I have a Carrier 410A heat pump that's about 10 years old. Last week, I noticed that that condenser fan was cycling on and off every three-to-five minutes, while the condenser continued to run. I don't think it was going into defrost mode, because I could see frost forming on the unit while the fan was off, then melting away after the fan went back on. The unit also started making an intermittent clunking sound I'd never heard before - three metallic clunks every few minutes.
Our contractor diagnosed a bad defrost board and replaced it. The clunking stopped, but the fan continued to cycle on and off, though not as frequently. With the new board installed, the fan repeatedly would run five-to-ten minutes, then turn off for about 90 seconds. (Outdoor temperature was about 40 degrees.)
This time, the contractor said the issue was the "Pressure Guard" vapor pressure switch. He bypassed the switch, and that eliminated the issue of the cycling fan.
In doing some reading on Carrier's website, the company says Pressure Guard was included on 410A heat pumps 10-15 years ago to comply with burst requirements in regulatory codes that were written for R22. (Though the contractor thought the switch was bad, I'm thinking it probably was functioning properly on our unit, cycling the fan off in regular intervals to reduce pressure in moderate weather.) By 2003, the codes had been revised and Carrier eliminated the switch, which it had concluded was unnecessary.
Carrier's site doesn't address whether it's okay to bypass the switch on older units, but our contractor said his company does so routinely to increase efficiency and prevent problems from malfunctioning switches. Does anybody have any thoughts/experience with this?