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Heat Trap Water Heater Nipples and Expansion Tanks

My dad always said there is no stupid question. But I feel this may actually be one...apologies.

Basically, all new hot water heaters have heat trap nipples on the hot and cold lines. Essentially check valves, correct?

Also, good practice says to install expansion tanks on heated hydronic systems.

My question is, how is one to add expansion capability to a water heater if the in/out connections are basically keeping water inside the tank?


  • ethicalpaul
    ethicalpaul Member Posts: 5,740
    They must not keep the water in the tank all the time or else you wouldn't get any water when you turned on the hot faucet in your bathroom ;)
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • mattmia2
    mattmia2 Member Posts: 9,753
    They allow flow in both directions, they just require a bit of pressure differential to open so they won't keep the system from flowing backward as the water is heated, they just stop gravity flow.
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
    "Also, good practice says to install expansion tanks on heated hydronic systems."

    Good practice? It's ,mandatory that there be an expansion tank, compression tank, or some other means to take up expansion.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,338
    Hi @Steamback , I’ve got the dumb question… are we discussing hydronic space heating or domestic hot water? The answers depend 😉

    Yours, Larry