hot water coming through cold taps

Discussion in 'Water Heater Forum, Tanks' started by askjim, Oct 28, 2013.

  1. askjim

    askjim New Member

    Oct 28, 2013
    witney uk

    I really hope you can help me.

    I moved house a month ago and noticed hot water coming through the cold taps. Not isolated to a single cold tap but both cold taps upstairs and most likely both cold taps downstairs. The hot water will last for 15-20 seconds.

    At first I was not too bothered but as we have a home care agreement I thought lets get this fixed and also because I think we had the same problem in the previous house which had the same boiler and was installed by the same guys but which we had previously put down to the cold and hot pipes being close together.

    Please see first image

    photo (2).jpg

    Cold in comes into the bottom of the manifold and cold goes back out on the left of the manifold, the pipe on the left above the cold out is pressure relief / expansion tank and at top is the blender where cold comes in at the bottom, hot at the top and hot going out on the left.

    The gas engineer put a device on the cold out and recorded the temp of the pipe at 45+ when the hot water was heating. After you have bled the cold tap and it goes from cold to very hot it then goes cold. If you leave the cold for 2 mins and then draw further cold it is hot and if left for longer would be very hot.

    He spoke to ideal and they suggested switching the blender. I think he was told to do this because the non return valve was likely to be broken.

    The blender was replaced and the problem remained. I don't actually think there is a non return valve in the blender.

    The engineer spoke to his boss and they decided to replace both blender and manifold in case the manifold had a non return valve. Both parts were replaced and it was noted that the manifold contains a non return valve on the cold in. Still the problem remained.

    So ideal came out to take a look who were perplexed and he rang his boss who suggested that the cold out was designed for a shower and that the hot out should have a non return valve inserted to fix the problem.

    Instead the cold out was re-routed so it comes off the cold in but below the non return valve in the manifold - this assumes there is a non return valve - I did not get to see the manifold. See photo 2

    But still the problem exists although I do not think the hot water in the cold is at hot. Weirdly when my wife ran the downstairs cold tap when cooking tonight the cold out seemed to go very hot (but I would need to check this)


    Please help as I would like to figure this out and especially as there is likely to be 35+ other houses where I live with this problem and as I would like to let the engineer know what the problem was.

    A reward is offered for a correct diagnosis icon_smile.gif

    Thanks in advance

  2. Smooky

    Smooky In the Trades

    Apr 4, 2011
    North Carolina
    The “blender” as you refer to it is called a mixing valve and the non-return valve is called a back flow preventer or a spring check or check valves etc. From your description it sounds like you need check valves on both inlets of the mixing valve so water can only go one way and not back up.
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  4. Murphy625

    Murphy625 Member

    Jul 7, 2013
    Here's a guess...
    The hot water in the water heater is so hot that it is building up a small bit of pressure. Not enough to cause a safety valve to pop, but enough that the pressure in the tank is greater than the pressure in the rest of the system.

    When you turn the cold water on, the pressure is using the cold water line as a relief until the tank pressure and the system pressure have equalized.

    Try this:
    Turn on hot water only for a couple seconds. Even some other tap on the other side of the house. (this will relieve any pressure build up)
    Then turn on the cold water and see if it turns hot.

    Your hot water tank must be really freaking hot... 170 to 180 or more...
  5. askjim

    askjim New Member

    Oct 28, 2013
    witney uk
    I think originally the engineer was advised that the mixing valve had a NRV but halfway up the manifold it supplies water to the water tank which I guess hot water could come back via. So I think that advice he was supplied was incorrect.

    But supposedly there is a NRV at the bottom of the manifold where the cold in goes. What I wonder is do you get any convection across a NRV as the water above it is 60+.
  6. askjim

    askjim New Member

    Oct 28, 2013
    witney uk
    Results from test. Heat hot water until boiler switches off (about 10 mins). Cold in / out is no longer boiling hot (probably due to the change in pipework).

    But wife ran cold tap downstairs and after 2 seconds the cold in went mega hot, too hold to hold pipe and then after 15 seconds reverted to cold touch.

    Do you suggest check expansion vessel / temperature the hot water is being heated too?
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