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This installation was on a 4-story home, and the tech had not yet gone to the basement to troubleshoot the “infinity” water source heat pump, as he was told he by the homeowner. He was in the attic at the infinity furnace above the 4th floor. He was getting his dcv in the proper range on his A and B green plug on the air handler, 3 to 4 vdc. His indoor board ohm reading was in range, 28k to 32 k. The tech ohmed out the A and B thermostat wires on the water source heat pump from the upstairs. His ohm reading was not in range of a normal infinity board. His reading was around 64.4k. The tech then made the long walk to the basement where the “geothermal heat pump” was located. Bingo.

Always keep in mind that the voltage to compressor terminals or motor connections during operation must be within voltage range indicated on unit nameplate. On three-phase units, voltages between phases must be balanced within 2%, and the current within 10%. Use the formulas above to determine the percent of voltage or current imbalance if there have been motor or compressor failures in a unit.

The voltage imbalance formula is in most three-phase installation instructions.

Sources:

48HC07-12 Installation Instructions

48TJ004-014 Product Data

HVAC SERVICING PROCEDURES

Compressors have evolved over the years from single stage to staged compressors. Staging the capacity of a compressor helps with energy cost, humidity control, and comfort. How is this accomplished in these units and how can you tell if it is switching and functioning?

The design of a basic scroll compressor has been changed with the addition of an internal unloading solenoid that opens a bypass port in the first compression section of the scroll compressor. The opening of this bypass port effectively reduces the displacement of the scroll compressor. The single speed, high efficiency motor continues to run while the scroll modulates between the two stages. Modulation is achieved by venting a portion of the gas in the first suction pocket back to the low side of the compressor, which reduces the effective displacement of the compressor. This unloading of the compressor reduces its capacity to approximately 67% which translates to approximately 75% cooling or heating capacity at the indoor coil.

To achieve 100% full capacity, the vents that were open to reduce capacity are closed. This is achieved by a DC solenoid inside the compressor. A 24-volt AC signal sent to the rectifier and converted to a 24-volt DC signal to operate the DC solenoid inside the compressor moves the bypass ports that allow the vents to be covered and uncovered. The loading and unloading of the two step scroll is done without shutting off the compressor between first and second stage operations. During startup of the compressor, even if full load is needed, it will always start unloaded, and after five seconds it will switch to the 100% full capacity.

While the unit is operating, your suction pressure will change as it switches from low to high capacity. The compressor current will also increase as it switches from low to high stage. When there is no 24-volt DC signal, the solenoid coil is de-energized, and the compressor is in low stage. When the compressor is operating in high stage, there is a 24-volt DC signal that energies the DC solenoid.

To test the operations of the loading and unloading of the compressor, there are certain procedures you can do. First, measure the compressor amperage while the unit is running. Cycle the unloader on and off from low stage to high stage, and then back to low stage in thirty second intervals. You should see at least a twenty percent increase or decrease in compressor amperage.

Second, you can remove the solenoid plug from the compressor, turn the unit on and make sure it is in high stage. Then, with a voltmeter, set it to the DC scale. The reading should be 24-volts DC coming out of the rectifier.

The third procedure is to measure the resistance of the compressor unloader coil. This resistance should be approximately 330 to 1640 ohms. If the coil is open or shorted, the compressor will have to be replaced.

This staging is not limited to these units. It is used in residential and commercial units.

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Source: Two Stage Heat Pump Installation Manual