Condensate drain line problem

nicholas bonham-carter · July 2016

Our 7 ton A/C coil blower has a condensate line of PVC pipe to the outside wall and through it to the parking area. The line plugged up, several times, even with blowing it clean each time, so we are changing the line from 3/4 to 1 inch pipe.
My service provider says we should put back in the P-trap in the line to prevent any negative pressure which might arise later on, (maybe from clogged filters), from allowing the water to build up in the condensate pan, and overflow.
My worry is that any elbows could be more likely to trap debris, and cause a later blockage. I do realize that a trap of some sort would keep any insects out.
How important is this trap for the drain line?--NBC

aircooled81 · July 2016

The trap serves the purpose of allowing condensate to drain when the condensate drain is located on the suction/low side of the fan. When it's on the high/discharge side of the pan, a trap can prevent your drain line from becoming a wasteful minuture duct.
To keep insects out is a valid point, but this can also be done by dishcarging your condensate drain into a pit submereged in pete gravel. Also known as a drywell.
With-out a vent, traps don't operate well. Make sure you instal the vent after the trap, not between the trap and the equipment.
You can add unions to make it easier to clean out, or female adapters with plugs.
Can you upgrade the merv rating on your air filters? What I find is that dust and dirt accumulate on a wet evap coil. The condensate coming down the coil tends to wash it down. When you have good filtration, that dirt gets thrown away with the filter, rather than washed down your drain.

Remember when sizing the depth of the trap weir, the lowest point of the drain leaving the pan, minus the distance to the lowest point of the outlet of the trap, will dictate your static height. So if your fan pulls 1" of negative static, but the exit of the trap is only 1/2" below the entry of the trap, you'll need 1/2" of condensate in the pan before it drains.

ratio · July 2016

There are also preformed PVC traps available, these are another bent from pipe material & I think they'd offer less chance of an edge catching crap & allowing it build up.

nicholas bonham-carter · July 2016

Thank you very much.
If I understand properly, the trap will prevent outside air from coming back into the blower cabinet due to the slight negative pressure. In order for this to work the vertical height of the shorter leg should be enough to prevent the column of water from being sucked up into the cabinet drain pan.
Maybe we can put the trap on the outside, where we have 8 ft. to have a nice tall column.
We seem to be restricted to 1 inch filters, unless we modify the filter holder. I would like to hear some discussion about how to choose good filters, as there is so much choice and hype, (like shampoo!).--NBC

aircooled81 · July 2016

Correct about the osa, but the worry of osa infultration is two fold. One is the osa could be something you don't want in your air stream, like by a dumpster or sewer vent. The other thing is, the suction of the fan pulls air in through the pipe, not allowing condensate to drain out. A perfect example of this is when someone installs the vent tee on the upstream side of the trap, air sucks into the condensate pan, while the pan continues to flow to the over-flow point because condensate can't fight it's way past the incoming air, to go down the drain.
The water leaving the trap is the same rate it fills. Heavy stuff will likely fall-out in a trap and lodge in there. I feel it's best to keep the trap near the discharge of the pan/drain, as the rest of the line will not be burdened with slow moving dirty condensate.
Regarding filtration, the denser the filter, the quicker it needs to be changed. I find Merv 13 and Merv 14 is a standard for a couple of my tech co.s out here. Air quality has become a huge deal. Merv 8 should be the minamum you use. When you go up to a merve 13 for instance, you'll want to make sure you don't cut the air performance, as these have a higher pressure drop. This is similar to creating a smaller return duct, so you may need to increase your fan speed, or change your rack to house 2" filters so you have more surface area.
If this isn't viable, try some rinsless evap foam coil cleaner every 4 months. Just pray it into the coil, rinse it off with a hudson sprayer (i know, rinseless, but) and then wash down your condensate drain at the same interval.
Instal a garden hose adapter on the condensate line, use it to hook up a garden hose for quick blow-down rinsing. When you're done, just put a cap over the hose adapter. Make sure if you do this, you don't blast back-up into the pan of the unit. Yikes, wouldn't want to make mess in there.
Poorly sloped condensate drains, or sags can make for poor drainage and dirt accumulation. If you can get 1/8" per slope your normal, anymore than that and you are in great shape.

JUGHNE · July 2016

I have used garden hose for AC coils that are under negative pressure. If space permits the hose is used to form the entire trap and drain line to the floor drain. The logic for the garden hose is for service, the HO or HVAC person is to remove the hose, connect it to the drain of the WH which is usually next to the heating/AC equipment. Drain the bottom of the WH for 5-10 minutes under pressure supply pressure. This drain the sediment, exercises the WH drain valve, assures the AC drain is open......also I don't have to bring a hose for WH draining.

The hose trap is held in place by loose fitting 2 hole straps. Sometimes a sharpie trace of it's loop so HO will maintain the trap. I have gotten calls with overflow problems and it is usually because the trap was lost.

At the coil drain port I use full size (3/4) fittings and connect the hose on the down flow side of a tee. I avoid concentric connections on the horizontal to get the water as low as possible in the coil pan. (just like the turn down on a steam header going to the equalizers).

Lately I use clear flexible tubing that fits tightly over 3/4" sch 40 PVC pipe. Short pieces are used for a visual check of flow and do the duty of a union in the piping for removal and flushing.

Using the clear tubing for the entire trap has been done. It allows a good training aid for helpers and HO's. On start up with the trap dry, after 10 minutes, you plug the end with your thumb and witness the coil suddenly drain into the trap. Many people do not really believe that the blower suction will hold that water from running out of an open pipe. This helps convince them.

jumper · July 2016

I dislike traps because they clog. What's wrong with a little bit of outside air?

JUGHNE · July 2016

Coil under is under negative pressure because of suction from blower being downstream. Drain pan will run over eventually.....promise.

ratio · July 2016

With a short drain line on suck side of fan, yes, pan won't drain & will overflow. With maybe 10 ft of PVC, trap is usually self-priming. On blow side of fan, air conditioning for the basement?

Condensate drain line problem

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