Yesterday afternoon Tam and I were busy at various, divergent tasks. She had been folding laundry in the basement and when she returned to the surface, she told me, "You might want to check that condensation leak all around the furnace...."
Uh-oh. The drain line from the drip pan under the A-coil* is ludicrously large for the amount of water: Roseholme Cottage is not a very large house. "Was there water on the top of the furnace?" I was hoping not. The furnace sits in a slightly-lower spot. Maybe it was just seepage from recent rain that hadn't evaporated.
"I think so."
Drat. Damme. Shit-oh-dear..
Investigation found a little water on top of the furnace, dripping down the outside to the floor; there was condensation on the outside of the section of duct where the A-coil lives. Frozen up? Probably. I killed the cooling, set the fan on continuous run, and used my wet/dry vac to force-drain the condensate line and pan. It's just a PVC pipe run to the the floor drain, so that's easy. In a few hours, the A-coil enclosure was down to room temperature and I wasn't getting any more water, so I opened up the furnace.
Either I missed the last filter change or the cats really stepped up their shedding: the filter was pretty well blocked.
I put in a new filter and ran the system on air-only for as long as Tam and I could stand it -- quite late, really -- then put the cooling back on with a higher set point. So far, so good, it's producing cool air in some volume, but I'm not going to push it until we get into the week and I can ring up our furnace guys to check refrigerant level. The forecast is for cooler temperatures and lower humidity, so here's hoping....
* What's an A-coil and why do they call it that? It's the inside-cold part of the cooling, where compressed gas from the compressor/heat exchanger outside expands and cools down as it does so, and then cools the air passing through the A-coil. In a central air-conditioning system, it typically lives in the ductwork at the output of the furnace, downstream of the fan and heating element. In order to get the maximum surface air to cool the air in the space available, the coil -- like a car radiator -- is usually arranged in two sections that form an inverted V or lambda in cross-section; or, in fact, an A shape. They can get clogged and do need to be cleaned occasionally, but are fragile and often hard to get to. They are one reason why your furnace has a filter -- and why you should change that filter more often than I did. So where's the water come from? It condenses on the A-coil just as it does on the outside of a cold glass. It drips off, is caught in the condensate tray, and drains down a pipe (or in case of a window unit, it just drips) -- unless your fan is running too fast, in which case it gets blown off the coil into the output air and you've got trouble. This stuff isn't as easy as it might look, nor as simple.
5 months ago