Jan. 23, 2006, was an historic day in the HVAC industry, for it was then that manufacturers had to comply with the Department of Energy's mandate, which states that from that day forward, all air conditioners and heat pumps that are manufactured must meet a minimum efficiency standard of 13 SEER.

Now that the deadline has come and gone, manufacturers are focusing on their new 13 SEER product lines and bracing themselves for the inevitable spate of questions regarding the new systems. Besides pricing and component size, the most commonly asked question of manufacturers is: Should the indoor coil always be changed when a new 13 SEER condensing unit is installed?

The answer is, it depends.

Rheem tells contractors that in most cases the existing coil should be replaced, as it’s very unlikely that it can be properly matched with a new 13 SEER outdoor unit. If a system contains mismatched components, it cannot operate as designed by the manufacturer and as required by federal law. (Feature photographs courtesy of Rheem.)


Considering that approximately 67 percent of the compressor-bearing units installed up until Jan. 23, 2006 were rated at 10 SEER, it is likely that contractors will face the mismatched question frequently. In most cases, manufacturers believe that the existing indoor coil should be replaced if a new condensing unit is being installed. As Tim Hawkins, engineering manager, residential air conditioning equipment, Rheem Air Conditioning Division, Fort Smith, Ark., stated, it is very unlikely that an existing coil is properly matched with a new 13 SEER outdoor unit.

"The only circumstance when a coil replacement is not necessary is when the indoor coil already has, or at one time had, an ARI-approved rating with the new outdoor unit. Rheem will usually recommend changing both the indoor coil and outdoor unit," said Hawkins. "A system that contains mismatched components cannot operate as efficiently as designed by the manufacturer and as required by federal law. A properly matched system operates more efficiently, delivers maximum customer comfort benefits, and minimizes energy use and utility bills."

Marc DeLaurent, product manager - split systems, Nordyne, O'Fallon, Mo., agreed that if the existing system is less than 13 SEER, contractors should always change the indoor coil at the time of installation or else the end user will not achieve a true 13 SEER rating.

"The only exception is if the contractor were replacing an existing 13 SEER condenser and the replacement unit is the same efficiency, type, and brand," said DeLaurent. "Our company has a list of ARI-certified matches that are necessary for the system to work properly. We also recommend on our technical specifications that contractors see the current ARI directory for certified combinations and ratings."

Tim Lashar, Luxaire brand manager and outdoor products marketing manager, Unitary Products Group of York - a Johnson Controls Company, Norman, Okla., added that the indoor coil should usually be replaced. "Most older evaporator coils are not designed for a 13 SEER condenser, and the energy efficiency of a 13 SEER system requires a larger condensing unit and indoor coil."

Lashar noted that when the 10 SEER standard was introduced, improved compressor technology alone accounted for a large part of the higher SEER. The same is true with 13 SEER equipment, in that these products contain advanced compressor technology, however, much of the efficiency increase comes from more coil surface area.

"The physical size of these 13 SEER outdoor units can be 50 to 90 percent larger than today's 10 SEER models. This means a 13 SEER outdoor unit will require up to 40 percent more refrigerant [R-22 or R-410A] than most of today's indoor coils are capable of holding," said Lashar. "Indoor coils have to be larger to hold the refrigerant charge necessary for peak performance. If the indoor coil is not replaced, it can become flooded, resulting in premature compressor failure."

Mike Miles, product manager - cooling, Lennox, Richardson, Texas, noted that if contractors don't change out the indoor coil, it is incumbent upon them to make sure the existing coil is going to work and perform with the newly installed condensing unit. "However, it's always our recommendation to change out the indoor coil with the outdoor unit."


While it's good practice to change out the indoor coil in a straight air conditioning system, it is mandatory in heat pump systems. "The indoor coil in a heat pump system always has to be changed if it is not a matched coil because of the coil volume ratio," explained Miles. "It's necessary to make sure the volume of refrigerant that the indoor coil handles matches what the outdoor coil can handle."

Miles stated that if the indoor coil is too small, then the heat pump could kick out on head pressure because it can't pump enough refrigerant through the system. In addition, it could get very hot, which could possibly trigger a high-pressure shut-off. On the other hand, if the indoor coil is too big, then the unit won't heat properly.

"Also, if the balance of the refrigerant isn't matched correctly and the volume ratio isn't right, it may be necessary to take out or add refrigerant to the system. Basically, if the coil volume ratio is not matched correctly, then it could cause a lot of issues," said Miles.

