Kitchen mixer tap – Cold water will not turn off

[accelerator]

I'm not an expert plumber but my view would be either the blue o seal has degraded, or the quarter turn cartridge itself is now faulty. So, it's probably new cartridge time. This video might help you out:

 

[accelerator]

Also, this might be helpful - how a ceramic tap cartridge works:

 

[Mr M]

I've thought about this a bit more and your test of swapping the hot cartridge into the cold side is not valid, because the cartridges are handed, that's why you need specific cartridges for hot and cold, so they can both open by moving the tap the same way (when they're on different sides).

Ok but would it not be the case that if the hot cartridge was fitted on the cold side, it would just mean that when the cold tap lever was in the 'on' position, the tap would actually be 'off' and vice versa? I actually think I noticed that was the case when I swapped them over earlier on today as part of my testing.

 

[accelerator]

I have ball-o-fix valves on both the hot and cold water supplies to the tap. When I turned the cold water supply back on, water started running from the tap head so I assumed I had reinstated the gland with it in the ‘tap on’ position. When I moved the tap on/off lever to its opposite way, the cold water continued to run. I turned the tap on/off lever fully both ways but the cold water continued running.

I just want to clarify. When you say "tap head" do you mean water is coming out of the tap spout, i.e. where water normally comes out, or do you mean it's coming out of where the tap attaches to the base of the mixer?

Also, wherever it's running from, is it normal mains pressure speed or reduced speed?

 

[Mr M]

When I said 'tap head', I was referring to the taps spout ie where the water normally comes out of.

The water that came out was normal mains pressure, not at a reduced flow.

 

[accelerator]

Ok but would it not be the case that if the hot cartridge was fitted on the cold side, it would just mean that when the cold tap lever was in the 'on' position, the tap would actually be 'off' and vice versa? I actually think I noticed that was the case when I swapped them over earlier on today as part of my testing.

It's hard for me to say without having one in front of me to look at. What I can say though, is that as you are looking down along the axis of the tap, the cold tap is opening clockwise, whereas the hot tap is opening anti-clockwise, that's the difference.

 

[accelerator]

To be honest I'm not that sure of how the internals of your mixer tap work. However, I'm assuming that the blue ring on your cartridge should sit on the metal around the hole shown and form a watertight seal. If it's not working when you try either cartridge, then it suggests maybe the ring isn't reaching the surface, so perhaps you have a problem with the thread that the cartridge goes into, meaning you can't tighten the cartridge sufficiently to get the blue ring to sit onto the metal surface tight enough to form a seal.