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Prior to 2006, most split system compressor failures resulted in a new outdoor unit rather
than a compressor replacement. This practice made good economical sense for the contractor
and the homeowner. The contractor received a higher margin on the replacement, and the
homeowner received a new, five year compressor warranty and half of a new system. This was
done for a price just slightly more than a compressor replacement alone. Efficiency did not suffer
because the 10 Seasonal Energy Efficiency Ratio (SEER) outdoor unit was replaced with a
comparable indoor unit. Fast forward to the spring of 2007; 10 SEER split equipment has all but
vanished from the warehouses. These warehouses are now full of 13 SEER and above.

A potential train wreck begins with the following questions:
• Can 13 SEER outdoor units be installed on 10 SEER indoor units successfully?
• Can 13 SEER outdoor unit operate “correctly” with a 10 SEER indoor coil?
• Will the result of mismatching 13 and 10 SEER units be ARI (Air Conditioning and Refrigeration Institute) rated? The answers to these questions are varied and vague. Most information available online seems borne of desperation and frustration. What seems to be the answer for one case is not the answer for another. In summary, reliable industry sources are not providing any clear-cut answers.

The contractor, homeowner, wholesaler and manufacturer will eventually suffer if proper
guidelines are not established and followed. The facts of efficiencies One of the primary principles of increasing efficiency is to make the coil surface area larger and to reduce the compression ratio. Higher efficiencies mean larger coils. It is reasonable to understand that a 13 SEER coil is going to be larger
in volume than a 10 SEER coil of equivalent capacity.

With a coil-volume increase there is a refrigerant volume increase. Here is where the problem begins. A 13 SEER outdoor unit usually contains more liquid refrigerant volume in the cooling mode than a 10 SEER unit. This liquid then must be metered into a coil that, in most cases is smaller in volume than a 13 SEER matching coil. The increase in the SEER of the outdoor unit likely will result in lower compression ratios, thus reducing the pressure drop across a fixed orifice-as shown in Test 2-with a factory charge. Raising the capacity requires adding more than six additional pounds of R-22-as Test 3 shows.

Both of these conditions will result in a rated capacity loss in cooling, higher-than-normal discharge pressures in heating, reduced efficiency and decreased indoor comfort. Also, the increased amount of liquid in the indoor coil will eventually lead to compressor damage and failure to liquid flood-back.
In the ever evolving HVACR industry, the ability and the integrity to do the right thing for the consumer must always be the cornerstone of our actions Bristol Compressors Inc. has performed laboratory testing on mismatched equipment. The unit size tested was a three-ton heat pump using R-22. As is evident in, Test 4 shows that installing a thermal expansion valve (TXV) on the indoor unit was effective at increasing the SEER rating and capacity in the cooling mode, but still fell short of the 13 SEER and 36,000 Btuh. This test also indicated that the heating mode opened the high-pressure safety at 410 psig and could not be tested with the amount of refrigerant in the system. The system charge would have had to have been adjusted to make the unit operate in the heat mode. The results of these tests answered whether or not it is possible to mismatch equipment and make it operational.

They also revealed how the systems would operate in such circumstances. It seems that most of these mismatches would be a constant problem to the technician and the consumer. Manufacturers also saw a sharp increase in compressor failures and service warranty claims due to the problems encountered with these conflicting systems.

ARI’s Web site ( featured a Directory on Certified Product Performance that allows
users to enter the model numbers of various units-indoor and outdoor-to search the ARI directory for ratings. While researching this topic, model numbers of 10 SEER indoor fan coils and 13 SEER outdoor units were inserted. Obviously, there was no ARI match rating of any of the combinations used. To indicate to a homeowner that changing only the outdoor unit will give 13 SEER is misleading. Only ARI matches are recognized when it comes to actual efficiency ratings.

The results are in
The results show that mismatching 13 and 10 SEER units would result in up to a 40% reduction in efficiency and cooling capacity, along with a need to adjust the refrigerant charge at least twice a year. Adding a TXV is not necessarily the answer to make the unit operate correctly in both its heating and cooling modes, nor would it make the unit a 13 SEER ARI match. It seems that the correct repair to make would be to upgrade both the outdoor and indoor units to an approved matched system. This would result in optimum system performance, high efficiency and, in the long run, customer satisfaction.

In the ever-evolving HVACR industry, the ability and the integrity to do the right thing for the consumer must always be the cornerstone of our actions. It is especially important to gain education and information as products improve and the market increase. History has always proved that doing it right the first time benefits everyone.