I overcharged and undercharged the system by 10%-15% to illustrate the effect on system operating numbers. The result of over/under charge is dependent on the metering device. Since TXVs maintain a constant superheat, overcharge won’t typically affect superheat or suction pressure, which the video confirms. However, if the metering is fixed (piston), you could expect the superheat to decrease and suction pressure to increase if the head pressure increases, due to the increased pressure differential across the piston.
Oftentimes in the field, you see low superheat due to someone overcharging a system to raise the evaporator temperature (pressure) as a stopgap solution for low airflow.
In this particular test, there was a small increase in head pressure (8 psi), significant increase in subcooling (8˚) and virtually no change in suction pressure or superheat.
The undercharge numbers showed decreases in head pressure, suction pressure and subcooling, with an increase in superheat. If the metering was piston, the superheat would probably have gone higher, with the reduced pressure differential across the piston but a TXV will open up in an attempt to maintain the superheat setting. And if a system has a sight glass, a sufficiently low charge will result in bubbles in the liquid line.
Wayne
HeatPumpClinic.viewmonster.com
