Arkansas HVACR NewsMagazine January 2025

HVACR NewsMagazine January 2025

Tech News

minutes for the Carrier defrost board; that can usually be configured when you have universal boards, but that timeframe is built into many OEM boards. On the other hand, demand defrost checks the coil temperature sensor and outdoor air temperature sensors at all times. Instead of going into defrost after 30 – 90-minute runtimes, it only goes into defrost when the temperature difference between the sensors exceeds a set difference. However, there is usually a maximum time between defrosts; six hours is a common timeframe. Demand defrost can offer major energy savings.

Heat Pump Defrost Troubleshooting Tips We’ve entered that season of service calls for “smoking heat pumps,” and I figured now is as good a time as any to share a few quick tips for troubleshooting defrost. Of course, the “smoking heat pump” calls can be solved over the phone with a quick explanation of normal defrost conditions — as can the service calls for a slightly frozen outdoor coil. However, problems with the defrost cycle can happen, and we should know the basics of how to solve them. The good news is that defrost issues can be narrowed down to just a few possible problematic parts. These parts are the circuit board, temperature sensors, and wiring or harnesses. Defrost control boards will also usually have fault codes for when things go wrong, so you should have a basic idea of where to start looking. Time and Temperature vs. Demand Defrost The defrost cycle requires the reversing valve to shift, the auxiliary heat to come on, and the fan to stop working. However, there are two different defrost methods: time and temperature (T/T) defrost and demand defrost. Time and temperature (T/T) defrost requires the coil sensor (thermostat) to close, which then starts a timer. (Carrier defrost thermostats close at 32°F.) If the equipment continues to run for a specific time interval — usually 30, 60, or 90 minutes — and the defrost sensor or thermostat remains closed, then it will initiate defrost. From there, the defrost cycle will happen, and the defrost cycle will terminate once the coil temperature sensor reaches a set temperature. (Again, the Carrier defrost thermostat will open at 65°F.) There is also a maximum timeframe for defrost, like 10

Checking Fault Codes

After you’ve done a visual inspection of the entire system, the first place to start in the “narrow” part of your diagnostics is by looking at the fault code. That fault code will tell you which switch has tripped, and you can start narrowing down your diagnoses from there. These fault codes will be in the defrost control’s manual (though some defrost controls, like the White-Rodgers 47D01U-843 universal defrost board, come with stickers of the fault codes and their meanings that you can affix to the unit). The following is an example of codes on the White-Rodgers 47D01U-842 universal defrost board’s display.