Since heat pumps work by extracting heat from the outdoors to bring it inside, the outdoor coil will almost surely freeze up. To mitigate the freezing, heat pumps enter a defrost cycle in which they run in cooling mode to send hot, high-pressure refrigerant back through the outdoor coil and melt the ice. While the heat pump runs a defrost cycle, the backup heat will kick on to keep the customer from becoming cold while the unit doesn’t actively move heat into the home. There are two different defrost methods: time & temperature (T/T or T-T) and demand defrost. Demand defrost is usually associated with greater energy savings and less wear on the compressor. Almost anytime you replace an OEM defrost board with a universal defrost control on a heat pump, you have an opportunity to choose how to configure the new control, and that could include upgrading the time & temperature defrost to demand defrost.

We’ll explore why and how to upgrade the heat pump defrost method in this tech tip, using the White-Rodgers Universal Defrost Control (47D01U-843) by Copeland as an example. This control replaces over 400 OEM part numbers, most single-stage heat pump defrost controls.

Differences Between Defrost Types

Time & temperature defrost requires the coil sensor (thermostat) to close when it detects a temperature below a specific set point, which then starts a timer. Defrost is initiated once the equipment has run for a 30, 60, or 90-minute interval without opening the thermostat. The defrost cycle will terminate once the coil temperature sensor reaches a set temperature or a certain amount of time has passed, whichever one comes first. 

Demand defrost checks the coil temperature sensor and outdoor air temperature sensors at all times. Instead of operating on a timer, it only goes into defrost when the coil and outdoor air sensors indicate a need for it. As such, the demand defrost strategy is usually better for energy savings and compressor longevity.

Two things must be true for the defrost cycle to begin: the differential between the coil temperature and outdoor ambient temperature must be large enough, and the coil temperature must be below the set point. Even if you have a temperature swing between the coil sensor and outdoor air sensor, defrost will not begin until the coil temperature has dipped below the programmed setpoint. For OEMs and the White-Rodgers Universal Defrost Control (factory default), these setpoints are as follows:

Other Advantages of Demand Defrost

Apart from the energy savings associated with fewer defrost cycles than a timed system (up to $400 a year), demand defrost could also lead to improved compressor longevity and reliability in the long run.

We’ve talked a lot about the dangers of short cycling. Apart from the comfort issues associated with short cycles in humid climates, more frequent startups will strain the compressor motor. Defrost cycles are no different; the compressor runs during defrost to pump the hot discharge gas through the coils. Anytime we run a defrost cycle unnecessarily, the compressor has to start up again, which is hard on the motor and energy-intensive.

The White-Rodgers Universal Defrost Control does, however, force a defrost cycle after six hours to ensure adequate oil return. Oil return is crucial because the oil lubricates the bearings and keeps the compressor parts from wearing out, which also leads to premature failure.

Sensor Location

Accuracy is the key for defrost cycles, so we want to put the coil and outdoor air temperature sensors in specific areas.

The coil temperature sensor comes with a clip and will typically be mounted to the lowest possible row of the condenser coil (not the liquid line). The goal is to make sure the sensor makes sufficient contact with the tubing so that it can pick up the coil temperature accurately. If you are replacing an OEM defrost board with a universal one, use the same spot as the previous coil sensor (unless there was something glaringly wrong with the original placement).

The placement of the outdoor air temperature is a little less straightforward. We don’t want it to be in direct sunlight, so we need to mount it to a place where solar radiation won’t skew the reading. It’s usually fine enough to route the sensor through the grommets of the condenser side panel and fasten it to the thermostat cable and/or electrical whip.

Programming the Defrost Control for Demand Defrost

If you install the White-Rodgers Universal Defrost Control, you could simply use its factory default settings, which are configured for demand defrost. With the control wired up properly and receiving power, press the “Option” until the display shows OE, and then press “Select.” From there, you can cycle through numbers 1–8 by pressing “Select.” Do this until you reach 8, and then press “Option” to confirm it. 

By choosing option 8, the system will work accordingly:

• Type: Demand defrost
• Short cycle time delay: 5 minutes
• Reversing valve: O
• Reversing valve shift delay: 30 seconds
• Maximum defrost time: 14 minutes
• Enable defrost set point (coil sensor): 35°F
• Defrost termination set point (coil sensor): 70°F

If you merely want to replicate the OEM’s settings, you can cross-reference the existing board’s model number on the Universal Defrost Control’s box or in the White-Rodgers app. Then, select the matching OEM preset. The table shown earlier has the exact numbers and corresponding settings.

Customizing Defrost Parameters

You can also set custom parameters by cycling through the menu and entering the time delay, reversing valve power method, reversing valve shift delay, max defrost time, and coil sensor set points manually. As with the OEM quick startup selections, use the “Option” button to cycle through the options and “Select” to pick the one you want.

The dF menu lets you select timed (t) or demand (d) defrost.

Et stands for enable temperature; this is the temperature that the coil sensor must drop below in order to enable defrost. The selections run from 30–36°F in one-degree increments (e.g., you could choose 32°F or 33°F). 

The tt feature sets the defrost termination temperature. The selection varies quite wildly, with 50°F on the low end and 100°F on the high end. Keep in mind that lower coil temperatures may result in more frequent (but shorter) defrost cycles.

You can set short cycle time delays with the SS feature. You can choose from 0, 3, or 5 minutes. Since we do NOT want short cycles, I usually recommend picking 5 minutes.

Reversing valve power is determined by the r setting. It should always be set to o (energized in cooling mode) EXCEPT when working in Ruud/Rheem heat pumps. These systems are unique because the reversing valve is energized in heating mode, and that call is on the B terminal (b).

You can also program a reversing valve shift time delay (rS) to reduce loud noises when the unit goes into defrost, as some customers may be alarmed by noise. You can choose to have no delay (0), 12 seconds, or 30 seconds.

The maximum defrost time (dt) can be configured to last for 8, 10, or 14 minutes. This setting may depend on your climate. 

Final Note

While some OEMs use time & temperature defrost, demand defrost is more popular among most manufacturers, and it has some very real benefits. Universal defrost controls offer the option to customers who have units that rely on time & temperature defrost and want to save more energy. 

Above all, the White-Rodgers Universal Defrost Board also gives you more freedom to customize the defrost cycles to suit your clients’ needs in your specific climate.