When connecting to a heat pump in heat mode, you will connect your low side to the common suction port. That port is located between the reversing valve and the compressor on the suction line, which will always be the suction line regardless of operating mode. The line between the air handler and the reversing valve is referred to as the “vapor line” regardless of operating mode; it can serve as either the suction line or discharge line.

You also can’t leave your high side connected to the expansion line in the heat mode on some systems due to the placement of the metering device.

Start your heat pump jobs by looking at the entire system, starting with a wide scope and narrowing your focus as you inspect the system. If there’s any frost on the outdoor coil, make sure you get it off the coil before taking any measurements. You can get rid of the frost by running the system in cooling mode, as the system will take its indoor heat to the frozen outdoor coil.

While you want to avoid using rules of thumb whenever possible, you can use heating check charts if the manufacturer has them. You can use heat mode’s low-ambient heating check guidelines when the temperature is below 65 degrees outside.

When it’s cold outside, your suction pressure drops, so you could mistake a system for being flat when it’s actually fine if you check it in heat mode while it’s cold outside. The suction pressure typically stays about the same as the outdoor ambient temperature, which is more of a fluke than anything else, but it’s still a decent rule of thumb.

Another rule of thumb is that the condensing temperature should be 30-40 degrees above the indoor dry-bulb temperature. When the outdoor temperature drops, the suction drops. When the suction pressure drops but the indoor temperature stays the same, the condensing temperature may change only a little bit, and the head pressure will stay close to the same. That’s where we can get higher compression ratios and reduced capacity.

Where possible, it is a good idea to test heat pumps without gauges in heat mode. However, you need the outdoor temperature to be between 65 and 15 degrees Fahrenheit and a frost-free outdoor coil. The suction line should be 5-15 degrees cooler than the outdoor temperature, the liquid line should be 3-15 degrees above the indoor temperature, and delta T indoors can vary wildly.

Using the FieldPiece charging jacket, the techs waited until the difference between the high and low side pressures was between 160 and 220 for the R-410A Lennox system. Then, they checked the target subcooling.

The heat pump needs to defrost itself from time to time because the evaporator coil can freeze when the temperatures get below 30 degrees Fahrenheit; the heat pump can’t absorb heat from the outdoors when the outdoor coil freezes over. The heat pump can give off steam in defrost, which looks like smoke and can alarm the customer. Heat pumps can also scare homeowners because they make weird noises in defrost.

The defrost board can usually detect when the coil is ice-bound. The two main defrost strategies are demand defrost and time/temperature defrost. Demand defrost looks at air and coil temperature to find trends that surround ice-bound conditions. The time/temperature defrost uses a temperature sensor and time to determine when to go into defrost.

During defrost, the system shifts itself into cooling mode, shuts off the condenser fan, and turns on the heat strips. Electric heat strips are quite inefficient, but it can be difficult to compare the economic benefits between electric and gas heat because of the changing cost of gas.

Bryan also discusses:

• Reaching the limit on the manifold gauge
• Metering device pressure drops
• Switching to cooling mode for testing
• Lennox’s recommended procedures with the FieldPiece charging jacket
• 100 degree over ambient discharge rule
• Head pressure, CTOA, DTD, and coil size
• Running lower airflow in heat mode
• Compressor noise
• Old defrost detection methods
• Jumpering out defrost thermostats
• Inverter-driven and ductless technology
• Climate zones and complex comfort solutions
• Gas furnaces in Florida
• Fuel oil furnace drawbacks
• Burning off heat strips during fall PMs