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Discuss Knocking / Banging when hot water switches on in the Plumbing Forum area at

  1. DNK

    DNK New Member

    Hi All
    I recently fitted a new shower and since then, everytime my hot water kicks in to get up to the set temperature, there is a knocking / banging noise that continues until it reaches the temperature. I have read a few things online about completely draining the tank and then turning the water back on. But I have also read about switching everything on in the house, all taps, dishwasher, washing machine etc... to flush out any air. So both kind of contradict each other. Any advise would be greatly appreciated because the banging is driving the whole family and next door neighbour insane. many thanks in advance.
  2. Best

    Best Trusted Plumber

    What sort of hot water system do you have?
    Is it a high pressure system (an unvented cylinder, normally white in colour)
    Or a low pressure gravity system (copper cylinder with a cold water tank usually above it in attic) ?
  3. DNK

    DNK New Member

    Thanks for your reply. I have the copper cylinder in the airing cupboard and a cold water tank in the loft - so I am guessing a low pressure gravity system :eek:)
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  4. Chuck

    Chuck Active Member

    Can you provide a few photos showing the tank and the pipework, please? When you say "I recently fitted a new shower" what type of shower and did you add new or modify any existing pipework?

    How is your hot water heated? What temperature does the stored water get to? Have you measured it at a tap? Did you change the thermostat when you were doing the shower?
  5. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Did the banging happen with the old shower ?
  6. DNK

    DNK New Member

    Hi Chuck

    Thanks for your reply

    I am unable to do pictures at the moment, but it was a straight swap of showers. Exactly the same model of Triton Power shower. No pipework changed in anyway. Just old one off and new one on. No thermostat has been changed. The water is heated through the immersion tank in the airing cupboard. Many thanks
  7. DNK

    DNK New Member

    Hi Shaun
    Thanks for your reply.
    There was no banging in the pipework before. I am guessing there is trapped air in the pipes from when i turned the water off to install the shower. But I have tried to clear it with no luck.
    Many thanks
  8. Chuck

    Chuck Active Member

    Does water come out of the shower at a decent rate?

    When the banging occurs, Is the tank being heated by (a) a boiler, or (b) an electric immersion heater?

    Does the banging depend on whether the shower is being used or not?

    Does the banging start pretty much immediately the water in the tank starts heating or is there a pause (tens of minutes) before the banging starts.

    Can you check that the level in the cold header tank is not falling significantly while the shower is operating?
    Last edited: Jun 29, 2017
  9. townfanjon

    townfanjon Trusted Plumber GSR

    Longshot . was the shower pump vented ? , there is a very small bleed valve , top right hand corner .
  10. DNK

    DNK New Member

    Hi Chuck

    Yes the flow of water from the shower is exactly the same as previous.

    The banging starts within a few minutes of the heating / water timer kicking in. So no water running in the house at all. If I reduce the temperature thermostat on the front of the cylinder, the banging stops pretty much immediately. It only bangs while it is trying to reach the set temperature, which is set to about 40.

    The cold water tank seems to be fine and at a good level.

    Many thanks
  11. DNK

    DNK New Member

    Hi Jon

    Thanks for the reply.

    I did not know there was a bleed valve on the Triton shower so I will certainly check when I get home. Would this help to clear the knocking in the pipes in the loft?

  12. Chuck

    Chuck Active Member

    A temperature of 40°C, if that's what it is, is dangerously low for stored hot water. It should be 60°C.

    If somebody has turned it down to 40°C it may indicate that the thermostat is an incorrect type, miscalibrated or faulty.

    Do you have a thermometer that you can use to check the actual temperature of the hot water, e.g. from a tap close to the tank while it is running? Leave the tap running continuously at a modest rate into a mug (so the mug is overflowing and check the temperature of the water in the mug. It should be at least 55°C.

    Based on your answers so far, I suspect that either the thermostat on your tank is faulty and it's getting much too hot or the vent/expansion pipe is not fixed correctly, i.e. in a way that allows expansion to occur quietly.

    Be careful. Faulty thermostats can result in a tank of boiling water that can be very dangerous.
  13. DNK

    DNK New Member

    Thanks Chuck. I do usually run it between 50 and 60, but due to the recent banging i turned it down to stop it annoying the neighbours. But when I went to have a shower this morning, it wasn't hot enough. Probably due to me turning down the thermostat last night to 40. I only turned it down to 40 because as soon as I lower the temp the banging stops!

  14. Chuck

    Chuck Active Member

    Okay, but you should normally leave it at 60°C to kill off legionella.

    So, it seems to me that you should be looking at the expansion/vent pipe arrangement. This is the pipe that travels up into the roof to above the level of the tank and then typically turns over and points down at the water surface but not submerged below it.

    Have a look at where it passes through holes, e.g in the ceiling. If it can't slide a mm or two reasonably freely a slip-stick effect will cause 'banging' with whatever it's rubbing against acting like a sound board. There are all sorts of remedies, but they depend on the site-specific circumstances. You might, for example, have found a mysterious piece of hemp on the floor of the cylinder cupboard that was dislodged when you were swapping the shower. If so, it used to allow the pipe to slide as it passed through the plasterboard...

    If this explanation seems reasonable to you (you're there and have heard the noise, we're not and haven't), we'll need some photos to advise further.

    Or just get a plumber to sort it out for you. If you can produce the noise to order it will probably only take them a couple of hours, if that, to diagnose and cure. Get them to check your HW system temperature at the same time.
  15. DNK

    DNK New Member

    Many thanks for your help Chuck. Do you think I can rule out air in the pipes then?