Arkansas HVACR NewsMagazine January 2025

HVACR NewsMagazine January 2025

Tech News

1. Disconnect the fan from the defrost board while the system runs in heat mode. 2. You can jumper out the defrost thermostat if the temperature sensor is a thermocouple, but this step will allow frost to accumulate on the coil and make sure the thermostat recognizes that it needs to go into defrost. 3. Speed up the defrost with the speedup tap, and then plug the fan back in. 4. Measure voltage between O and Common (24v). 5. Measure voltage between W and Common (24v). 6. Watch the system come out of defrost and verify that the condenser fan comes back on. Electrical Diagnostics Connections to the board must be tight and configured properly for the climate and application (e.g., for demand defrost). When you do your visual inspection, make sure the connections are snug and that there are no signs of arcing. During the defrost cycle, the reversing valve switches over to cool mode and the auxiliary heat comes on (usually electric heat strips in our market, but the aux heat may be gas if there is a dual fuel setup). When you test defrost, you can verify these by measuring the voltage between their respective terminals and common. • Voltage between O (reversing valve) and Common = 24v • Voltage between W (backup heat) and Common = 24v Ruud and Rheem heat pumps utilize the B terminal instead of the O terminal, and they energize the reversing valve in heating mode. In ALL operating modes, you should read around 24v between Y and Common and R and Common at the defrost board. Y connects to the contactor coil, and R runs constant 24v

power. The board will run without R because there will be a Y call, but defrost can’t occur because R redirects on O and W, and your heat strips and reversing valve can’t work if there isn’t 24v on R. Coil Sensor Problems Defrost boards like the 47D01U-843 universal defrost board have a fault code for coil sensor faults. In our experience, temperature sensor failure is relatively uncommon. Many of the problems that deal with coil temperature sensors involve damage to the conductors (such as by animals or lawn equipment) or shorting out. That’s why a visual inspection is one of your most valuable skills when it comes to troubleshooting defrost. You can spot damage, loose wiring connections to the board, or sensors with poor contact on the coil. When we have snap-action thermocouples as temperature sensors, our main goal is to make sure that they close at the defrost control’s set point to begin the defrost cycle and open at the temperature needed to terminate defrost. Thermistors behave a bit differently, and you will need to ohm them out and compare your resistance reading to the temperature per the manufacturer’s recommendation. If you’ve verified that the connection is solid and still notice that the sensors aren’t closing properly or have resistance values that don’t match up with the manufacturer’s recommendations, you can condemn the bad sensor and replace it. This tip isn’t a comprehensive guide to heat pump defrost by any means, but it goes to show that a lot of the issues with defrost will be electrical, and you’ll usually have fault codes or visual indicators that something is wrong. Each manufacturer is a little bit different, so be sure to read the manual each time you need to mess around with a heat pump’s defrost board.