P Trap

Techman

For a regular AC unit a minimum of 4" down and a minimum of 2" back up is proper

John Barry_2

P-Trap

What is the proper size of the drop piece compared to the up piece in a P-Trap? DoesEet SP have anything to do with it?

Jim_47

P traps

the static pressure does have an influence.

its very important when you are draining an air handler.

I worked at a huge cleanroom facility that had several additions and upgrades over the years. As the plant would expand the engineers balancing the cleanroom whould change the fan speed of the air handlers(over 135 of them) to balance the clean room. As they would change the SP by changing the fan speed using the VFD drives, the SP would change and the trap design was no good.
Inches and inches of water would accumulate in the air handlers and then the 3phase motors were below water.
Took me a little time to figure it all out.

Brad White

It is all about static pressure

Design standards I have seen have the total static pressure defining the depth of the trap differential. The "rule" is "TSP plus at least one inch".

In actual practice it has to do with the suction static pressure on a draw-through unit (coil upstream of the fan) and the discharge static pressure on a blow-through unit where on the downstream side of the fan.

Think of it this way on a draw-through unit. The supply fan is trying to suck back or "sip" back your condensate. It is pulling so many inches of water column (self-defining) so your drop depth has to at least exceed that generated by the supply fan at that point. You want to give the condensate a place to "fall" beyond the reach of fan static pressure.

What I like to do is to raise my units as high as I can or absent that, have the GC create a pit (if the unit is in the basement), to get enough height to do this right.

Then, I use an adjustable P-Trap with a spud nut, like a trombone. I can adjust the trap depth until water starts to flow visibly. As filters load up the suction SP increases so I will add another inch or two to compensate for that. This is for larger institutional or commercial air handlers where you may have 6 to 8 inches total pressure and are drawing in about 4 inches through wet coils and several filter banks.

For residential and smaller commercial projects, a trap depth and differential of 2 to 3 inches should be plenty.

Techman

Could you?

Hi Brad! Just a curious question here , On an air handler that is very low to the floor,could you have a 1' or 2' horizontal run in between the drop leg and the up leg?Does the weight of all that water count?Thank You!

Brad White

Only in the vertical does that count

unfortunately. The water needs to "stack" in height to get any traction for outflow. I suppose if your static pressure at the point of connection is low it may work but you really need the height. Maybe a siphon system? You got me thinking (not that I know much about them) but there are siphon type roof drains which reduce rain water leader pipe sizes. I wonder if that principle could be applied?

Just what I need, another project!

No time now, but I will ask my plumbing engineers... whew.