So I have a central A/C unit that was installed last year, charged with R456a. Yesterday, my wife noticed that the blower was running continuously. Compressor was off. My first thought was that the thermostat was fer****zen. So I replaced it with my spare, problem not solved.

Next, I opened the electrical panel on the air handler. The control board was flashing blink codes, like an old Bosch fuel injection system. I looked up the code and it was "Bad dissipation sensor." I had no idea what that meant, I had never heard of a dissipation sensor.

It turns out that R456a is flammable, and heavier than air. If a leak occurs in the evaporator coil, the refrigerant would pool in the air handler, and a spark would make it explode through the ducts. So for A/C handlers charged with this stuff, they add a dissipator sensor and control card.

The dissipation sensor is there to detect tiny leaks. When it does (or when it malfunctions, as happened here), the control card interrupts the thermostat signal and turns on the blower. The idea being to mix the refrigerant with enough air to make it not a problem, and push it out of the ductwork. The blower will run until the problem is fixed.

What you need to know is that new A/C units will come equipped for this stuff. The dissipation sensors can go bad. And when they go bad, a perfectly good a/c system will be unusable until they are fixed. I think they need to be cleaned from time to time, we'll see.

Oh, and the a/c blower may occasionally come on to calibrate the sensor.

The future isn't what it's cracked up to be.