I'm looking to trigger defrost on my home HVAC system whenever the indoor coil freezes over. It happens rare enough to not warrant replacing the system.
Easiest way that I can think of doing this, is to measure duct air temperature before and after the evaporator coil. If the delta T decreases below say 10 degrees for 10 minutes, open a relay to interrupt the compressor for 30 minutes while the blower defrosts the coil.
I've never used an Arduino, so I'm looking for recommendations on cheap reliable temperature probes, and if there is any pre built code to handle something simple like this. The probes would be monitoring 60-75F approximately, And accuracy of about 1-2F should be fine.
My recommendation is to get a professional to check the unit and it's charge.
Measuring the duct temperature will be an interesting task. Consider as the coil freezes it restricts the air flow. This reduced air will warm quicker and the duct will get warmer unless it has some super insulation.
I would use an airflow switch. This freezing probably happens when it is very humid. Be sure all of the filters are clean and the vents are open. Sometimes there are filters in the vents, be sure they are clean. If you want to use4 a temperature sensor use it on the return refrigerant line. When the temperature goes below about 35F turn on a warning light. Freezing is one of the signs of low refrigerant, as time passes it will get worse.
An eazy way to fry a compressor is to cycle it. Consider the 6 minute rule, if it goes off it must stay off for at least 6 minutes. If turning on and no fault it must stay on for 6 minutes.
When a heat pump goes into defrost mode it actually turns on the cooling so the warm Refrigerant is going back out to the condenser and defrosts the condenser. The cool air while in cooling is offset by the electric heating element in the airhandler. Most defrost control boards reverse the refrigerant and can run from 30-60 min. Now, this is on a heat pump not a straight cool system