I have a mixer shower connected to a combi boiler (the model is RD628) and the water runs intermittently completely cold for approx 15 seconds approx every 2 mins. When I look at the boiler the ignition light is going on and off. The fault light is not turning which suggests its not an overheat cut off issue.

Every other tap in my home doesn't have this problem, I can run the hot water constantly and the temperature is consistent, and the ignition light on the boiler remains constant. This is also true for the shower providing I turn the water up to be very hot.

I have already tried changing the thermostatic value in the shower and this did not resolve issue, and I bought a completely new bar shower as well and this also did not resolve the issue.

I have tried changing the hose and the shower head also. I have been living with this for 7 years and am getting a bit sick of it.

Please help, even if the just advise on what sort of professional to engage. Is this a plumbing problem or a boiler problem?

  • Are you on well water with your own pump or city water?
    – crip659
    Commented Oct 1, 2023 at 14:32
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    Do you use the shower fairly cool (or with quite low flow)? I suspect the boiler is reaching its maximum output temperature and cutting out, giving a pulse of cold that travels down the pipe, before its thermostat turns it back on again
    – Chris H
    Commented Oct 1, 2023 at 15:42
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    Some boilers have a feature like a shower timer: they intentionally interrupt the hot water flow to signal you should end your shower. Could it be your boiler has such a feature and set to a short time? What is the make and model of your boiler? Commented Oct 1, 2023 at 17:59
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    @BartvanIngenSchenau If I had that, I would get some exercise with a sledge hammer, to reset it.
    – crip659
    Commented Oct 1, 2023 at 19:44
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    @crip659, when such a feature is available, it will also have a setting "off". It could be that someone fat-fingered when trying to set it off and set it to the shortest time instead. If it is present, it should be mentioned in the manual. Commented Oct 2, 2023 at 6:07

1 Answer 1


I will take a SWAG and say your shower is not drawing enough hot water to keep the boiler operating. Try running the hot water in your sink and take a shower, If that solves the problem the easiest solution would be to use a shower head with a little more flow.

Not sure where you are from but in our area the amount of water allowed to flow through a faucet, shower head, etc is limited. Years ago I remember 5GPM (Gallons Per Minute) flows rates. Depending on your location retailers are now limited to shower heads with a maximum of 2.5 GPM, 2.0 GPM, or 1.8 GPM flow rate.

  • 1
    Yes! Running the tap at the same time means the shower stays hot! Also removing the shower head means the water stays hot, the water in fact becomes too hot. How to know how much flow a shower head has before buying it? (a low pressure shower head?) Are there any other variables I can play with to make the boiler draw more water, or enable the boiler to run?
    – Dan
    Commented Oct 3, 2023 at 20:43
  • You can cool the water down by adjusting the temperature using the shower mixing valve or opening the cold some. Using one of the non eco friendly shower heads with more flow would probably solve it for you and give you a more enjoyable shower.
    – Gil
    Commented Oct 4, 2023 at 4:54
  • Sorry if I am being dumb. What specifically do I need to search for. If I put 'showerhead with good flow' into google this doesn't return anything useful. What do you mean by adjusting the mixing value. I know there is a thermostatic mixer the shower, but ultimately that can only be adjusted by making the water hotter or colder, like you do when anyone operates a shower. If I set the shower to a colder temperature the water is just cold all the time.
    – Dan
    Commented Oct 4, 2023 at 17:51
  • @Dan - pull the screen out of the head. Pull the rubber o-ring out with a pin or something. Remove anything else that will remove. Now you have a working shower head. How can I improve the water pressure in my shower?
    – Mazura
    Commented Oct 5, 2023 at 0:27

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