Loft tank leaking via overflow

Uplander1111

We have a small water tank in the loft that occasionally drips through an overflow - not badly but I am concerned that it might need looking at.  I wonder if somebody might be able to explain what this tank does so that if we need to speak with a plumber I have a little bit of understanding.

The bungalow has a gas boiler that heats the hot water in a Joule tank in the airing cupboard.  The only hot water we use is for two showers each morning and as far as I can tell it is this usage that triggers the leak through the overflow (presumably the ballcock is not shutting it off quite soon enough).

Could somebody just give me a quick overview on how this tank interacts with the rest of the system?

Thanks in advance!

Exodi

Given what you've said, I'd recommend a plumber or heating engineer to take a look at it.

Rodders53

Small (4 gallon, 20 litre) tanks (aka cisterns) are known as Feed and Expansion (or header) cisterns for old-fashioned vented central heating systems.  

The water level should be quite low when cold, but when the CH (or HW tank coil) is running as the circulating water heats up and water expands the volume in the tank will increase.  It should never reach the overflow pipe level.  
The water will over time evaporate into the loft space and need topping up via the fill valve, automatically.

If the fill valve never shuts off completely it may overfill (it may only be a slow drip, drip )...  The valve washer may just need replacing or the ball valve arm adjusting to ensure when cold there is not a lot in it.

Any plumber or CH technician should be able sort that.  If you can't DIY it.

Modern CH systems are sealed or unvented, with red expansion vessels to absorb the water expansion (often built into the boiler).

AFAICT the Joule is simply an unvented hot water cylinder -- that will have a white pressure vessel to absorb the water expansion inside that cylinder as it is heated from cold to hot... there will be safety relief valves that can let by water in various scenarios... those usually discharge into a 'tundish' overflow in the airing cupboard so as to be seen by the householder.  Mains water pressure for HW so a better shower experience.  That probably requires an unvented (G3?) plumber if it's that 'overflow' you see in the black plastic tundish.

Uplander1111

Thanks Rodders53 - your last paragraph describes our system including the expansion vessel so if it is unvented (and sorry if I am being thick), what is the small loft tank for?  

WIAWSNB

Uplander1111 said:

Thanks Rodders53 - your last paragraph describes our system including the expansion vessel so if it is unvented (and sorry if I am being thick), what is the small loft tank for?  

"Small (4 gallon, 20 litre) tanks (aka cisterns) are known as Feed and Expansion (or header) cisterns for old-fashioned vented central heating systems."

Your DHW (Domestic Hot Water) may be pressurised, but the primary system - the boiler and rads and heating loop - could be vented. In which case you'll have a small F&E tank in the loft, and that's likely to be the culprit. 

Can you get up there for a looksee?

If it's a stuck ballvalve, then it's a simple job for a plumber. Quite DIYable too, but prob not a good first task as it's up in the loft where it can do damage if it goes wrong... 

Uplander1111

Thanks WIAWSNB.  I guess therefore it makes it a system that is part vented / part unvented.  Reading Rodders53 I thought it meant that a system was one or the other.

I assume that my impression the dripping was linked to hw use was therefore erroneous.

Anyway, thanks for the post. 

  

WIAWSNB

There are two separate circuits, the primary and secondary. They can be different in terms of being vented or not.
If there isn't a pressure gauge on the boiler, then it'll be vented, so a wee tank in t'loft.

Uplander1111

Thanks for the help!

danrv

Uplander1111 said:

presumably the ballcock is not shutting it off quite soon enough)

It can be adjusted. I changed my loft tank ball valve a while ago as the overflow pipe was dripping. 
It was in bad condition and wasn't shutting off the water completely. You can just change the washer but a new valve is quite cheap. The plastic ball can be reused on the new one: Straight forward job for a plumber if access is good.

https://www.screwfix.com/p/flomasta-brass-part-2-float-fill-valve-1-2-/352JN?tc=ED6&gclsrc=aw.ds&gad_source=1&gad_campaignid=22605979379&gclid=CjwKCAjwprjDBhBTEiwA1m1d0rlwMiGIw6aSDiqP8AdrBlqg0c_5J-M-KEg34cpt19h0BddIVa3ygBoCZc8QAvD_BwE

silvercar

If you are in a hard water area, it could just be that the ball of the valve has got too much crud and limescale on it weighing it down and stopping the valve shutting completely. Just using your hand to scrape some of the crud off may solve the problem.

Uplander1111

Thanks to all for taking the trouble to post.