When testing multi-head variable speed heat pumps all the PTCS specifications still apply including; equipment specifications, control specifications, airflow between 325-500 CFM/ton or manufacturers documented recommendation, and the PTCS Temperature Split chart is still used to confirm appropriate refrigerant charge in the heating mode. In addition, a single indoor centrally ducted unit as part of the system must serve at least 50 percent of the home.

 

The primary difference between non variable and multi-head variable speed systems is the unit that is used to calculate the CFM/ton. Usually, the capacity of the outdoor unit is used to calculate CFM/ton. However, for multi-head variable systems the capacity of the centrally ducted indoor unit should be used to calculate CFM/ton. During the CFM/ton calculation confirm that the indoor ducted unit is running at full capacity and that it is the only unit calling for heating or cooling. Measure airflow using the TrueFlow plate and meter. The CFM/ton will then be determined from the indoor unit capacity.

The PTCS specification requires the refrigerant charge to be tested in heating, anytime the outdoor temperature is 65 degrees or below and in cooling anytime the outdoor temperature is 65 degrees or higher. The heat pump should be operating for 15 minutes before any measurements are taken.

When testing the refrigerant charge in the cooling mode, the technician should calculate subcooling or use a manufacturer approved method, such as approach or superheat. The tested subcooling value is compared to a manufacturer-provided target value to determine if the unit has the correct amount of refrigerant to operate properly and efficiently. This is accomplished by measuring the liquid line temperature at the outdoor unit and subtracting that value from the saturation temperature that corresponds to the liquid line pressure. Using a temperature/pressure chart and a calibrated refrigerant manifold gage set is crucial in assuring the measurement is accurate. Typically, if the temperature difference is within three degrees of the target, then the unit is considered properly charged.
 

When testing the refrigerant system in the heating mode, the technician should use the PTCS Minimum Expected Temperature Split Chart to confirm that the system is delivering the expected capacity for a specific outdoor temperature. The capacity of the heat pump in heating mode varies based on outdoor temperature and airflow. The technician needs to measure the airflow, the outdoor temperature, and the return and supply temperatures. The measured temperature split (supply temperature minus return temperature) should be compared to the value found on the Minimum Expected Temperature Split Chart. If the measured temperature split does not meet program specifications, the refrigerant charge should be corrected by adding or subtracting refrigerant and then re-measure the temperature split test.

Following these procedures will help to confirm that the heat pump is properly charged and works effectively and efficiently.