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Heating Mode Refrigeration System Evaluation

When confirming that a heat pump is performing properly in the heating mode, there a two factors to keep in mind regarding the process of evaluating the refrigeration system.

First, as with all equipment, it’s important to use any specific service information that is available from the manufacturer to evaluate heating cycle performance of the refrigeration system. And second, checking the operation of the equipment is warranted even if the system seemed to be operating normally in the cooling mode. The fundamental reason behind this is that the indoor coil of a heat pump is not as large as the outdoor coil, which means that small variations of air flow or refrigerant charge that may not have noticeably affected system performance in the cooling mode, may have a more dramatic effect when the indoor coil is acting as a condenser in the winter operation.

Accomplishing a system evaluation requires measuring the outdoor ambient temperature, refrigeration system pressure, and the indoor air dry bulb temperature. In our first example, the chart in Figure One is used to check the discharge pressure of the equipment.

                                  Figure One

In this example, which is an R-410A system, the outdoor ambient we’ve selected is 35° F on the outdoor ambient scale at the bottom of the chart. The three indoor dry bulb curves shown represent 80, 70 and 60 degrees, top to bottom. In this situation we have plotted from the 35° F outdoor temperature to the 70° indoor air curve on the chart.

When we read to the left to determine the recommended discharge pressure of the equipment, we find it to be at 347 PSIG. And, when we apply the manufacturer’s acceptable deviation of ± 10 PSIG, we determine that our gauge pressure in this specific situation may range from 337 PSIG to 357 PSIG. If our system test showed a pressure within the acceptable range, we would consider the system as operating normally. In the event that our pressure reading was outside the acceptable range, we would know that the equipment efficiency is being affected.

The possibilities to consider here are:

1. Refrigerant level incorrect.
2. Air flow not correct.
3. A combination of the two factors above if previous service was not accomplished properly.

In addition to checking the discharge pressure while the equipment is in the heating mode, a test of the suction pressure can also be accomplished with a chart such as the one shown in Figure Two.

                                     Figure Two

In this example we’re employing the same factors relative to outdoor ambient temperature and indoor air entering curve selection, to determine the recommended suction pressure, which is 84 PSIG.

In the case of suction pressure acceptable deviation, a manufacturer’s recommendation is commonly ± 3 PSIG, which would mean that in this case, a suction pressure reading between 81 and 87 PSIG would indicate that the equipment is operating properly.


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