SRHuston Dot Net

How many roads must a man walk down
Before you call him a man?

–Bob Dylan, “Blowin’ In the Wind”

I had some questions about our air conditioning system, and now a chilling mystery has finally been solved.

We had our air conditioner fail just before the start of the last summer season. Fortunately we figured that out very early, and had the technicians come out and verify it was dead (lost its charge, and given the age it wasn’t deemed worth fixing.. I balked, but kinda agreed). They were able to schedule us for a new install pretty quickly, and we had everything wrapped up within a week of the summer temps getting pretty outrageous here, so I was happy with how it turned out.

One thing I noticed with the new system is that it uses a newer refrigerant as well. Now I’m used to R-22 not being “the thing” anymore, and even R-410A is on the way out, but I’d never heard of R-454B. The Wikipedia page on it is an interesting read, if only because it mentions one of the constituents of it is R-32, which is being phased out in some places and thus endangering R-410A’s usage (which makes you wonder why use another blend containing it). And of course the bit that as of last year, there’s a shortage of it. If I’d read that before they suggested this unit I might have suggested another model…

Anyway, things worked great for the summer, and now it’s winter time. We’re not using the A/C of course, and our heating is from hydronic baseboard so we have no need to run the indoor unit at all. So imagine my confusion some time ago when I was sitting at my desk and suddenly realized the fan was running. I wondered if the thermostat was messed up and checked outside, but the compressor and outdoor unit wasn’t engaged, just the fan. I flipped the fan switch from its normal “AUTO” to the “ON” position and back, but no difference. So thinking the thermostat might be more broken than I thought, I pulled it off the wall. When the fan kept going, now I knew something more interesting was going on. And yes, for those keeping track, that would be the Confucius definition of “interesting.”

After a bit of research, I came to a conclusion. See, since the refrigerant in this unit is slightly flammable, there’s a “dissipation sensor” inside the indoor unit. If it detects a flammable gas present, it forces the compressor to stay off and turns the fan on for about 15 minutes. Sure enough, when one of these episodes happened I was able to note the start time and the end time was almost exactly 15 minutes later. Ok, now I know why the fan is coming on, but why does it think it has to come on? Well, the board in the unit would tell me via a test light – if it was flashing a certain pattern, that would mean it thought it detected gas present and was turning on. So I’d now have the information needed to tell the service folks that either we have a leak in our system and need it fixed and recharged, or we need the sensor or board replaced because it’s faulty. But next time it went off, I couldn’t get upstairs in time to check it, and by the time I did get there I realized I didn’t know what side of the unit to open to get to the control board anyway, so I went back to the computer to look up more information. And that’s when I made the next discovery.

After searching for more information on the dissipation board I found a Reddit post that sounded like I could have written it myself based on what was happening. But it was the OP’s follow-up in a comment that struck the right chord:

While my sensor was installed wrong, it turns out that wasn’t the cause of the problem. Carrier just issued TIC2025-035 that says this is the intended behavior of the sensor calibration electronics. The new coolant is more flammable and requires more vigilant monitoring. If the fan doesn’t run for 15 minutes over a 1 week span it does so to clear the vents and recalibrate. Unfortunately this was apparently designed without thinking about setups like mine where the vents are closed during the winter and the air in the vents is approaching 5°F.

–u/dvogel on Reddit

Unfortunately I’ve not been able to find that TIC myself, but this all seems to make sense. So at least now I have an answer on WTF is going on, and why my air conditioner thinks it’s appropriate to blow cold air on me once a week. And now I have plans to wire up a switch in the room below where the air handler is so that I can manually switch it off during the winter when it’s not needed (yes I could use the breaker in the basement, but that’s harder to get to, and so if I decide I want to turn it on one day it would be harder to get to it easily.. and breakers aren’t meant to be switches).

Sonsabitches.