(The content of this post is intended for consideration by trained service personnel only)

The time-temp type controls are, simply put, an electronic timer with a mechanical coil temperature sensor (defrost thermostat, DT, DFT etc). The timer “runs” all the time, or at least during the compressor “on” cycle. If, after the timer reaches the end of its preset period between calls for defrost (30, 60, 90 min etc) and it sees the DFT “closed”, it initiates the defrost. The timer doesn’t know if it’s summer or winter. It’s the DFT’s that decide what the seasons are, and normally close at some temperature around 30F. So, if the DFT fails closed for some reason, although the coil temperature is actually closer to 100F than 30F, the “board” will initiate the defrost cycle…

Demand type controls are a little more sophisticated. They use two thermistor devices to monitor both the coil temperature and the ambient temperature. The “board” then becomes master of the seasons. When the difference between the coil and ambient temps reaches some design value, the board interprets the difference as a need for defrost. It’s important to keep in mind, as the outdoor coil frosting increases, less heat transfer can take place due to decreased airflow, and subsequent reduction in available heat energy. So, the boiling or evaporation inside the coil slows, resulting in a lower suction pressure and lower saturated coil refrigerant temperature. The temperature differential is an indirect measurement of frost build-up. And most, many, maybe all, demand controls include a design outdoor temperature value, above which, the board won’t initiate a defrost…those values I’m familiar with are around 40F. So if the outdoor temp stays above the “upper limit”, excessive frosting that could result from, say a failed condenser fan motor, won’t produce a defrost cycle.

The obvious difference between time-temp and demand controls is the frequency of defrost cycles actually taking place. The time-temp controls can initiate every 30 minutes under the right conditions, with maybe every other defrost being unnecessary. The demand control, on the other hand, will initiate only when the coil actually needs a defrost…which of course, improves the heating efficiency of the heat pump system.

You can get a full explanation and illustrations of all the common electrical failures in the “Troubleshooting Heat Pump Electrical Systems” videos for rent:

Troubleshooting Heat Pump Electrical Systems

(The content of this post is intended for consideration by trained service personnel only)