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.