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# Math time: condensate drain on common exhaust

Posts: 31Member
Gang,

I have 2 Navien NFB-200s that are exhausting via a common vent. Per Navien's instructions, we put a tee facing downward just downstream from the last unit to catch condensate (otherwise, condensate can collect on top of the little dampers in Navien's special kit, preventing them from opening properly). Venting is perfect. Drain line is 1" PVC.

However...

Based on Navien's instructions to put a loop in some flexible plastic tubing drain line, my plumber built an impromptu -- and short -- PVC p-trap (he and I both like that better than plastic tubing). Purpose of this loop / trap is to ensure you aren't venting exhaust into the house via your drain line. But, since neither he nor condensate had filled the trap yet, we ended up ..... blowing exhaust out the drain pipe into the house.

This prompted some debate and I'm not finding the right sites to tell me the math on this: how many inches of water sitting in 1" PVC will weigh enough / provide sufficient head to prevent exhaust from blowing the water out of the trap? Please point me to the math so that I can over-engineer for safety. Thanks!

TC

• Posts: 31Member
One edit: math is one thing, experience is another, so PLEASE tell me your thoughts on how many inches "deep" the p-trap should be to ensure the exhaust doesn't blow it out -- with my thanks!
• Posts: 11,885Member
Any idea what the pressure is in that exhaust line? That would enable doing the math...

But there is another way. Get some flexible, clear, tubing. About 20 feet of it. Hook it up to the drain outlet and let it loop down to the floor (hold the other end up at the level of the drain outlet). Fill it with water. Now start the boilers -- both of them -- and raise the open end of the tubing high enough so that the water doesn't come out... add water as needed and keep raising it. You are making an impromptu trap. Then you note how high the water stands in the raised side in relation to the other side (which is still down on the floor, right?) and there you are...
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Posts: 31Member
Jamie, that is exactly what Navien wants us to do with tubing - you are spot on! We both just hate the tubing part of it as a solution and would prefer PVC.

I'm looking at specs and they don't give an indication of CFM on exhaust. Figuring out the air pressure won't be easy.

One thing he and I realized was that by reducing drain "aperture" from 1" down to 3/4" or 1/2", we naturally will reduce the exhaust air pressure. Reduce pipe down / then back up, and the higher diameter water then offsets much lower air pressure.
• Posts: 2,181Member
Reducing the diameter of the pipe used will have no influence on the depth of the trap necessary.

I think Jamie was intending that you merely use the tube to measure the necessary trap depth—it'll be the difference between the two water levels—not necessarily leave the tubing in place. A manometer connected to the drain port while the unit fires at max will give you the same number.

• Posts: 11,885Member
ratio said:

Reducing the diameter of the pipe used will have no influence on the depth of the trap necessary.

I think Jamie was intending that you merely use the tube to measure the necessary trap depth—it'll be the difference between the two water levels—not necessarily leave the tubing in place. A manometer connected to the drain port while the unit fires at max will give you the same number.

Exactly. My suggestion is a poor man's manometer!
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Posts: 6,441Member
I used to think that the clear tube loop trap looked hokey.
But you can see your water level, see when it needs cleaning and if it gets real crappy just replace it.
You don't have to prime it if you hold your finger over the end on the initial run. Remove your finger and the loop will fill.

The pvc looks proper, but you cannot tell if it is dry or plugged without opening up.
• Posts: 31Member
@ratio - you are right on depth, but the smaller aperture will reduce the PSI from the exhaust, which reduces the downward pressure exerted on the water (and then we get into Pascal's law). Take the diameter back up and you have a larger volume of water per inch to displace in your trap. Trying to get the most favorable physics for an uncertain situation.

That said, @Jamie Hall and @JUGHNE, you are right: tubing has its merits for both diagnostics and maintainability.

Given the newness of this system, you have all three convinced me to test, measure, observe, repeat rather than cast my fates to opaque PVC.

Thank you, gentlemen, for the proper nudge! I'll report back on this in a week or two.

TC
• Posts: 6,441Member
The tubing gives that unique look of a "Moonshine Still"
• Posts: 11,885Member
Downward force, yes, @tc60045 -- but not downward pressure (psi). Even if it were just a soda straw, the pressure (psi) does not change, and it would take the same height of water column to balance.
Br. Jamie, osb

Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
• Posts: 999Member
Navien really needs to figure out how to handle the condensate. Their flapper design is something to be desired also. Yes one can do the trap design, but who is going to remember to fill that trap each and every fall.....
D
• Posts: 183Member
ratio and Jamie have it correct.

"you are right on depth, but the smaller aperture will reduce the PSI from the exhaust, which reduces the downward pressure exerted on the water (and then we get into Pascal's law). Take the diameter back up and you have a larger volume of water per inch to displace in your trap. Trying to get the most favorable physics for an uncertain situation."

this is wrong, there is no velocity, therefore the pressure is static and will equalize.
~28 inches under the water in a small hot tub will create 1 PSI, the same as you'd get 28 inches under lake superior.
enough water to raise Lake Superior 2.31 feet weighs a whole lot more than it does to raise your hot tub 2.31 feet.

as always,,,,,,
• Posts: 6,441Member
The larger the loop and the larger the tubing ID, the more liquid it would retain and be less likely to evaporate between heating seasons. (Concerning the seasonal refill of the trap.)
• Posts: 6,310Member
@tc60045

What @Jamie Hall said is correct. Use the tubing to see the water level and determine the trap size. Then you can pipe it in pvc. Do not reduce the diameter.

The trap should come out od the drain and drop low. The lower the better.

How much static pressure the trap will hold is determined by how high the riser from the trap rises back up before termating.

1" wc pressure in the stack would require 1" rise in the trap + a smige more to make sure it is sealed
• Posts: 31Member
Just coming back with an "epilogue" here -- we ended up using a pretty significant trap on 1" PVC tubing and haven't looked back. Took a couple of hours for the condensation to fill the trap and it has been fine since.

I was also worried about some condensation getting past the drain and sitting on top of the flappers -- hasn't happened.
• Posts: 23Member
Way late here:

I found a "solid" trap with clear U. Got from Amazon.
Rectorseal 83114 2 EZ 113B Trap Kit with Brush

Works fine. Fully cleanable. The pressure differential is under 1 inch, but my vents are over-ample for my burner; another installation may have more back-pressure.