We have an older home that has a large living-area/fireplace room; the AC unit sits over it in the attic. Earlier this year, in that room, we noticed a crack beginning to form in the ceiling sheet rock, forming a fairly straight line toward the fireplace. When we called someone over to repair it, he went into the attic to investigate and found that the ductwork coming from the AC was likely to be causing the problem: the ceiling insulation beneath the plenum (or what he thought was the plenum at the time) was wet. He figured that the duct insulation was no longer preventing condensation from forming, and the resulting water was causing the ceiling crack…we would have to get that issue repaired before fixing the ceiling.
Last Friday we called an AC tech to look at the situation. He showed me that instead of a plenum, we actually have a large, square portion of duct, less than a foot long, emerging from our AC coil to form a “T-junction” with the main duct line running to the rest of the house. The tech explained that such duct-line arrangements were originally designed for heating only…when AC was first brought into our home, it was simply connected to this arrangement, likely because current methods weren’t in use at the time. He explained that this arrangement isn’t what is done nowadays: a real plenum would use static pressure to help improve the cool-air flow to the rest of the house. Because the T-junction isn’t helping to move cold air as quickly, it lingers in the junction, and so it increases the chance that condensation will form.
The tech said that he didn’t know anyone who would re-insulate and re-vapor-barrier the existing setup, since that arrangement isn’t done nowadays. However, according to his measurements, the area where our heater, AC coil, and T-junction are in the attic is too small to simply install a proper plenum and redo/reinsulate our duct lines. He said we need to buy a new heater, which will be smaller and more efficient…that will give them room to put a new coil next to it and finally, insert the plenum where it belongs…then run new ducts throughout the whole house. Because we have an older AC system, that would mean we will also need to purchase a new outside condenser as well (to match the new coil and its new refrigerant-type). All of this is to the tune of almost $11,000.
Both my wife and a friend of mine suspect that we are being sold a “Cadillac” solution. I plan on getting a second opinion anyway, but I’d love to hear some input from any other knowledgeable HVAC folks who could tell me if someone is trying to hose us. Thanks!