Hot water problem in shower and taps

I have a problem getting hot water. The boiler is working fine and regularly serviced and the heating is working fine.

I have mixer taps and when I turn the hot on it takes ages for the hot water to come through (its a lot faster if the heating is on at the same time).

The shower is the same, a mixer and that runs cold for ages and takes a long time for the hot to come through. It will often suddenly go cold mid shower as well. There is no problem with the water pressure coming though the shower.

Any ideas what the problem is please.
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  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    How long does the hot water take to come through if the mixer is turned to 'hot'?
    Is this a recent development, or has it always been like this?
    I'm guessing it's a combi boiler? Make, model, age?
  • Thanks for the reply.

    It's always been a bit temperamental. It can take a coupe of minutes for it to run hot. It is a combi boiler, t's a Baxi, not sure of model as I'm not at home at the moment so cant look but it will be about 10 years old.
  • Is there a fair distance from the taps to the boiler ?.... don't forget it has to push through all the cold water in the pipes first when it flares up.... then it's followed by the cold pipes taking away the heat initially from the hot water as it flows through
  • turnitround
    turnitround Posts: 715
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    edited 8 November 2023 at 10:35AM
    Is there a fair distance from the taps to the boiler ?.... don't forget it has to push through all the cold water in the pipes first when it flares up.... then it's followed by the cold pipes taking away the heat initially from the hot water as it flows through
    Not too worried about it taking a while to come hot downstairs but the shower is a pain to use.
    I would say it is about 5-6 mtrs from boiler to shower. The thing with the shower is that you can wait a minute or so for the hot to come through before getting in the shower which is ok, but then it will intermittently go cold when you are under the shower so you then have to twiddle the lever on the tap (the lever is not loose or slipping) and step back till you get the warm shower again. This happens constantly while you are showering.

    It has got a lot worse recently, I know this happens if someone turns another tap on downstairs but that is not what is happening here.
  • We have had 2 different boilers in the last 20 yrs and they both have done the same thing, similar to yours 5/6 metres away.... takes a while to go hot, then goes to a coldish phase then slowly heats up again to desired temp...... but it only does that once, not like you say though  "This happens constantly while you are showering"
  • rob7475
    rob7475 Posts: 836
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    When was the boiler last serviced? If you run hot water through a tap, does it stay hot once it gets up to temperature? If it's just at the shower, it could be the thermostatic cartridge on the shower that's playing up. 

    Also check if your boiler has a 'pre heat' mode for hot water, this keeps a small amount of water hot in the boiler's heat exchanger ready to use and speeds up the time it takes to get hot water at the outlet.

    If the water cycles hot and cold at all your taps, it could be the plate heat exchanger that's clogged on the boiler. 
  • sheramber
    sheramber Posts: 18,651
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    When I had that problem of going cold when showering it was because the boiler was cutting out. the plumber checked the boiler with the shower running and  could hear it  cutting out.

    As it was anol boiler of undeterminable age- BG said 5 years old, plumber said they stopped making that model 15 years ago- I bought  a new boiler. 
  • Thank you all for the replies.

    Just thinking about what I said about the water being much hotter when the heating is on. Hubby went to have a shower, same problem, cold for ages then ran warm then less than 20 seconds later went cold again, So I put the heating on and he tried the shower again. Perfect hot water and it stayed at the right temp throughout the shower.

    Have done the same again since in case it was a fluke but got the same result so from now on that is going to be the answer, heating on for 5 mins while in the shower. 
    The boiler is serviced annually and it is due in Feb so will have a chat to the guy when he comes and then spend the next 30 mins trying to convince him we don't want a new one. The service is done by Baxi and I would shop around first before making a decision.
  • Micron
    Micron Posts: 78
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    You could try turning down the domestic hot water temperature setting to 50 deg, this might improve the hot water flow to the showers.

  • ThisIsWeird
    ThisIsWeird Posts: 4,458
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    Thank you all for the replies.

    Just thinking about what I said about the water being much hotter when the heating is on. Hubby went to have a shower, same problem, cold for ages then ran warm then less than 20 seconds later went cold again, So I put the heating on and he tried the shower again. Perfect hot water and it stayed at the right temp throughout the shower.

    Have done the same again since in case it was a fluke but got the same result so from now on that is going to be the answer, heating on for 5 mins while in the shower. 
    The boiler is serviced annually and it is due in Feb so will have a chat to the guy when he comes and then spend the next 30 mins trying to convince him we don't want a new one. The service is done by Baxi and I would shop around first before making a decision.
    That's interesting - and weird!
    A combi boiler has a diverter valve inside it, which switches the main boiler flow to either the CH system or to heat your DHW. They can't do both at the same time.
    As Rob says above, if a tap or shower runs hot, then goes cold, then hot, then... that is classic blocked Secondary (or PlateToPlate) Exchanger symptoms. (In essence, because the boiler cannot pass the produced hot water into the P2P exchanger fast enough - because it is partially blocked - then the excess hot water trips the overheat 'stat, so the boiler shuts down until it cools again - ergo, the water runs cold.)
    However, you have this additional weird thing going on - if you turn on the CH, the DHW seems to then work fine. That's different, and shouldn't be the case, as both don't run at the same time! (
    If you turn on the CH, and then run a hot tap, the combi immediately diverts away from the CH to provide DHW - the latter has priority. Turn off the hot tap, and it rediverts to the CH.)
    So, what I suspect is happening is that your diverter valve is stuck, or sticky. This means it's partially stuck between the two positions, and probably doesn't reach the full extent to each side. If you run just DHW, then not enough boiler water can get through to the P2P as the valve is partially closed to that side, so it overheats - hence goes cold and hot and cold and. However, when you also turn on the CH, the valves moves a bit more, so now allows boiler water to the rads, and that gives enough overall flow to keep the boiler running.
    That is pure guesswork, of course.
    Your other tap in the bathroom is also a mixer? Is it also thermostatic (I presume the shower is)? If you turn it to just hot, does it work fine? 

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