Hot water stops once it gets up to temperature

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Mander

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This just started a day or two ago but it got really bad this morning. When I'm running the hot water in the kitchen mixer tap it cuts out once the water gets hot. In order to keep it going you have to turn it on more, but if you leave it it will turn on and off again very rapidly.

What should I be looking for to fix it? Or do I need a whole new tap?
 
Do any other taps in the property suffer from the same symptoms ?
 
And does it stay hot if you run another tap at the same time?
 
Also, what kind of boiler (assuming combi)?
 
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Yes, a combi. All the other taps are fine and both work and stay hot both when it's running and when it cuts out.
 
You mean the water stops flowing? The tap internals are expanding with the heat. New washer and regrease might cure it if not it's a new cartridge
 
This may be the cause if the mixer is a thermostatically controlled mixer

When the mixer tap is first turned on it will have it's hot port wide open and there will be a good flow along the hot pipe, enough to cause the combi to fire up. When hot water reaches the thermostatic controller the hot port will reduce the flow in the hot pipe to obtain the required temperature for the mixed water. If the flow in the hot pipe is reduced to a flow rate less than the minimum required to keep the boiler's flow switch active then the boiler will shut down. Alternatively the flow switch stays active but the flow is low enough that the water temperature exceeds maximum and the over heat sensor shuts the boiler down.

The same would apply if there is a blend valve in the hot supply to the tap.
 
The tap itself stops flowing, which causes the boiler to shut down but only because the water stopped.

I'm assuming I need to change the washer but I tried to take it apart yesterday and I can't get the valve to come out. I tried the usual tricks like WD-40 and whacking the spanner with a hammer but it won't budge. Any other tricks I should try?
 
The offending tap:

I'm probably using the wrong terms for everything. I can't turn the large brass nut, but surely it must come out?

IMG_20210712_150457347.jpg
 
Usually 17mm of some kind, be it open end spanner, ring spanner, deep socket. If none of those shift it, try pouring some boiling water on it, and try it again.
 
I used an adjustable one because I was being too lazy to dig out the proper wrenches but it fit fine. I'll try hot water.
 
That doesnt have a cartridge, the whole brass part will come out, as @CBW says pour hot water on the chrome part and then put a ring spanner on the brass part and hit it with a hammer, the sharp shock will make it come out, sounds like it is chocked with calcium
 
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