What I have is that when I turn on the kitchen faucet to warm while washing dishes and someone uses the toilet or cold water on another faucet then the kitchen faucet gets very hot. I am on city water and pressure is at 90 and 3/4 mains. Thank you

  • the main may be 3/4", but what is it in the house? .... how did you measure the 90 psi?
    – jsotola
    Commented Jan 29, 2019 at 6:21
  • Everything 3/4, it's 1/2 only at the branches to individual fixtures. Measured with a gauge.
    – aofkj
    Commented Jan 30, 2019 at 1:22
  • 1
    now we know what is happening, but we do not know what your question is .... i can think of several questions, so it is difficult to guess which one is yours.
    – jsotola
    Commented Jan 30, 2019 at 1:46
  • Simple, I don't want the water to get hot because someone else flushed the toilet now, is there something I could do?
    – aofkj
    Commented Jan 30, 2019 at 3:41
  • try turning down the toilet shutoff valve .... the toilet tank will fill slower, but it will not drop as much pressure in the cold water supply line whenever the toilet tank is filling
    – jsotola
    Commented Jan 30, 2019 at 5:14

1 Answer 1


It sounds like you have an unequal (probably low) pressure hot water system, which is common in older homes.

Some taps have a "venturi" system which helps with this by using cold water flow to suck water from the water tank. Alternatively limiting the flow of the cold water (probably not what you want) or adding a booster pump to the hot water will help. I imagine the "gold standard" solution though would be to convert to an equal pressure system.

  • 1
    The problem is the opposite, it looks like the 'cold' side is loosing pressure.
    – aofkj
    Commented Jan 30, 2019 at 1:32
  • I don't think so - When someone flushes the toilet they are reducing the pressure of the cold water, while the hot water pressure remains low, so more hot water mixes and it becomes warmer. If you use a booster or similar the hot water tap will not be as open for a similar target temperature so the flow difference will be less. Of-course, if you change to an equal pressure system the issue will go away altogether.
    – davidgo
    Commented Jan 30, 2019 at 2:03
  • Thanks, what's a equal pressure system?
    – aofkj
    Commented Jan 30, 2019 at 3:39
  • As I understand it (and I could be wrong), a low pressure system is where the hot water cylinder is not sealed off from the air and the pressure in the tank is different to the mains pressure. Equal pressure (most modern ones) systems are sealed off and the pressure going through the boiler is the same as the pressure coming in. I believe they require different tanks.
    – davidgo
    Commented Jan 30, 2019 at 3:47
  • If it's not sealed off from Air then it's probably not sealed or from water either, no?
    – aofkj
    Commented Jan 30, 2019 at 14:45

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