Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Series vs. Parallel ?

Recently replaced 2 40 gallon water heaters that were in series at same temp. Owner wanted Parallel because his Plumber buddy liked it better. I started asking any Plumber or Hydronic Contractor what they liked. I read your water heater book Larry and didn’t see anything about having 2. Love to hear your take!

also, I pull out old water heaters with dialectic unions that are always nearly plugged. When I demo ones that use brass or copper unions they look much better! Love to hear your thoughts on that too! Thanks!

My apologies if this has been discussed elsewhere! I am new to sight and didn’t see anything


  • hot_rod
    hot_rod Member Posts: 22,158
    I generally like series, you can set the first one lower to preheat, or turn it off when you don’t expect a high demand.
    With parallel the piping needs to be equal, so you draw the same from both tanks. If one goes out you will get a blending of temperature

    I prefer copper or stainless flex connectors, some areas require them for seismic flexibility, same with the gas connection.

    Most tanks have dielectric nipples now, just screw on a copper to female adapter. I’m not sure unions serve much function, you use them once to install.
    When time comes for a new tank they get cut out anyways, as the new tank probably will not match up

    The plastic lined dielectric nipples don’t tend to scale up like galvanize or brass.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,304
    Hi @steampunk_2 , I agree with @hot_rod . I prefer to put tanks in series if it doesn't create a flow restriction. This is in part because even if you could get perfectly balanced flow with tanks plumbed in parallel, things change. Dielectrics or dissimilar metal connections form rust and imbalance the flow. I like to use a three valve arrangement on tanks, with valves on both hot and cold, and another valve above and between the two lines from the tank. This way, the tank can be isolated and service to the building can still come from the other tank.

    As for dielectric corrosion problems, I use lined steel nipples from the tank and go directly to corrugated copper or stainless flex lines that have true dielectrics built into the ends. This gives a good dielectric with no exposed steel to rust, and puts distance between different metals. Do re-tighten the flex lines after about six months. The rubber washers shrink a bit. ;)

    Yours, Larry