P-Trap with Primer

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by chezball, Jan 4, 2009.

  1. chezball

    chezball New Member

    Jan 2, 2009
    Can someone explain to me how the primer for a floor drain p-trap works? In particular, is it always dripping (wasting) water, or does it only drip water when the water level drops below a certain level (from evaporation)?
  2. sixlashes

    sixlashes Plumber in Previous Life

    Aug 6, 2008
    Retired USAF Aircraft Maintenance Superintendent,
    Pensacola. FL
    They connect to a cold water SUPPLY line running to a fixture - usually a sink or toilet. A clothes washer is also sometimes used. When water is used by the fixture, the resulting change in pressure within the line activates the trap primer and a small amount of water is discharged by the trap primer for the floor drain p-trap. You have to ensure the fixture demands enough flow in relation to the static pressure of the line the primer is connected to. They also need to stay accessable.

    If there is no demand by the fixture they are connected to, the p-trap will dry up. The operation has nothing to do with the level of water in the trap. There are other solutions for a p-trap which rarely gets any water in them. Some floor drains use balls or openings that close up when dry. I have only used trap primers and they work well, if the parameters are met.

    Here is the skinny on Zurn's unit - http://www.zurn.com/operations/specdrain/pdfs/installation/t1.pdf
    Last edited: Jan 4, 2009
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  4. maintenanceguy

    maintenanceguy In the Trades

    Aug 18, 2008
    The ones I'm familiar with flow water whenever the supply pipe has a pressure drop of 5 to 10 psi. That is, if the trap primer is fed from a pipe leading to the washing machine, it will open every time the washing machine fill valve opens. This means a small squirt of water happens a few times a week in this application.

    I don't like trap primers because they fail after a few years, they're not something that gets inspected so when they fail, it's as if they were never installed.

    So it probably does drip more than it is needed but it does not run continuously.

    Other tricks to solve problems with dry traps: a few drops of baby oil in the trap will make a film over the water in the trap and significantly slow evaporation.

    Or install a "barrier type" drain trap into the drain. These are also called "duck bill" valves and use a flat rubber tube that opens whenever there's water running into the drain and closes against sewer gasses when there is not. Not as good as water in the trap or for use as a substitute to a trap but they've solved a lot of sewer gas smell complaints for me, especially in hard to access air conditioner condensate drains that dry out in the winter. These barrier traps just get pushed into the drain opening with some caulk to hold it in place and seal it.
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