Can we talk about trap seal primers in residential use?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by speede541, Mar 14, 2011.

  1. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    One of my "to do" items is to install a trap seal primer to replenish my Laundry and Boiler Condensate traps, both of which are in the boiler room, so I am led to understand that they're prone to drying out. Ok, gotcha, I'm down with that.

    So I've got a dandy Mifab MR-500, and I've got my usual list of questions.

    1. Does it matter if I connect it on a line serving a single, highly used kitchen faucet? The main cold water supply line to the boiler is a closer run and terminates nearby. Will pressure drop further back on the line register here at the end of the line, or does this function require water to be flowing past the Tee where the Mifab is connected?

    2. Air Gap: Mifab makes one, but not for the reason I expected. They say it's to allow backflow to escape and not re-enter the clean water supply. I see it as helpful to allow the "squirt" of water to flow down to the trap, essentially acting as a vent.

    3. Distribution Unit: I need one, since I'm feeding two traps. But does the air gap go on top of the distribution unit? Or do I need two air gaps, one under each of the two ports exiting the distribution unit? Both the Distribution Unit and the Air Gap are shown in Mifab's literature, but not used together.

    4. Connections to Drain: For the condensate line, I'm just planning on Tee-ing in to the 1/2" PVC where it feeds into the trap. For the laundry, the plan is to install a Wye at the top of the laundry drain pipe, up near the top near where it connects to the box, and reducing the Wye to 1/2" to connect to the primer. I guess I just feel a little weird connecting water lines directly to drains; is this how it's done?

    5. Should I NOT use a 1/2" copper check valve as backflow prevention? This was my first inclination, but I can't find any examples of anybody recommending this. Do these check valves require a certain amount of water pressure to operate in the desired "Flow" direction, effectively blocking the squirt of water sprayed out by the trap primer?

    As always, thanks for any and all input -- it's much appreciated.
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,811
    Location:
    Cave Creek, Arizona
    1. The "pressure differential" style can go anywhere, but the problem with them is that SOMETHING has to create the pressure drop, and in a residence there are few appliances that will "tax" the water system to cause the initial pressure change. They are also ineffectual in commercial systems when the water piping is large enough to minimize pressure changes.
    2. You can create an "air gap" by using a reducing coupling below it as a funnel. The air gap is to prevent "fungus" growing in the line from contaminating the primer device. The hole on its side is a vent to prevent back siphonage.
    3. The amount of water discharged is so small, during each cycle, that it is doubtful that there would be enough to prime more than one trap, IF you could even separate the flow into two, or more, streams. For multiple drains you would need a "flow through" or "continuous flow", model which discharges while the faucet it is connected to is being used.
    4. Usually the primer is "hung" above the drain with a space between the pipe and the drain, which creates an air gap. When the primer is connected directly to the floor drain, then the air gap is placed below the primer, see answer #2.
    5. A check valve would NOT be an effective backflow preventer. You do not need it ahead of the primer, and if installed after it could cause problems. Such as, water would be forced out of the primer's vent holes, and the check valve could inhibit the flow to the drain.
  3. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    Hmmm... so regarding servicing of two traps: The MR-500 states it'll service up to 6 drains via a distribution unit, dispensing 1/2 oz of water when it senses a 3 PSI pressure drop. In your experience, are these claims overly optimistic? Because, frankly, it would cost me about the same money and time to just install two separate primers, rather than a single primer and a distribution unit.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    They may reliably dispense water when those conditions exist, it's just that reproducing those conditions may not be reliable in a typical home.
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,811
    Location:
    Cave Creek, Arizona
    Unless the water piping is "small" enough to induce that 3# pressure drop when a faucet is opened, it will NOT dispense any water. My experience with them is that they look good and make the inspector's happy, but SELDOM perform as advertised.
  6. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    HJ and Jad, thanks, I guess I'll have to just give it a go and see how it works.

    As I mentioned, nearest line I can connect to is a 3/4" feeding the boiler/DHW, but also Tees off to 1/2" to a half-bath just a foot or so past where I'd be connecting. If I can't get the pressure drop I need across the 3/4" line, perhaps I'll try the 1/2". Unfortunately, I'll have to do some testing before putting up drywall.

    Would I be shooting myself in the foot (or even code legal?) if I were to momentarily reduce the 3/4" line to 1/2" -- maybe for a total distance of 4 inches -- in order to "enhance" the PSI drop whenever water flows? Again, this 3/4" line is filling the water heater & pushing hot water to the whole house, but in the grand scheme of things, would such a momentary restriction have any apparent effect to the users?
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,811
    Location:
    Cave Creek, Arizona
    You would be guessing as to whether it would create the pressure drop. For the pressure to drop, several things have to occur simultaneously. First, the faucet has to open VERY quickly, Second it has to have a high flow, Third the water in the system has to have "inertia" so it takes a moment to start moving, and Finally, it has to start moving BEFORE the momentary pressure in the system becomes ZERO, or you will induce a "reverse water hammer".
Similar Threads: talk trap
Forum Title Date
Plumbing Forum, Professional & DIY Advice what type of fitting am I talking about? Aug 9, 2012
Plumbing Forum, Professional & DIY Advice Ahhh...my toilet keeps talking to me! Sep 30, 2011
Plumbing Forum, Professional & DIY Advice Bottle Trap or P-Trap Sep 17, 2014
Plumbing Forum, Professional & DIY Advice I'm fed up trying to get my p-trap and riser plumb. Please help me before I spontaneously combust! Sep 2, 2014
Plumbing Forum, Professional & DIY Advice does double kitchen sink require two traps? Aug 17, 2014

Share This Page