Murky water gushing out of central heating expansion tank

MirandaMiranda7

I have had murky water gushing out of my central heating expansion tank on and off since June. It only happens when I switch on my water but it doesn’t happen every time. I hear a rumbling in the loft and then I can see murky water gushing out of the overflow pipe. As soon as I switch of the water it stops. I have had a new pump, new valve and a powerflush none of which solved the problem. The boiler is only 7 years old. Last week I had all the radiators and pipe-work changed and had a new hot water cylinder installed. Everything was fine for about 5 days and then the problem started again. I switched on my hot water at the controls (the boiler was on but the thermostat was turned down low so that my radiators were not on) and then I heard the rumbling and the water started gushing out of the overflow pipe. Can you help – I’m at my wits end and my plumber doesn’t seem to have a solution?

latecomer

Did you have all your pipework replaced or just the stuff associated with the radiators? Sounds to me like you have a blockage of some description in the hot water system which is preventing the water returning to the boiler and hence its being forced up into the expansion tank.

Note that I'm just a DIYer not a plumber. And if you plumber cant figure it out then I'd look for another one.

Canucklehead

Hi

You seem to have a new system, all but the boiler, so the only thing I can suggest is a basic pipe layout problem or maybe a wiring problem.
A photo of the cylinder and pipework would help and the boiler make/model.
Corgi Guy.

catkins

When you say you switch on the water do you mean that you are switching on to make it hot or are you running a a tap? Has the thermostat on the hot water tank been checked or changed?

EliteHeat

MirandaMiranda7 wrote: »

I have had murky water gushing out of my central heating expansion tank on and off since June. It only happens when I switch on my water but it doesn’t happen every time. I hear a rumbling in the loft and then I can see murky water gushing out of the overflow pipe. As soon as I switch of the water it stops. I have had a new pump, new valve and a powerflush none of which solved the problem. The boiler is only 7 years old. Last week I had all the radiators and pipe-work changed and had a new hot water cylinder installed. Everything was fine for about 5 days and then the problem started again. I switched on my hot water at the controls (the boiler was on but the thermostat was turned down low so that my radiators were not on) and then I heard the rumbling and the water started gushing out of the overflow pipe. Can you help – I’m at my wits end and my plumber doesn’t seem to have a solution?

The first thing to do is change your plumber. Boilers and heating systems tend to obey the laws of physics and are not supposed to be a mystery to those who make a living from installing and repairing them.

Overflowing expansion tanks are caused by:-

• A sludged up system
• A failed ball valve
• An incorrect open vent, cold feed and pump configuration
• Perforated hot water cylinder heating coils
• Incorrct wiring
• Failing motorised valves and missing automatic bypasses
• A failed boiler thermostat or high limit thermostat
• A failed boiler PCB or sticking gas valve
• Reversed circulation caused by an incorrectly fitted pump

Getting a qualified and recommended heating engineer to look at your system would be a good start.

MirandaMiranda7

The plumber I have is corgi registered - but have taken on board your recommendation and will seek a second opinion.

MirandaMiranda7

Thanks to everyone for their help on this.

My plumber replaced a final pipe and all seemed to be well for 4/5 weeks, but on Monday this week the same problem returned: I switched on the controls to heat up the water and a few minutes later there was a terrible rumbling and hot, brown water started gushing from the expansion tank out through the overflow pipe! I switched the hot water (and central heating) off while my boyfriend inspected the tank. Ten minutes later, I switched the hot water control back on in an attempt to reproduce the problem while my boyfriend was in the loft, but everything was fine again.

Since then my plumber has come out to have a look, and noticed that the circuit board in my Potterton (Kingfisher MF) boiler had burnt out at one end. This has now been replaced and the boiler is much quieter than it has been recently (it's not making the constant clicking sounds it was before).

Since the problem is intermittant, it's difficult to tell if replacing the circuit has cured the problem. Is there any possibility that the faulty circuit board could have caused my expansion tank overflow problem in the first place?

Would be grateful for your thoughts.

EliteHeat

It is possible that the boiler was sporadically not shutting down due to a faulty PCB. If you have an "S" plan system i.e. two zone valves then this would certainly result in your symptoms.

The problem occurs when both zone valves are shut and the boiler continues to run, if there is no bypass in the system the only open path for the system water is the open vent over the header tank.

This is a relatively easy problem to diagnose as it is easy to check whether either zone valve is actually calling for heat. Hindsight is a wonderful thing though!