Central Heating Expansion Tank Problem

Hi there,
I'm new to this forum but I'm wondering if anyone can help. I have just had my central heating system flushed because the rads had cold patches and I was told that sludge had built up over the years and needed flushing. This is a very old system, must be 20/25 years and probably the most basic setup: oil fired boiler, no zoning (so no motorised 3 way valve to switch between CH and HW), no thermostats except that on the boiler. Despite it's age, the system worked OK apart from the radiator cold spots.
However, since the flushing (by what was supposed to be a professional plumber), the overflow has been leaking. He has been back to it 3 times now and replaced the ballcock, balanced the system and tried further tweaks. It was fine for a couple of hours and then the problem started again. So, I decided to go into the loft to have a look myself. The ballcock is fine (there is no water dripping from it). I part emptied the expansion tank (rusty, yellowy water) to a couple of inches below the overflow outlet, enough so that the ballcock didn't start letting water flow. However, within a couple of hours, the water level has risen and the starts overflowing again. This is without firing up the boiler, in other words, just leaving the system idle. I notice that when I turn the stopcock off at the mains, the water level does not rise. There are 3 pipes in the expansion tank (you guys know this anyway!) - the ballcock assy, the overflow and an outlet at the bottom which I am assuming is from the boiler/pipework. So, it would seem that water is coming in via this pipe. But how can this be with the system idle? Any help you guys can give would be so much appreciated. Thanks in advance.
PS: I've remembered something else about this: while the guy was flushing the system, he asked me if I wanted to pump replacing because the existing one was 13 years old. He put one on and after he left, I noticed hot water spewing from the overflow. The radiators were stone cold. I turned the boiler off and the pump was red hot. I got him back and he said that the pump must be faulty so I went to the local plumber's merchant and bought a brand new one myself. Don't know if this helps anyone diagnose the problem. Thanks

Possible the coil in your hot cylinder is split (not uncommon old old systems) allowing the cold water tank to feed up into the expansion tank which is often lower (water finding it's own level) turning the mains of and then running cold water like flushing the loo might be lowering the level in the main cold tank so confusing the issue.
I suppose another possibility although I have never known it is a faulty mixer tap that somehow allows mains water to bleed into the hot side having the same effect.

Thank you for your reply, much appreciated. So this presumably would mean a new cylinder? Is it a big (and expensive!) job? I forgot to mention that the guy fitted a new radiator and replaced two old ones. One of the replaced ones is tepid at best. I tried bleeding the rads and air came out of a few.

Do you think the the coil has split due to the flushing of the system? Cheers.

Would be a moderately expensive job, new cylinder and about a days labour no idea what the rates are where you live.
Did cross my mind that the flushing may have been the last straw, but not really down to the engineer I would say.
Try what you did before turn off the main and expansion tank bail a bit out and without using any water (fill the kettle first) keep a check on the water levels in both tanks.
Is there much difference between the water levels in the two tanks?

I'll nip up into the loft and have a look at both tanks and do what you suggest- thanks again. Your time and consideration of this problem is much appreciated

Just a further thought: if I replaced the boiler with an oil fired combi, I wouldn't need a hot water cylinder would I? can existing pipework be used? Is it a big job?

A combi is a good idea given the age of your system but worth replacing any old rad valves at the same time the increased pressure will make old ones leak. Combi is fine as long as you haven't got multiple bathrooms ideally only one hot tap at a time will be supplied by a combi.

Thanks again. There's just the two of us. Would I need to replace existing pipework as well if the increased pressure would be an issue? We've just had a new bathroom fitted, the shower is a Triton SR electric shower. Would this still work with a new combi arrangement even though the electric shower is fed from the cold supply? Cheers

Yes your electric shower can still run off the mains water, pipework should be fine, you do not say why the rads were replaced if they were leaking due to rust you need to be confident about the condition of the remainder. That said a combi only runs at around 1 to 2 bar not as high as the standing mains water which may be around 5 bar so not a risk with normal old pipe work just the seals on valves etc as I said. A good solution to your problem better control faster warm up and cheaper to run. Frees up cupboard space but you do lose the heated airing cupboard.

PC452 said:

I notice that when I turn the stopcock off at the mains, the water level does not rise.. Don't know if this helps anyone diagnose the problem. Thanks

Click to expand...

Then the water must be creeping in before the ballvalve maybe where it connects to the pipe inside the cistern. Are the bases of the tanks on the same level ?

Hi Nige, Yes, they are. I have a plumber calling round this evening so hopefully he'll be able to solve the problem. Thanks