own home. They are now supplied with integral
NRVs.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html