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Why does my electric water heater sizzle when I drop the pressure ?

Standard top ports 60 gallon electric water heater.
When I need to change out my whole-house filter element, I shut off the main valve (between the pump+pressure tank and the rest of the house) and walk to the nearby bathroom and run the cold for maybe 5 or 10 seconds to reduce the standing pressure. Dropping the pressure makes it easier to put the filter housing in bypass before I open it up to swap out the filter.
When I get back to the util room, the water heater is always making a sizzling sound. I'm certainly not dropping the water level in the tank by more than a 1/16" if anything.
Im not sure if it sizzles at other times (its not very loud), and I guess Im only noticing it because Im working right beside it. But every time, its no sizzle before pressure drop and sizzle right after.

Is it just the elements (or build-up ?) reacting to the lower pressure ?
30+ yrs in telecom outside plant.
Currently in building maintenance.


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,485
    edited August 2022
    Boyles Law

    Or is it Henry's law. I can't remember.

    It has to do with the amount of gas (like oxygen) that is dissolved in a liquid (like water). As the temperatures and pressures change the water has less or more ability to keep gas dissolved in solution. By opening a hot water tap, you reduce the pressure and release micro-bubbles especially around the elements if they are (or were recently) hot. Think of it as water boiling as a result of lowering the pressure like water boils at 180° in Denver but will boil at 212° in New York City.

    Try the same test by having someone open a hot water tap while you are next to the tank, with the cold supply valve closed. I bet you the sizzle will be more noticeable as the pressure difference is greater.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Dave Carpentier
  • Dave Carpentier
    Dave Carpentier Member Posts: 577
    edited August 2022
    Hauling me back to high school, but that does sound familiar.
    Same basic deal why an opened bottle of pop should stay in the fridge, since it holds the carbonation inside the liquid better than if it was at room temperature.

    With the water heater, Im dropping the pressure. With the pop bottle, Im increasing the temperature.
    Both ways release some gases.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,228
    Hi, I’ll add that we know water boils at 212F at sea level. At fifty psi, it boils at 298F. If heater elements are dry fired, they will melt in seconds. Putting this all together, when you greatly reduce the pressure, water can more easily boil and we know that the element wants to be much hotter than the boiling point. It can’t get there normally because heated water convects up from the element, being replaced with more water to take the heat. The sizzling you hear is bubbles of steam forming and collapsing. Take pressure off a gas fired heater when it’s firing and you may hear some loud thumping as water at the bottom of the tank flashes to steam and then collapses. Sediment in the tank helps create more noise 😜

    Yours, Larry
    Dave CarpentierEdTheHeaterManJHKhot_rod