Hot Water / Heating / overflow problem

theukmoog

theukmoog

theukmoog

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This started a couple of weeks ago but I've been away and this morning was the first chance I've had to investigate.

One of the 2 overflow pipes has been dripping onto the patio of a morning, starting soon after the heating/hotwater comes on and stopping soon after the timer ends.

It's coming from the smaller of the two tanks in the loft... I've lifted the lid to have a look inside in this picture...
Tank01.jpg


and inside looks like this...
Tank02.jpg


I was expecting the problem to be the float valve, as I had to replace the float valve in the bigger tank last summer when it started constantly dripping, leading to a constant overflow drip.

This time, it's the small tank (heating/hot water expansion tank?) and it's not the float valve - that turns off completely - but rather the pipe coming into the tank from above is pumping hot water into the tank when the heating/hot water system is on. The water in the tank was quite warm and causing condensation on the outside of the cold water tank next to it.

I have a memory of looking inside this tank when I replaced the other float valve and the water level was nowhere near the top/overflow then (last summer).

We've lived here for over 3y and it has only started this behaviour in the last couple of weeks.

Before I call a plumber out... any idea what's causing it?

Thanks for any advice :)
 
IIRC that smaller tank is normally the header for the central heating, basically allows for the central heating water to have somewhere to go when it expands if it's not a combi boiler, and allows it to automatically top up if say you bleed the radiators (modern combi boilers use a sealed expansion vessel to allow for the expansion, and a manual feed tap under the boiler to do the job of the float valve).

I'm not a plumber but it might indicate there is air/a blockage in the heating pipes causing the water to back up when the circulation pump is running.

Basically it's for the radiators, so go round and check for any signs of cold radiators to give you an idea if there is a blockage.
 
Yes - to both.

Hot water at all taps is ok.

Rads are as expected, although not all are on.

Thinking about it, there's a chance this started soon after that very cold snap a couple of weeks ago when I turned the bedroom radiator on - don't generally like a heated bedroom but it was too cold to ignore!

I'll turn all the radiators on tomorrow and see if they all get hot.
 
What is your system? Is it a gas boiler with a hot water tank upstairs?

Some suggestions, just guesses to help with your investigation - it could be a broken valve somewhere, or it could be a broken thermostat. If the overflow is getting activated, them something is not shutting off properly and causing the water to get overheated.
 
Yes, gas boiler (ground floor) with hot water cylinder upstairs and 2 tanks in the loft (large cold water tank and this small expansion tank).

I'll be experimenting tomorrow with just hot water and then heating with all radiators on and see if/when it still overflows.

Just to add, the system was in when we bought the house, 3 and a bit years ago, and the boiler was last serviced in Sep 2023 (and was 9y old at that point but in very good condition, apparently).

Looking at the pictures of the inside of the expansion tank, there is a lot of, what I believe the technical term is, gunk in there - is that expected for a system this old?
Or is it indicative of a gunked up heating system?
 
Last edited:
theukmoog said:
Just to add, the system was in when we bought the house, 3 and a bit years ago, and the boiler was last serviced in Sep 2023 (and was 9y old at that point but in very good condition, apparently).

Looking at the pictures of the inside of the expansion tank, there is a lot of, what I believe the technical term is, gunk in there - is that expected for a system this old?
Or is it indicative of a gunked up heating system?
Click to expand...

It looks like it could be limescale build up. This is probably what has caused your failure, the build up has caused a valve to stick or as mentioned above, a switch motor to fail.

Your heating would probably benefit from a flush and addition of inhibitor.
 
theukmoog said:
Yes, gas boiler (ground floor) with hot water cylinder upstairs and 2 tanks in the loft (large cold water tank and this small expansion tank).

I'll be experimenting tomorrow with just hot water and then heating with all radiators on and see if/when it still overflows.

Just to add, the system was in when we bought the house, 3 and a bit years ago, and the boiler was last serviced in Sep 2023 (and was 9y old at that point but in very good condition, apparently).

Looking at the pictures of the inside of the expansion tank, there is a lot of, what I believe the technical term is, gunk in there - is that expected for a system this old?
Or is it indicative of a gunked up heating system?
Click to expand...
The small tank is the radiator header, so if you're getting hot water returned to it that'll be gunk from the radiators, or possibly the heating coil in your hot water tank (the boiler in this sort of system normally heats the hot water indirectly by running heated water through a copper coil that indirectly heats the water that'll come out of the taps).
As radderfire says emptying the rads, flushing them and refilling with an inhibitor would probably be a good idea at some point, it's not normally too hard to do yourself but can be time consuming and something best done when it's warm enough you can hook up hoses to the drain points (there should be several at the low points) and run them out the door to a drain, then when you refill you want to open the bleed valves on the highest radiator.
There should be some good guides online, as it's something that is quite common.

The big tank is the header for the hot water as it needs a lot more volume to maintain pressure to the taps/shower as the water in the hot water tank gets drawn out.
 
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