Q When you install an a/c 20- to 40-foot line set with a thermal expansion valve, you use subcooling to check the charge. If you use a cap system, you use superheat to check the charge. If you run a 100-foot line set, what would you do differently or what new things would you use to check the charge?

A It is true that charging is properly done by subcooling for systems with expansion valves and by superheat for fixed-orifice systems. Manufacturers usually will provide a charging chart and/or subcooling requirements.

A Carrier residential unit typically will be factory charged for 15 feet of interconnecting tubing, and .60 ounces of refrigerant should be added per foot of additional tubing length. This information may vary by manufacturer. Always verify the system operation with
the proper charging process to prevent overcharge.

Remember that long lines may take longer to show temperature and pressure changes. Adding or removing charge must be done slowly to allow time for the system to stabilize. Another caution is to consider that the load may be high while the charging process is being achieved providing plenty of superheat. Make sure that the superheat is correct at low load before leaving.

Manufacturers should be consulted for specific long-line application guidelines. There may be considerable differences in recommendations specific to equipment application and design.

For example, Carrier residential air conditioners and heat pumps (nominal capacity of 18-60k Btu) will consider anything more than 50 feet a long-line application or when a 20-foot differential exists in elevation between the indoor and outdoor coil.

Long-line guidelines may include tables for capacity losses dependent on line diameter; metering device selection; recommended use of crankcase heaters; start assist recommendations; and possibly liquid-line solenoid.

For example, a metering device may be required to be a thermal expansion valve or an increased size in piston sizing. This is not a general question and is dependent on the system characteristics and manufacturing requirements.

Question from Charles Desnick, Minneapolis . Answer by Sherri R. Wilkerson, CM, product support engineer, North America residential engineering, Carrier Corp.