Hot Water Expansion Tank Overflowing

[Peter Cherry]

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-c*ck 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 c*ck 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

[G&M]

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 students then :-)

[Dave Liquorice]

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 hotwater 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?

[Peter Cherry]

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

[ARWadsworth]

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

[Dave Liquorice]

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.

[Peter Cherry]

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

[Dave Liquorice]

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.

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

[Ed Sirett]

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