Thread: AC Condensate Line Blowing Cold Air Through Sink?

Hi Everyone,

I've researched this as much as I can find online, but am still not 100% clear on if my AC condensate drain is properly installed. Hopefully you can help! I just bought a new home that is 4 stories and has the AC unit located outside the attic on the 4th floor. A thick insulated line from the AC unit runs inside the house and to what I believe is the condenser/furnace. Out of the condenser comes two small PVC lines (one being the primary drain and the other being the secondary I believe). There is cold air coming out from the condenser from around where the two drain lines enter, but I assume that is probably minor? The primary drain line then goes to a T shape with a cap on the top of the T (the clean out I assume). The bottom of the T goes to the third floor guest sink and ties in between the sink and the P trap. I disconnected this rubber line and poured water/bleach through the attic clean out to make sure it is not clogged. No problems here. However, the drain line is blowing a fairly large amount of cold air up through the sink and into the house. Since there is a P trap on the sink between the drain line and the sewage, do I have a problem? Do I need a P trap on the drain line in the attic and does not having one damage anything? I attached a very rough diagram I drew that might be helpful. Forgive me for any ridiculous assumptions... Google can only teach one so much!

Thank you very much for all your help. Please know it is greatly appreciated!

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Slip "Evaporator" in place of your condenser wording and you have it mostly correct.
The condenser is the exterior portion of the air conditioning system since it condenses hot refrigerant (Freon) from a vapor to a liquid.
The evaporator coil is the indoor part of the a/c since it vaporizes cold liquid refrigerant to a gas.

Both the primary condensate (water) drain and the secondary condensate lines should have traps to stop air from moving through the drain lines along with the liquid.

All of the above assumes you have a air conditioner being used in the cooling mode since a heat pump or condensing furnace will have different answers.

condensate switch and trap.jpg
This is a photo of a properly configured primary condensate drain and trap with a water sensor in the secondary drain port of an up flow furnace. A trap can also be used on the secondary drain but it will need to be primed (I prefer mineral oil) to prevent loss of trap seal due to evaporation during off season.

Cond%20-%20Trap%20Lennox%20Diagram%20B.gif

I can tell you the majority of installers in the north Texas area do not install traps properly.
But for what it is worth, this is primarily an energy efficiency issue, not a functionality problem.

Install a trap and you will be fixed.

Originally Posted by JTravis

Hi Everyone,

I've researched this as much as I can find online, but am still not 100% clear on if my AC condensate drain is properly installed. Hopefully you can help! I just bought a new home that is 4 stories and has the AC unit located outside the attic on the 4th floor. A thick insulated line from the AC unit runs inside the house and to what I believe is the condenser/furnace. Out of the condenser comes two small PVC lines (one being the primary drain and the other being the secondary I believe). There is cold air coming out from the condenser from around where the two drain lines enter, but I assume that is probably minor? The primary drain line then goes to a T shape with a cap on the top of the T (the clean out I assume). The bottom of the T goes to the third floor guest sink and ties in between the sink and the P trap. I disconnected this rubber line and poured water/bleach through the attic clean out to make sure it is not clogged. No problems here. However, the drain line is blowing a fairly large amount of cold air up through the sink and into the house. Since there is a P trap on the sink between the drain line and the sewage, do I have a problem? Do I need a P trap on the drain line in the attic and does not having one damage anything? I attached a very rough diagram I drew that might be helpful. Forgive me for any ridiculous assumptions... Google can only teach one so much!

Thank you very much for all your help. Please know it is greatly appreciated!

You cannot have a direct connection to a waste line, period. This connection has to be through what is known as an indirect connection, meaning there is an atmosphere gap.

Likewise, a trap is required on the condensate lines.

All HVAC systems that convey air as in air distribution are required to be 100% sealed.