Scenario: my 9yo natural gas 40gal Whirlpool hot water heater has been "draining" (i.e., being used) for 5+ minutes from someone taking a hot shower or washing dishes with hot water. If someone else runs cold water (e.g., flushes a toilet) anywhere in the house during or immediately after the hot water has been used for these 5+ minutes, the hot water tank sounds like it gurgles loudly (often leading to knocking sounds as well -- but not always).

What is going on here? Why would a hot water tank gurgle/knock from using cold water?

Is there something wrong with the hot water heater? Something possibly wrong with my plumbing? Is this normal?


  • We're on city water.

  • Cold and hot are definitely not reversed.

  • no discharging, leaking, or anything else peculiar. No other symptoms other than sounds (which are loud enough to hear anywhere in the 1600 ft2 house).

    • Though, the sounds are very clearly coming from the water tank and do not appear to be generated or audible coming form other parts of the plumbing.
  • This has been occurring for 12+ months

  • no recirculation system or anything else fancy. Copper supply line splits at T joint to go into HW tank (with rest of cold water continuing on to 2 toilets, 3 sinks, and two showers) and copper line from HW tank carry hot water to 3 sinks and 2 showers.

  • We only have the issue if hot water has been running for a while. If no hot water has been used (or, say, only a small amount has been used from washing hands for example), toilets and other cold water outlets have no impact on the tank.

  • could be the hot/cold are reversed. Jul 16, 2023 at 22:35
  • 2
    Water heaters don't actually drain. As hot water is used, cold water is pushed into them by street pressure. They are always 100% full of water 100% of the time. Do you have a recirculation system (to speed up hot water arrival)? Is water discharging from the emergency relief valve and do you have a well or city water? Is there a check valve from city water? Jul 16, 2023 at 23:04
  • regarding check valve: not that I know of. Unless one is built in my water meter (immediately inside my house) or somewhere further up the city line underground outside my house Jul 17, 2023 at 3:06
  • FWIW I had this exact same problem and replacing my water heater (it was 19yo 40gal natural gas) fixed the issue. After showering, cold water usage anywhere in the house would result in alarmingly loud bubbling/gurgling noises in the water heater. After having my heater replaced, the problem no longer occurs.
    – Chase
    Dec 21, 2023 at 17:27
  • Has the water heater been cleaned/drained on a regular basis. Calcium deposits and rust may cause boiling at the bottom of the tank on older units (gas). If it is that, replacing may be the best alternative, but you can try draining and scraping bottom through drain, or use a de-calicifier in tank. That probably won't help much... too late, and might create new problems with old tank.
    – RG Hughes
    Dec 21, 2023 at 18:21

1 Answer 1


I will take a SWAG and say it is the hot water line expanding and contracting with the temperature change. It will make noise when rubbing against the house structure where it is supported.

  • thanks for the suggestion. but it's definitely a gurgle from within the tank. I can stand next to it and here it coming from the tank Jul 17, 2023 at 3:01

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