Help me diagnose an HVAC problem

[Retirednoguilt]

I've seen that wall switch issue before. My parents lived in an expensive condo on Long Island. After my widowed mother died, I inherited the unit, which I didn't want to retain. The HVAC system had

2 zones with 2 separate heat pumps, one zone for each level with the heat exchange and blower for the upper level in the crawl space attic. In the process of selling it, the real estate agent's inspection determined that the upstairs zone wasn't functioning. An expensive troubleshooting visit by a HVAC company tech finally found the problem: an unlabeled wall switch on a bedroom wall that had been painted the same dark color as the rest of the wall and located in the shadows near the baseboard underneath a vanity with no cabinet underneath. Very hard to see. It was in the off position. No one had any idea what it controlled; but turning it on brought the upstairs zone to life. I often wonder what other hidden goodies I overlooked that my paranoid parents never told me about.

[micky]

LOL. Your parents knew what the switch was and where it was, and the color didn't fool them.

I've thought I should write up a list of this house's idiosyncracies, before I sell it or die, and before I forget what they are. Maybe I will some day.

[trader_4]

It prevents some cold pressurized air from being wasted down the pipe.

No. A vent on a sanitary drain line is to prevent back pressure and allow multiple volume sources to drain without interfering with each other. You just have a single source trickle down a pipe.

[rbowman]

Do you say program-me? I'm reminded of the Teatime character in one of Terry Pratchett's novels.

[TimR]

Answering the last paragraph first, the vent. I don't actually know if I have a single volume source, as I have not found the discharge from those lines. Hopefully when I go back with the system running it will be obvious. Either way I don't intend a vent at this point, the system apparently ran without it, sometimes better is the enemy of good enough.

The trap. I have some experience inspecting commercial construction (for government jobs) and if that trap was not deep enough the pan wouldn't drain. The fan was downstream of the cooling coils and the cooling unit was therefore under negative pressure. I missed it on the first project and learned the hard way. I looked at some YouTube videos and most had a trap similar to the shallow one at this house, so maybe residential is different. (For sure I was never in a mechanical room where I had to crawl.)

[TimR]

Here's an update a month later. When we got it running the weather was cool enough not to know if the drains were clogged. I suspected the main pan drain was blocked, or the secondary pan wouldn't have had water, and hoped the secondary drain was not blocked, or the secondary pan would have overflowed before rusting through.

Alas. My wife's friend had water through the ceiling again. It's hot and humid enough to be producing some condensate.

So I went over today. The secondary pan was full up to but not over its drain outlet. I looked again for the end of the drain lines without success, even pulling a vent off the crawl space, but could not find anything. The outdoor unit is about six inches off the wall with a vent behind it, I suspect they are in the crawl space. In the attic I used a shop vac (on a chair with two hoses end to end) to suck the water out of the pan. Then I cut both lines and put the vaccuum on them; both were clear from the cut to unit and blocked from the cut downstream. Well I think they were blocked, the motor whined like when you put your hand over the end. I tried my snake but could not get past the 90. I left the shop vac on the secondary line a long time and got an occasional burble feel, but it never slowed down like it had cleared.

Anyway, I've reached the end of my skill set, unless somebody has another suggestion. I patched the pipes with compression couplings and figure the next guy will at least have a place to start. Maybe the secondary pan is draining slowly. When I cut the main drain line it ran water, but the secondary did not. They were both slimey and gloppy inside.

[Retirednoguilt]

Our HVAC service person ensures the condensate drainage line is clear by using a small compressed air cylinder on a fitting that enables a pressure-tight fit into the drain line. You might want to try pressure rather than vacuum to clear the line. Too bad you don't have a long fiber-optic inspection scope available. Perhaps you can rent one somewhere although an inflexible 90 degree elbow might prove to be barrier unless the scope is super flexible.

[Hiram T Schwantz]

On Tue, 23 May 2023 09:40:32 -0400, Retirednoguilt posted for all of us to digest...

Wait until the pipe inside the wall explodes, you'll find the clog.

[TimR]

The pipe goes down the wall vertically and exits presumably into the crawl space. I'd say there is a good chance the clog would never be found and the AC would drain fine forever.

Might have some mold growth I suppose.

[Retirednoguilt]

This tech has been doing this for decades and told me that he's never disrupted a condensate drain pipe using compressed air and has successfully cleared numerous clogged condensate pipes with that technique. I didn't ask him what PSI he's using but he seems to know his business.