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Backflow preventers

Wayco Wayne_2
Wayco Wayne_2 Member Posts: 2,472
Hey y'all. I need a simple explanation of why backflow preventers are needed on domestic potable hot water tanks. About 4 years ago our area started requiring them along with potable expansion tanks of course. I understand why a boiler system needs them, because boiler water is nasty stuff. But not why a potable water heater? I'm hvac so I'm not so much into the plumbing, but I've seen several systems where the expansion tank pressure has not been maintained and the result is a flooded basement. What problem is being dealt with and is it worth creating the flooding problems? Thanks for your information.


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    why backflow preventers are needed

    I am not a pro, but I can guess.

    Where I live, they are not required. I never had a backflow preventer in my (former electric) hot water heater, and even had none in my (former) hot water boiler. I suppose they were not required when my house was built in the early 1950s, if there even was a code then. When I had my boiler and water heater replaced, they did put a backflow preventer  on the boiler, but none on the indirect hot water heater. This system was inspected by a plumbing inspector, and electrical inspector, and a gas inspector. So I guess I do not need a backflow preventer on the hot water heater here.

    It seems to me that there could be two reasons for requiring one on the hot water heater.

    1.) If the hot water heater overheated, and the P/T valve was all calcium deposited up, steam could go down the supply line into the city water. I am not sure how realistic this is, because it might condense in the supply pipe. But I would not wish to guarantee this.

    2.) Perhaps my hot water heater could be contaminated by my science experiment of running it at 125F and growing who knows what bugs in it. Then overpressure in the water heater could push the contaminants into the city supply lines.

    In the installation manual for my system, they suggest piping and anti backflow valve in the domestic side of the indirect. First the valve, then an expansion tank, then a vacuum breaker. Now that opens up the question, why the vacuum breaker.
  • EricAune
    EricAune Member Posts: 432
    On the tank or the water service?

    Do you mean the city water service or actually at the water heater? 

    Some municipalities in my area require a check valve on the water service in the home or business, in turn this requires an potable expansion tank (properly sized of course...and I doubt anyone has ever considered sizing one) because of this.

    The requirement of the valve in this situation is for protection of the supply in the event a negative pressure is created if/when the fire department where to draw too much from the hydrant supply.
    "If you don't like change, your going to like irrelevance even less"
  • hot_rod
    hot_rod Member Posts: 16,293
    protecting the public water supply

    The EPA holds water providers to a tight requirement when it comes to keeping public water safe. The CWA Clean Water Act also requires the protection of systems with backflow devices.

    In many cases funding for public water systems is tied to the backflow prevention devices being installed on every connection.

    In my former town the public works department decided to install meter yokes with backflow preventers at the streetside meter pits. Problem was the homeowners were not notified of the installation, or the homeowners did not call a plumber, and plenty of WH relief valves started discharging around town as they changed out the yokes.

    The two choices they offered were DHW expansion tanks or the toilet ballcocks with pressure relief valves built in. But the ballcocks wasted a lot of precious water and were quickly dropped as an option and expansion tanks were the final answer.

    This is a good example why relief valves should always pipe to a working floor drain, as the expansion tank will fail someday.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordan
    Gordan Member Posts: 891
    Is this an issue at all with low water content heaters, though?

    Say, (certain) tankless or reverse indirects. Not a lot of water in there to expand, compared to the water volume in domestic pipes.
  • ScottMP
    ScottMP Member Posts: 5,884

    Water tanks have much more volume than the domestic piping.

  • ScottMP
    ScottMP Member Posts: 5,884

    Water tanks have much more volume than the domestic piping.

  • ScottMP
    ScottMP Member Posts: 5,884

    Water tanks have much more volume than the domestic piping.

This discussion has been closed.