tank overflow

[rach85]

Hi, I could do with some advice as I'm getting more and more confused with the advice I've had from central heating engineers from my utility company lately. I've posted this on other forum websites but thought the more advice the better. I have no central heating experience whatsoever, so I apologise if I have to use small and basic words!!
Since moving into our home last yr (approx 20yr old), we had a problem with water overflowing into our garden from the smaller of the two tanks in the attic. My fiance changed the ballvalve and washer and set it to correct height (I'm just taking his word for that, like I said I have no clue), which worked fine until a few weeks ago. We noticed that a steady drip starts to overflow again, and we are pretty sure its only when the boiler is running and our hot water is on (as its still relatively mild we havent had the heating on in ages so I dont know if it would do the same when that is on we havent tried). It seems worse when we run a lot of hot water, like filling the sink to wash up or running the bath. He had a look in the attic again and he noticed that the water level in the larger tank is dropping slightly when the water is overflowing. As we have central heating cover with out utility company we called them out. The first engineer called didnt have a look at anything in my house except a pump in the airing cupboard. He turned that down and said if that didnt work then we definitely have a "blocked cold feed" and seeing as our airing cupboard is downstairs under the stairs, its going to involve a lot of carpet and floorboards coming up and possibly making big holes in my wall upstairs....which incidently isnt covered by policy. I decided to seek advice from other people including two gas engineer friends, they said that another likely option is that there is a crack or small hole in the coil in the cylinder (which is covered in my policy). When the second engineer came for a 2nd opinion he again didnt look at my system whatsoever and said it was definitely a cold feed blockage and would "never in a million years" be a cylinder problem.
Any ideas on the likely answer? Or maybe how to test if its one or the other, without destroying my lovely bathroom wall tiles?

 

[david williams]

it could be several things ,from pump over into f@e tank (small tank) leak from cylinder into primary coil.i have had this problem several times most times pump over due to poor design and once due to leak between cylinder and coil

 

[doitmyself]

Probably a hole in the cylinder coil; but let's eliminate the blocked feed first.

My fiance changed the ballvalve and washer and set it to correct height

When the water is cold the level should be between one and two inches above the lower outlet (feed). When the heating is on the water level will rise, but should stop well clear of the overflow pipe.

Drain some water from the heating system (temporarily lock motorized valves in MAN) and check that the level in the tank goes down and the ball valve opens before the level reaches the lower outlet. This will prove if there is any blockage in the feed pipe. Reset motorized valves to AUTO.