Losing Efficiency

A system that contains mismatched coils isn’t something that Pedro Portillo, president and lead service technician of ACU Air Heating and Air Conditioning, Conroe, Texas, likes to see.

“A system that is properly matched will operate at its fullest efficiency, providing the customer with a more comfortable environment at a lower operating cost,” he said. “If a system is not properly matched, customers are paying more than what they are getting — their electric bills are a lot higher.”

Portillo said that he sometimes sees contractors undersizing the condenser coil in order to get more efficiency out of the system. For example, if a house requires a 5-ton system, a contractor might install a 5-ton evaporator coil and a 4-ton condensing unit.

“According to some people, you do get a little bit more efficiency out of that. The problem I have seen out in the field is that the unit runs a lot longer, because it’s not able to keep up with the load. Then it becomes a real problem because the house is too big for that 4-ton condensing unit, so the homeowner isn’t getting the efficiency he or she is looking for,” said Portillo.

“Some contractors may use an undersized condenser unit to help reduce humidity,” he added. “The problem with doing so is that the condenser, in many cases, just isn’t large enough for the heat load the house produces. The condenser may lower the humidity, but it may not sufficiently cool the house for human comfort, or the unit will have to run for a prolonged period of time to cool the home. For these reasons, I disagree with the practice of installing an undersized condenser.”

Portillo said the best way to deal with humidity is to properly size the air conditioning system for the house. “Each system should be customized for the home in which it is to be installed. In some cases, it may be necessary to install a dehumidifier to help the air conditioning system reduce the humidity in the conditioned air. Also, a system which is properly sized will run more efficiently.”

Proper maintenance will improve the system’s ability to reduce humidity in the conditioned air. “The best way I can help my customers deal with humidity in their homes is by keeping their equipment in top operating condition by providing them with the best service and maintenance for their units: keeping the units properly charged with refrigerant, keeping the condenser and the evaporator coils clean, cleaning the drain lines, and properly sealing the airstream.”

Another reason experts cite for changing both coils if one fails is that today’s indoor coils are larger, which increases their efficiency. In addition, enhanced heat transfer surfaces such as grooved tubing and enhanced fin designs are utilized more often.

“If a system contains an older evaporator coil from a lower efficiency unit, it will not have enough heat absorption (or rejection for a heat pump) capability to generate the performance rating of the new condenser coil,” said Dan Arnold, director of Product Planning and Government Affairs, York International, Unitary Products Group, Wichita, Kan.

“In addition, the pressure drop through the expansion device, distributor tubes, and circuiting will not be optimized for the outdoor unit,” he said.

Arnold also pointed out that coils that are not optimized will not remove the amount of moisture they should, therefore sacrificing comfort as well as efficiency. Arnold added that heat pumps should never be mismatched, as an undersized indoor coil may not have enough internal volume to adequately handle the system charge in heating mode.

Potential Problems

Manufacturers want their coils to match mainly because this allows them to guarantee performance. Arnold stated that for an OEM of matched systems, the highest sales volume tested combination (HSVTC) must be tested, and the performance validated with a statistically significant number of systems tested.

“While we are allowed to computer simulate combinations other than the HSVTC, we actually test most matches in order to optimize system performance by determining the refrigerant charge and fixed-orifice size or TXV setting. This provides confidence to the end user that the system meets the published performance certified by ARI,” said Arnold.

If one coil is changed and its replacement is either smaller or larger than the remaining coil, the charge and expansion device may need to be adjusted in order to keep from flooding or starving the coil. In severe cases, a lack of superheat results in floodback to the compressor, which can cause compressor failure, noted Arnold.

He added that heat pumps should definitely have matched coils, because in the reverse-cycle mode the internal volume and heat transfer characteristics are critical. “Refrigerant floodback to the compressor, high-pressure lockouts due to undersized indoor coils, and problems due to differences in check valves on the indoor coils are all potential problems.”

In this business, the customer usually has the last word. Portillo said some of them just want him to fix what’s broken and that’s it. “I do advise them that they will get a lot more efficiency by replacing the condensing unit as well if the evaporator coil has failed. That’s especially true if the system is over seven years old. But everything comes down to money, and the customer is the one who makes the last decision.”

Portillo said that people are more educated about their cooling systems, especially in regards to efficiency, so more often than not, his customers will trust his judgment and replace everything if he suggests it. “All we do is advise the customers as well as we can, so they can make an informed decision. The way I personally look at it is the more my customers know about their equipment, the better the decision they can make.”

The Consequences

For those who read the manuals, manufacturers often place warnings in their literature that the company cannot be responsible for the performance of a mismatched system and that all combinations must appear in the ARI Directory of Certified Air Conditioners and Heat Pumps.

If that’s not enough to scare the installer into using a matched system, Arnold warns, “If a system is sold that will result in an SEER below the federal minimum of 10, then if it’s not an actual violation of the law, it is certainly against the spirit of the federal minimum energy efficiency standards passed by Congress in order to save energy for the benefit of the nation and the consumer.”

So the next time you’re tempted to replace one coil and not the other, or you want to undersize or oversize one coil, the consensus is: don’t do it. If one coil has failed, it’s probably only a matter of time before the other coil fails too. Then you will have to come back, open up the system, and charge the customer for additional labor and refrigerant recovery they already paid for the first time. That’s not going to make a good impression.

If the contractor’s goal is to leave the customer with a cooling system that is operating at peak performance, Portillo and Arnold suggest that the system’s condenser and evaporator coils should be matched perfectly.

Publication date: 02/24/2003