Don't Cap the Vent!

Similar to how our heat went out on the coldest night of the year in 2015 (the first winter we were in the house) our air conditioning went out on the hottest night of the year last weekend. I had heard it shut off at one point mid-evening and when I looked over at the thermostat it was completely blank. Based on what happened with the heat I knew immediately this meant that the moisture sensor in the back up drain pan underneath the HVAC unit in the attic had been tripped.

Sure enough once I climbed into the attic I discovered the pan to be full of water. I hauled my shop vacuum up there and once I had sucked up enough water to get it below the sensor the unit came on again. (We’ve had the thermostat set at 74 and thankfully it had only climbed to 76.) Between my vacuum not being very big and getting heavy with water it took me three trips up there to remove all of it.

So how did it happen? The day before this happened a local HVAC company had come out to check over the unit and make sure everything was working properly. I’m on a twice a year preventative maintenance agreement (which is cheap insurance against bigger problems) but I had been worried the last few days that something wasn’t right and with the temperature scheduled to nearly 100 degrees for several days with even higher heat indexes it was worth the $57 cost for a little extra peace of mind. (Everything ended up being fine.) When the technician came out I had asked him about how there was cold air blowing out of the condensation drip tube and if it should be capped. He agreed but said it should be left loose so it could vent and still allow the water to drain. But the cap ended up being on too tight and prevented the tube from venting. From there the water backed up to the secondary drain and out into the pan.

Arrow marking the vent where the cap was placed.

Back up drain pan underneath the HVAC unit.

So the lesson learned from this, while it might seem like it makes sense to cap the tube, don’t do it. On the flip side, I know for sure once again that the back up drain pan and moisture sensor work that keep water from overflowing and leaking through the ceiling.