Mixer tap running cold after a short time

[bod1611]

Hi,

We've had a problem with hot water to the kitchen sink for some time now and i've just decided to check whether it's a boiler problem (which i don't think it is).
When the tap runs hot, the hot water light appears on the boiler and the temp moves up into the 80's but after about 60-90 seconds, the hot water light drops off and temperature drops. It looks as if it tries to re-engage but fails and it subsequently drops back into the 50's and the water turns cold. If i leave the tap for about 2-3 minutes and try again, the same thing will happen, if i try immediately, there's only cold water. Feels like somehow by running the tap, it's feeding cold water back into the boiler somehow and by keep trying immediately, i'm just feeding the problem.

I've tried with 2 other mixer taps downstairs and they can maintain a hot temperature which is why i think it's something to do with the tap rather than the boiler but not sure how this could happen?

Any advice would be greatly appreciated, really annoying when trying to fill the sink for dishes - it takes about 10 minutes in total!

 

[jeff the gasman]

You probably have a mixer or blending valve passing somewhere.
What showers do you have and is there a blender on the bath supplies?

 

[bod1611]

I've just read my description back and i haven't mentioned that the tap in the kitchen that is running cold is a mixer tap too - is it likely that the valve in the tap itself is the problem?

I have to admit, i have no idea about a blender on the mixer shower and we do have a mixer on the bath too.

 

[bod1611]

Just did a few more tests by running multiple mixer taps at the same time and the kitchen tap stayed warm. I've subsequently removed the tap filter and ran the hot water which also stayed warm but when the filter was put back, the original problem came back. I couldn't see any build up on the filter itself so i think this is probably a pressure thing and the filter is just tipping the balance - could this be right?