Hot Water / Heating / overflow problem

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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 :)
 
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.
 
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?
 
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