Can anyone tell me why we have hot water everywhere except here?

We have a shower which has a mixer tap. 

When you are under the shower it starts hot then goes cold. When you turn the shower on and then feel the pipes which are inside the cupboard behind the shower you obviously feel the hot one is running hot but then it suddenly goes cold.

Someone has had a look at it and says it could be the regulator inside the mixer which has gone faulty and therefore it is cutting off the hot water. 

He says he can put a part in which may work but it may need a new shower. Its quite expensive to try the part if it does not work.

But we have a concern which may be completely unfounded, but, about 3 weeks ago we had our boiler serviced and the problem began after that service.  Could it be something relating to the service that has caused the problem?

Someone else has told me that if the regulator inside the shower is not working then there is a safety thing inside the shower which will automatically cut the hot water off as a safety thing. I have also been told there could have been something grit or something in the pipes which was dislodged during the service and may be in the mixer so I should keep fiddling the dial back and forth to try to dislodge it which I have done without any success. 

So I'm in a quandry now, do I risk changing the part only which I have been told may or may not work?
Do I need to call back the service guy which will result in a charge if it is not a problem with the boiler caused during the service?
 Do I need another plumber for a second opinion. 

Every other tap in the house is fine, its just the shower that is the problem. Any advice please?
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  • Where is hot water from? Cylinder or combi boiler? If boiler, does shower still go cold if you also run a hot tap (anywhere in house) at the same time. It's not too difficult to take the thermostat out of a shower valve and clean it or change it.
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  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    edited 13 December 2023 at 12:58PM
    Combi boiler, Swingie? Make & model?
    And, are you sure it doesn't happen with other taps? Could it be that the shower is just being run for longer than other hot taps? 
    And, if you are brave enough to stay in the shower - pointing it away from you - does it heat up again after, ooh 30 seconds or so?
    At the moment, it could be the shower mixer if it's a thermostatic type - is it? Or it could be the boiler, but I can't see a service having done this.
    Oh, if it isn't a combi - if you have a hot cylinder - then chances are much higher that it's a faulty shower mixer.

  • grumbler
    grumbler Posts: 58,591
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    swingaloo said:
    We have a shower which has a mixer tap. 

    When you are under the shower it starts hot then goes cold. When you turn the shower on and then feel the pipes which are inside the cupboard behind the shower you obviously feel the hot one is running hot but then it suddenly goes cold.

    Someone has had a look at it and says it could be the regulator inside the mixer which has gone faulty and therefore it is cutting off the hot water. 
    Even it it's true, the pipe cannot get cold suddenly. If HW flow stops it takes time for the pipe to cool down.
    Are you sure the hot and the cold water supplies have the same pressure? If hot water is gravity fed (vented HW cylinder) and cold water is mains, then it's very hard to make the mixer shower working. High-pressure cold water will overpower low-pressure hot water and will start filling the HW pipe from the mixer valve.


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  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    edited 13 December 2023 at 1:10PM
    grumbler said:
    swingaloo said:
    We have a shower which has a mixer tap. 

    When you are under the shower it starts hot then goes cold. When you turn the shower on and then feel the pipes which are inside the cupboard behind the shower you obviously feel the hot one is running hot but then it suddenly goes cold.

    Someone has had a look at it and says it could be the regulator inside the mixer which has gone faulty and therefore it is cutting off the hot water. 
    Even it it's true, the pipe cannot get cold suddenly. If HW flow stops it takes time for the pipe to cool down.
    Very good point.
    It seems like that the shower runs cold because the hot flow to it has been replaced by cold, and not because the shower has shut the hot off at the mixer. 
    If it's a combi, I suspect a sludged P2P. If it's a vented cylinder, I'm liking your theory. 😜
  • swingaloo
    swingaloo Posts: 2,638
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    Where is hot water from? Cylinder or combi boiler? If boiler, does shower still go cold if you also run a hot tap (anywhere in house) at the same time. It's not too difficult to take the thermostat out of a shower valve and clean it or change it.
    Its from a boiler. The shower always did go cold if you turned on another tap at the same time.
  • swingaloo
    swingaloo Posts: 2,638
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    Combi boiler, Swingie? Make & model?
    And, are you sure it doesn't happen with other taps? Could it be that the shower is just being run for longer than other hot taps? 
    And, if you are brave enough to stay in the shower - pointing it away from you - does it heat up again after, ooh 30 seconds or so?
    At the moment, it could be the shower mixer if it's a thermostatic type - is it? Or it could be the boiler, but I can't see a service having done this.
    Oh, if it isn't a combi - if you have a hot cylinder - then chances are much higher that it's a faulty shower mixer.

