Help identifying water tank / container in attic.

4 Sep 2022
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United Kingdom
Hi all,

Just wondering if someone can help me identify this water tank / container, and let me know how I should be maintaining it.

It sits alongside my main water tank, and, as can be seen from the attached images, it's quite dirty. The overflow pipe leads out the back of the house, but hasn't been overflowing at all.

It was suggested to me by a neighbour (who didn't see it) that it might be related to my heating system, the radiators and such.

To me, it looks like I should be bailing out some of the dirty water and filling it with fresh water. Or maybe just bailing out the dirty water altogether.

Any help and advice would be appreciated.


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1. It is the Feed and Expansion (F&E) tank for your central heating. As the name implies, it feeds water into the heating system, and allows for expansion of the water as it is heated.
2. There is very little movement of water through the tank. The system rarely needs water feeding into it, and it is relatively rare for expansion put water into the tank. Hence the water tend to go stagnant.
3. It would be advisable to clean the tank to avoid muck being drawn into the systems, although this is a fairly remote possibility unless the system is drained down. To clean it:
3a. Stop new water entering the tank. If there is a stop cock on the inlet (pipe leading to inlet and float valve at top of tank), turn it off. If no valve, put a piece of wood across the top of the tank and tie the float valve arm to it to close the float valve.
3b. Use a bucket, old mugs, sponge to get all the old water out. Clean the inside of the tank as well as possible.
3c. Allow water back into the tank. It should settle to cover the outlet at the base of the tank with about 100 mm of water above.
4. Cleaning the tank will very slightly dilute the level of inhibitor in the system. If inhibitor was recently (last 2 or 3 years) replaced, ignore dilution.
5. If dilution is an issue and you want to add inhibitor:
5a. Carry out 3a and 3b.
5b. Allow a few (say 5) litres of water out of the system by opening a radiator drain off ( or other drain off) valve on the central heating system, running the waste to a bucket or outside.
5c. Close the drain off.
5d. Pour a 1 litre bottle of Sentinel X100, Fernox F1 or similar inhibitor into the F&E tank.
5e. Carry out 3c.
5f. The reason for draining water from the system is to allow the inhibitor to flow into the system, rather than sitting for months / years in the F&E tank.
Wow, thanks so much for the concise answers. This has put my mind at ease that it's a fairly common thing. Cleaning should be easy.

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