In last month's Tech Tip, we discussed some very critical "best practices" that perhaps should be considered the baseline for a proper installation - those that won't result in a callback or worse, premature system death. Our Tech Tip looked at the continued importance of coil matching and airflow at the evaporator.

This month we're going to discuss the refrigerant charge and airflow as it's related to the duct system.


Most central air conditioners are running with an incorrect refrigerant charge. How about the one you just installed? How can you make sure during the installation?

The line set. Most of today's systems come with a precharged line set. When you're replacing an R-22 system with an R-140A system, you also need to replace the line set.

A typical condenser comes with enough refrigerant charge for a 25-foot line set and matched evaporator coil. However, even if the installation is standard, you may need to adjust the charge.

Check the superheat and subcooling. Superheat is measured to make sure liquid refrigerant won't enter the compressor. Subcooling is measured to make sure the expansion device has a solid column of liquid fed, so the metering device will be able to control the load at its peak efficiency. With a TXV, the subcooling will follow a manufacturer's curve based on a given set of operating conditions.

Cleanliness counts. Many 13 SEER and higher systems use R-410A, which is less forgiving when it comes to contamination - due in part to the POE oils used with 410A. POE oils have a much greater affinity for water, they are hygroscopic, so you can't leave the system open.

You also need to use a proper refrigerant line size and proper brazing techniques so that condensation cannot get into the POE oil. If a tech uses sloppy techniques on 410A systems, he will get condensation in the system, callbacks, and may have to go through several filter-driers. Those kinds of callbacks can eat up the profits from the job.

Duct conditions: If you're installing a new, high-efficiency system on a home with poor-quality ductwork, the homeowner will not, cannot, get the efficiency you may have promised. And that could be the least of your problems.

First, make sure there's enough airflow in the system. As we mentioned previously, the airflow at the equipment must be correct across the evaporator coil - airflow should be set at a nominal 400 cfm/ton unless otherwise specified by the manufacturer. Poor ductwork can result in insufficient airflow across the evaporator

Disconnected or otherwise damaged ductwork can draw in unconditioned air, raising the sensible and latent (moisture) load on the system. The system will need to work harder and run longer to handle the sensible load, and again, the homeowner won't see the savings he or she expects. If the latent load gets too high, moisture may become re-entrained in the airstream, leading to a severe moisture problem in the home.

Do your best to explain to the customer the importance of well-sealed ductwork. Point out areas of damage, if possible. Explain that fixing the ductwork could well solve the customer's comfort problems, or at least reduce the size of the air conditioner they need.

For more information, click on the Emerson Climate Technologies logo above.