    Combi boiler, Swingie? Make & model?
    And, are you sure it doesn't happen with other taps? Could it be that the shower is just being run for longer than other hot taps? 
    And, if you are brave enough to stay in the shower - pointing it away from you - does it heat up again after, ooh 30 seconds or so?
    At the moment, it could be the shower mixer if it's a thermostatic type - is it? Or it could be the boiler, but I can't see a service having done this.
    Oh, if it isn't a combi - if you have a hot cylinder - then chances are much higher that it's a faulty shower mixer.

    It just occasionaly the kitchen tap will go cold mid flow but then within seconds gets warm again.  Its a Baxi boiler, not sure of the number. 
    If you stay in the shower with it turned away and wait it does not get hot again, once it is cold it stays cold.
  • swingaloo
    swingaloo Posts: 2,638
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    grumbler said:
    swingaloo said:
    We have a shower which has a mixer tap. 

    When you are under the shower it starts hot then goes cold. When you turn the shower on and then feel the pipes which are inside the cupboard behind the shower you obviously feel the hot one is running hot but then it suddenly goes cold.

    Someone has had a look at it and says it could be the regulator inside the mixer which has gone faulty and therefore it is cutting off the hot water. 
    Even it it's true, the pipe cannot get cold suddenly. If HW flow stops it takes time for the pipe to cool down.
    Are you sure the hot and the cold water supplies have the same pressure? If hot water is gravity fed (vented HW cylinder) and cold water is mains, then it's very hard to make the mixer shower working. High-pressure cold water will overpower low-pressure hot water and will start filling the HW pipe from the mixer valve.


    Right to explain a bit clearer as all this is above my pay grade lol. 

    When I say it suddenly gets cold I mean - If someone gets in the shower it is hot when first turned on. When someone got in the shower and turned it on I stood at the side of the shower where the pipes go through the wall to the mixer on the other side and had my hands on the pipes. I feel one get red hot and one is  cold. A minute or so later the hot one will go cold (it my cool gradually but its only  seconds before it feels just the same as the cold pipe).

    I dont know about the pressure being the same on hot and cold but there is no change in the flow to the bathroom sink, both taps are fine. The sink is further away from the boiler than the shower. The boiler is a combi and is in the next room to the bathroom. 
    The shower is a Mira with 2 levers, one turns on and off, the other turns one way for hot and one for cold, very basic.  

    We have been told that there is a part inside (cartridge) which can be replaced but may or may not solve the problem but as its quoted a couple of hundred to try it we dont want to pay for that and find its not the problem.  To change the valve is double the price and to change the shower itself is treble the price so need to be certain we are doing the right thing.  To change to a different type of shower would be worse as the tiling would be affected.
  • sheramber
    sheramber Posts: 18,605
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    When my shower was doing that it was becasue the boiler was cutting out.

    The plumber turned on the shower then listened to the boiler and could hear it cutting out.
  • FreeBear
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    ThisIsWeird said: If it's a combi, I suspect a sludged P2P. If it's a vented cylinder, I'm liking your theory. 😜
    Sludge in the heat exchanger would affect all the hot water taps, not just the shower. So i would suspect the mixer/cartridge first.

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  • swingaloo
    swingaloo Posts: 2,638
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    sheramber said:
    When my shower was doing that it was becasue the boiler was cutting out.

    The plumber turned on the shower then listened to the boiler and could hear it cutting out.
    I just tried turning on the shower and listening tom the boiler kick in. The first time it did kick in and thewater stayed hot in the shower, second time I turned the shower on the boiler di not kick in so I have called Baxi back.
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