Hawkins agreed, noting that for a heat pump, the indoor coil must always be changed out to ensure the proper charge balance between the heating and cooling modes. "In some cases, a different quantity of refrigerant is needed during the cooling mode or heating mode. The wrong coil could result in too much refrigerant in the system during either the heating cycle or cooling cycle. Also, the UL certification is voided if the indoor coil is not changed to one matched to the outdoor unit."

In some cases, it may also be necessary to change the line set, so contractors should refer to the manufacturer's installation and operating instructions for recommended line sizing for particular models.

As DeLaurent noted, "The existing line set may be used but a loss in capacity and efficiency may occur. Most larger tonnage systems will require a larger line set. Also, if the system being replaced used R-22 refrigerant and the new system uses R-410A, we'd recommend that the line set be changed to ensure there are no contaminants such as incompatible refrigerant lubricants in the line."

If for some reason it’s not feasible to change out the indoor coil, it is possible that a coil equipped with a thermal expansion valve may be acceptable, but Rheem does not recommend it. That’s because even with retrofitting the expansion valve, there is no guarantee that the modified system is reaching it’s designed 13 SEER ratings and the capacity may be less than the manufacturer’s design specifications.


Rick Roetken, director of marketing, Carrier Corp., Indianapolis, stated that when determining whether or not to change out an indoor coil, consideration must be given to the existing coil's surface area and metering device. "We require that the indoor coil be newer than 1992 to avoid complications from capillary tube metering devices as well as for the potential of the coil surface area to be undersized for the needs of the 13 SEER products."

Roetken added that as a matter of good practice, consumers deserve a new coil with their new condensing units, due to the many problems that can arise if an existing coil is left in place. Those problems could occur in the following areas:

  • Surface area. If the coil is older than 1992, the coil surface area will be inadequate. It will substantially reduce efficiency while increasing the rate of failure for compressors. These coils should always be replaced.

  • Metering device. Capillary tubes should not be used, as these metering devices create efficiency and reliability concerns. A thermal expansion valve (TXV) is recommended with a 13 SEER system due to additional refrigerant charge and to achieve higher efficiency ratings.

  • Status of the coil. If the coil is dirty or it is an older coil, both efficiency and reliability can be affected by reducing airflow. Additional concerns may be the condition of the drain pan and coil construction.

    DeLaurent added that simply changing the outdoor condenser will result in a system that does not meet the federal government's mandated 13 SEER minimum efficiency. "A frozen coil is one of the possible outcomes of this type of configuration, and higher head pressures may exist, resulting in potential nuisance trips as well as causing unnecessary wear and tear on the compressor."

    Hawkins agreed, noting that the superheat and subcooling may not be correct with fixed restrictor coils. "That condition impacts the performance and reliability of the system. Lack of superheat can cause liquid floodback to the compressor, causing the compressor to fail. Too much superheat can result in an overheated compressor that will trip on its protector and eventually fail."

    All these potential problems have led manufacturers to look long and hard at their warranties. Lashar noted that according to compressor manufacturers, an improperly matched system gives compressors just a 50/50 chance of making it through the warranty period. This is why a mismatched system could possibly void a warranty.

    "Equipment manufacturers assume great risk when offering warranties, and they must ensure that systems are being installed in a manner that allows them to stand behind the product they offer," said Lashar. "There will likely be a higher level of accountability required of dealers and distributors when filing a warranty claim. If a circumstance arises where an old evaporator coil is left on a 13 SEER system installation, the original equipment manufacturer may disallow the claim, especially if a TXV is not used."

    Nordyne stated that while the company offers warranties on individual components, a mismatched system would void its Quality Pledge and Dependability Promise programs. According to the provisions of these programs, the company will replace a condensing unit during the first five years of operation should the compressor ever fail. However, the new 13 SEER ruling presented a twist to that program in that after Jan. 23 there won't be any equivalent 10 or 12 SEER systems for replacement, so Nordyne modified the policy.

    "Our new Quality Pledge and Dependability Promise policies offer consumers either a new 13 SEER outdoor unit as part of a matched system or a complete compressor changeout. Either way it ensures our customers receive the full efficiency and comfort benefits of a properly matched system," said DeLaurent.

    Roetken noted that Carrier's warranty is not voided if the product is applied correctly, while Miles stated that the Lennox warranty is tied to the condensing unit, and as such, a matched system is not a requirement.

    Given all the new pricing, sizes, warranties, etc., there's a lot to learn in this new era of 13 SEER equipment. All contractors would do well to take the time to familiarize themselves with manufacturer guidelines to ensure they are installing the correct system each and every time.

    Publication date: 02/27/2006