We show a Carrier chart for checking the charge in heat mode; manufacturer information will always be one of your most helpful tools when checking the charge in heat mode. In this case, we check the indoor dry-bulb and outdoor (wet-bulb preferred) temperatures to find the corresponding suction and head pressure ranges. Note that lower outdoor relative humidity also results in less defrost.
Many times, it’s best to weigh the charge and then use the manufacturer’s chart, especially if the system has a known leak or long lines. When charging from scratch, ALWAYS try to weigh in and then use the chart.
When connecting gauges, connect to the always-suction port on the low side. On quite a few systems, you can get your head pressure on the liquid line, but some systems may require you to connect to the discharge line to check the head pressure.
A common rule of thumb is the 100-degrees over ambient rule; the temperature of the discharge line on a running system should be 100-110 degrees above the outdoor air temperature. Higher temperatures may indicate an insufficient charge or a restriction. There is also an evaporator DTD rule of thumb that states that your suction saturation temperature should be 20-25 degrees below the outdoor ambient temperature. The CTOA rule of thumb states that the CTOA should be 30-40 degrees above the indoor dry-bulb temperature. You can also check the system without gauges; using non-invasive temperature clamps on the suction line takes DTD and superheat into account. Indoor delta T values will vary wildly.
Overall, the design conditions in heat mode vary a lot more than they do in cool mode, so you’ll see those wider ranges of variations reflected in your readings.
To learn more, go to https://hvacrschool.com/checking-charge-heat-pump-winter/.
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