A puzzling knocking sound in a water heater at a camp fed by lake water sent a plumbers' forum into troubleshooting mode, but the conversation quickly widened into a debate over both plumbing problems and the safety of untreated water sitting in a warm tank.
Here's what to know
In a post in Reddit's r/AskPlumbing community, the original poster explained that the property uses lake water and has a pressure tank between the pump and the house.
They said the knocking appeared to be coming from the cold-water line at the water heater and asked if a check valve might be pulsing when the pump draws from the lake, rather than the pressure tank buffering that effect before it reaches the home.
The OP also asked, "Could it be an indication of a problem with the system or the water heater?"
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Replies in the thread focused heavily on pressure behavior. After the poster said the faucets pulse along with the pump and that the tank gauge reads 25 psi, one commenter replied, "There really shouldn't be a pulse with the pump."
Other users suggested trapped air, a swing check valve, low pressure, or trouble with the well-style pressure tank as possible reasons for the noise.
The conversation did not stay limited to the knocking. One commenter shifted attention to sanitation and wrote, "Unless that thing is cranked up to 140, it's a bacteria farm."
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The concern was that untreated surface water held in a warm storage tank can allow harmful bacteria to multiply if the system is not properly treated and maintained.
So while the sound itself may trace back to a valve or pressure problem, commenters raised the separate question of whether the lake water feeding the heater is being adequately filtered and disinfected before it is used for cooking or bathing.
Not everyone in the thread viewed a lake-fed system as automatically unsafe. Another commenter wrote, "Some lake water is safe to drink. There are also filtration and treatment systems for lake water for people who live where a well isn't feasible."
So while the thread never produced a confirmed answer, the comments kept pointing back to the same two concerns: a pressure-related plumbing issue and the safety of warm, stored lake water.
What can be done?
A licensed plumber or water-treatment professional can inspect pump cycling, bladder tank pressure, check valves, and whether the plumbing layout is allowing pressure surges into the house.
Water testing and confirmation of proper treatment should also be part of that inspection, especially when the source is a lake.
If the system is old enough that replacing the water heater becomes part of the conversation, Cala is one company worth knowing about and could be part of a smarter long-term fix. The company's intelligent heat pump water heaters help homeowners save money while providing hot water exactly when they need it.
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