Vented central heating, tank overflowing

[Danno1800]

I've done a lot of reading here on similar threads and think I know what's up but I would like to double check my understanding.

Recently the overflow pipe from the feed tank that tops up the central heating loop has been leaking into the garden. The water is black which suggests corrosion. I checked the tank and there is warm water in it (not hot enough to melt the tank - i've read those stories and am aware).

There is a lot of air in the system as evidenced by the whooshing etc.. noises that happen when it's on. I suspected that the expansion and contraction of that air when the heating goes on and off might be causing volume changes which is pushing water back into the feed tank and causing the overflow (I stood and watched the tank and could see a water upwelling just after I heard a lot of gurgling)

I turned off the valve that leads to the ballcock in the feed tank to see if either its that which is replenishing the water or if (worse case scenario) fresh water was potentially coming from a cracked/leaking coil in my domestic hot water cylinder into that loop.

At first the water level in the feed tank dropped which seemed like good news - but today after heating both radiators up and also domestic hot water I see the level in that tank is back to overflowing and that extra water is not coming from the ballcock filling mechanism.

Is there anywhere else the water can be coming from? It seems like a new cylinder for me but I just wanted to run it past someone before I find a plumber to fit me a new cylinder (or maybe go to a combi boiler).

Cheers

 

[Notch7]

The pipe that goes over the small expansion tank is a vent for the primary circuit

It could caused by pump failing or faulty valve

A bunked up system with lots of air could cause it

If cylinder coil has split that would likely mean dirty water coming out of hot tap


It sounds like you need a powerflush once you’ve sorted immediate issue

 

[Danno1800]

If cylinder coil has split that would likely mean dirty water coming out of hot tap

I did wonder why i'm not seeing any dirty water out of the hot taps.

Interestingly since I last posted and the heating and hot water has been off the level of that tank has gone down again so it may just have been expansion of water& air that I saw causing the level to rise after all (feed is still switched off).

The radiators need bleeding big time and judging by the colour of the water I wouldn't be surprised if there's some level of blockages. Current plan is to get some X800 tomorrow, drain system and run the X800 through for a few hours. Bleed all the radiators. Flush it through and then add corrosion inhibitor (or maybe x400 for a few weeks) and see how it goes. I'm really hoping it's not the cylinder coil because replacing it will be a major upheaval in the house.

 

[denso13]

A blockage where the feed from the cistern and the open vent join the heating system are common and cause these symptoms.

 

[Notch7]

I did wonder why i'm not seeing any dirty water out of the hot taps.

Interestingly since I last posted and the heating and hot water has been off the level of that tank has gone down again so it may just have been expansion of water& air that I saw causing the level to rise after all (feed is still switched off).

The radiators need bleeding big time and judging by the colour of the water I wouldn't be surprised if there's some level of blockages. Current plan is to get some X800 tomorrow, drain system and run the X800 through for a few hours. Bleed all the radiators. Flush it through and then add corrosion inhibitor (or maybe x400 for a few weeks) and see how it goes. I'm really hoping it's not the cylinder coil because replacing it will be a major upheaval in the house.

Do your radiators get hot all over or are there cold spots especially at the bottom around the middle - it’s a sign of sludge build up.

Sometimes a strong magnet can be tried against the pipework, say in horizontal runs, and especially where Denso mentions. If there’s lots of sludge you will feel the magnet sticking.

Gurgling / whooshing noises and frequent need to bleed rads is a sign of a system needing a power flush.

My guess is it’s a pretty old system if it’s not a sealed, pressurised system.

 

[Danno1800]

Thanks guys. I will check the things you've mentioned to get a measure of what and where is sludged up. The magnet trick sounds fun and I have a thermal camera gadget that plugs into a smartphone I will dig out - it might help too.

The system is about 25 years old at a guess.

 

[Madrab]

Is the cistern (CWSC) that feeds the hot water cylinder higher than the small F&E cistern? Normally it would be and that's by design, that allows the CWSC to feed into the system, rather than the F&E cistern (CH) feeding into the cylinder, if the HW coil becomes pinholed, thereby contaminating the HW.

In that case, If the cylinder coil has become pinholed, the F&E cistern would normally overflow all the time as it's being constantly fed by the CWSC, so as suggested first place to look would be the H section of the system where the feed and vent are connected and has become blocked up. Some cleaner through it may sort it out.

 

[Danno1800]

Is the cistern (CWSC) that feeds the hot water cylinder higher than the small F&E cistern? Normally it would be and that's by design, that allows the CWSC to feed into the system, rather than the F&E cistern (CH) feeding into the cylinder, if the HW coil becomes pinholed, thereby contaminating the HW.

The cisterns are on the same level next to each other. I suppose the pipes may be different heights though i'll have a look next time I'm up there and see if I can figure out which pipe is which.

I've been running X800 for a few hours overnight and will be draining and flushing shortly. I discovered a couple of radiators that we never use were stuck closed (TRV pins were jammed, i've been working them loose slowly with WD40, gentle hammer then pressing in with a spoon) though I don't know if they would affect general flow in the system. I got out the thermal camera attachment and it was interesting seeing some pipes under the floor not warming up until after a few hours of X800 treatment so I suspect there was indeed some pipe blockages.

I also have 2 radiators with broken bleed valves, only the bottom gets warm - i'll have to replace the radiators totally but one step at a time..

 

[Madrab]

I've been running X800 for a few hours overnight and will be draining and flushing shortly

Be very careful using X800 as it is very aggressive, if there is any risk that the coil is pinholed then it can find it's way back into HW supply. Just something to keep in mind.