Question about a drain Problem.

[Brent Ridley]

I installed a new carrier comfort series 5 ton system yesterday. I installed a horizontal air handler and didn't have a return plenum so I installed a flat piece of metal over the end and tapped into it with a 18" elbow. Everything went fine, the unit fired up and was working properly but it didn't want to drain. After a few minutes of looking at it, another service tech came in a told me that it was because I didn't install a return plenum box. He claims the suction on the blower motor was too much and was holding water in the pan. He claimed if there was a plenum then there would not be as much suction. I personally think it was because my trap wasn't deep enough. Anyhow, I adjusted the fan speed down and the unit started draining. Is he full of it? or is this possible?

[fincomber569]

Sounds to me like someone needs to put an incline or digital manometer on the system and find out what the static pressure differential is. I have seen undersized ductwork and/or restricted airflow cause this problem many times. Carrier provides fan performance charts to determine airflow based on static press. readings. If nothing else airflow could be determined by temp rise method provided you have a heater package installed in that air handler.

[jdavis72]

Sounds to me like someone needs to put an incline or digital manometer on the system and find out what the static pressure differential is. I have seen undersized ductwork and/or restricted airflow cause this problem many times. Carrier provides fan performance charts to determine airflow based on static press. readings. If nothing else airflow could be determined by temp rise method provided you have a heater package installed in that air handler.

I agree. System could be pulling so much static that it's keeping the drain from draining correctly. Just for giggles, have you tried pouring water in the drain pan with the sys off and see if it drains ok? Is there a vent on the drain before the trap? After the trap? Is it an R trap, or a P trap? Newer air handlers with higher static pressure can sometimes actually suck the water out of an R trap.

[Jevans]

I came across a big carrier the other day that was not draining well and the trap was different then what I normally see. The vent pipe was after the trap and without a cap. Could you explain a little the difference between the two.

[Brent Ridley]

It was a P trap and had a vent pipe after the trap. I poured water into the trap without the unit running and it did drain. So, I am going to try testing the static pressures and go from there. Does anyone know where can I find the fan performance charts? Thanks for all the help!

[beenthere]

They're in the install manual.

Your return set up is a lot of the problem, as the tech said.

[jdavis72]

I came across a big carrier the other day that was not draining well and the trap was different then what I normally see. The vent pipe was after the trap and without a cap. Could you explain a little the difference between the two.

When the evap is on the negative side of the blower, there should be no atmosphere between the drain pan and the trap. Remember, the blower is actually sucking on the water, kinda like sucking on a straw without your lips actually on it, so no venting of any kind before the trap. It's recommended to put a vent after the trap, because a lot of the times, the drain line is not properly supported and thus can create a natural trap in it. Thus, you have a dead air space between two traps, creating a positive pressure that will cause gurgling if it drains at all.

[jdavis72]

My last employer sold a lot of Trane units with the Clean Effects package. He would size his return air without considering the static pressure increase from the air cleaner. Thus, I was going back on a lot of his installs, and seeing the condensate pouring out of the unit, and into the secondary drain pan, because there wasn't enough return air to drop the static pressure. The VSB was actually sucking so hard, it was keeping the water from draining into the drain line, and overflowing the primary pan. You'll see the same thing with a VSB where the customer hasn't changed there air filter in six months or so.