overflowing F & E tank

21 Mar 2006
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United Kingdom
Can any of you pro's out there shine some light on my little problem ?

Water has started to drip from my f & e tank overflow continually when the heating is on. I syphoned some of the water out and all was fine for about 6 hours then the dripping started again , checked the f & e tank and it was full again. Thought there may be a restriction in the return pipe to the boiler and the water was going back up the feed pipe into the tank , checked the pipes with a magnet and it stuck to them. Removed all the rads from the system and flushed them out with a power washer in the garden , thick black gook all washed out , refitted rads and put Fernox DS 40 into the system. Followed the instructions on the Fernox to the letter and have flushed all black gook out of pipework. Replaced the ballcock valve in the F & E tank and bent the arm of the float so the water level in the tank is approx 1" above the feed pipe. Heating working better than it ever has but the next day the dripping from the overflow had resumed , checked the F & E tank and it is full again ( heating run for 6-7 hours and the water level in the tank has risen approx 4"). Any ideas as to where the water is coming from ? Does the water expand that much when heated ?Could it be the Aerjec II thingy in the loft below the F & E tank ? have tried solating the feed into the F & E from the cold water tank in the loft and checked the vent pipe from the Aerjec II thing but it is always dry. Only other observation is that the feed pipe between the F & E tank to the Aerjec II thingy is hot up to approx 4 " below the F & E. Thanks in advance RGBD.
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Is the water level in the large cold feed tank higher than the level in the CH header (F&E) tank? If so, it could be leakage from the HW cylinder into the heat exchanger coil inside the cylinder and thence to the header tank.
Others may be able to give you other suggestions but a common fault seems to be a blockage in the feed pipe where it joins the main pipework.

Test this as follows with heating off:
Mark the water level in the F&E tank.
Tie up the ballcock or isolate it if there is a valve fitted.
Drain off a couple of bucketfulls of water from the system through the draincock at the boiler or at a low radiator.
Check that the water level in F&E has dropped accordingly.
If not you have a blocked feed and any water going up the vent or leaking from the ball cock cant return. System may eventually empty.

The Fernox may not have been any use to this problem because there was no flow thru' the feed pipe when you did all the flushing.

If feed is blocked you will have to drain donw sufficiently to cut the feed pipe, clear the blockage and then rejoin.

Let us know how you get on
turn boiler off isolate water to f and e tank and drain if it fills with water again the coil in your cylinder is punctured
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Chrishutt - yes the F&E tank is below the level of the water in the big cold water tank.

Jobloggs - should have said I put the fernox in the F&E tank and then ran the heating for a couple of days before draining the system , refilling it and adding the fernox neutraliser and running the system for a couple of hours then draining & refilling the system 5 times to flush all the sludgy stuff out.
No water getting up through the vent pipe as it is always bone dry and definately not getting in through ballcock valve as I have isolated this by closing both the valve coming out of the cold water tank and the valve which feeds the ballcock valve ( which I also replaced) but will try your measuring tip tomorrow out of curiosity Cheers RGBD
holty said:
turn boiler off isolate water to f and e tank and drain if it fills with water again the coil in your cylinder is punctured

Very well put.

It sounds like a new hot cylinder is on the cards.
Holty - Cheers , will try that tomorrow . Stupid question , Do I need a Corgi registered heating engineer to swap out the HW cylinder or a plumber or just do it myself ??
Your latest post suggets the problem is not a blocked feed.

Unfortunately for you, Chrishutt and Holty are probably right on the mark. If so you will need a new cylinder which is an expensive fix.

Fernox DS 40 is apparently quite aggressive and can remove corrosion product which is sealing pinhole leaks. However your first post suggests that the problem was there before all the cleaning out was done.

Fernox do a leak sealer. I have never used it but there has been a post on this forum within the last 2-3 days praising it. Some of the experts may have an opinion.
rgbd said:
Holty - Cheers , will try that tomorrow . Stupid question , Do I need a Corgi registered heating engineer to swap out the HW cylinder

rgbd said:
...or a plumber
Preferable a heating engineer.

rgbd said:
...or just do it myself ??
You could do. Just take your time, learn about it first, and ask if you need advice.

The only stupid questions are those that aren't asked.
Just a bit of oar-sticking from me:

I had a faulty cylinder replaced not long ago, took the opportunity to have the biggest one that would fit in my airing cupboard, so it holds more than enough for a big bath and some washing up. They aren't much more expensive that small ones, and it's more efficient to run the boiler once to produce a lot of hot water, than a several times, each producing a little (especially in summer). the new ones also come with built-on foam insulation, but I also smothered mine in two red jackets that I'd had from the old one, and plenty of Climaflex pipe lagging.

Had I thought about it enough, I'd have had one with an upper and lower immersion heater as well, for the infrequent times when my boiler isn't working. it seems you can't buy a dual-element immersion any more.

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