In this live episode, we talk about heat pumps, why Bryan likes them, why other people don't, charging and diagnosing them, and defrost.

Even though heat pumps work best in warmer climates, they can theoretically work as long as the temperature is above absolute zero (-460 degrees F). Viewers across the USA install heat pumps in their markets, even in places with cold winters like Wisconsin. Ideally, the discharge line should be around 100 degrees above the outdoor temperature in heat mode. Although this rule of thumb appears to work in many different climates, it is only really applicable on single-stage equipment.

When charging heat pumps from scratch, check the manufacturer data in heat mode. Airflow for comfort or efficiency is something else to account for when you're commissioning a heat pump; the CFM should be higher if you want the system to be efficient, but the building will be more comfortable if you have a lower CFM per ton. Airflow is especially important to control in heat mode, as small changes can noticeably affect head pressure.

When it comes to defrost, heat pumps typically use a time and temperature strategy. Defrost cycles usually run at a certain temperature for a fixed time period. Heat pump defrost boards usually look a lot more complicated than they really are; when you come across them, stay calm and remember that they're just like any other board.

We also discuss:

• Absolute zero
• Climate zones
• "Vapor line"
• Discharge superheat vs. over ambient
• W calls
• Supplementary heat and dehumidification
• Confirming airflow on a heat pump in heat mode
• Controlling mean radiant temperature (MRT) vs. blowing hot air
• Using in-duct psychrometers and manufacturer charts to assess system performance
• How reversing valves may fail or get stuck
• Thermal imaging applications
• Copeland compressors and mobile app
• Testing defrost boards
• Carrier vs. Trane & Rheem defrost strategies
• Demand defrost
• Suction pressure and compression ratio under frost buildup

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