Contractors often lament the fact that a customer who is already getting cool air is a hard sell for maintenance and service work. What these customers may not realize is that this “If it ain’t broke, don’t fix it” attitude can be expensive.

The performance of air conditioners is sensitive to service-related issues. Degradation faults, such as fouled heat exchangers, commonly make air conditioners run inefficiently and lead to premature equipment failure. However, if the unit provides cool air, the customer may not appreciate the need for service.

Imagine a unit that produces cool air but, based on the measurements, is not operating well. How can you get the customer’s confidence that servicing the unit, even when they are happy with the cool air, is in their best interest?

Our suggestion: Define the goals of servicing a unit. The reasoning behind each goal can be discussed with the customer when justifying needed services.

There are three reasons for servicing an air conditioning unit:

1. To maintain or restore reliable comfort;

2. To protect the unit from expensive premature failure; and

3. To save energy.


Of the three listed goals, comfort may be the only one the customer is aware of. The unit has to provide enough capacity to cool the indoor air to the desired temperature and remove humidity. The evaporator has to be cold enough with the proper airflow to provide sufficient cooling and dehumidification capacity.

Ideally, you may like to provide the customer with a measure of the current capacity and compare it with the ideal capacity under the current load. A capacity index (CI) may be defined as:

CI = (Current capacity)/(Ideal capacity at current operating conditions)

Industry standard capacity measurements are complex and expensive, but capacity can be estimated from more easily made measurements. However, calculating capacity manually is time-consuming and technically difficult.


Some customers are more aware than others of the benefits of catching problems early. Unless they are particularly stubborn, most can be taught.

If you detect signs of liquid refrigerant entering the compressor, explain that there should be no liquid refrigerant entering a compressor.

The refrigerant washes lubricant away and breaks valves and pistons as the compressor tries to compress a liquid when excessive liquid causes slugging.

The compressor should not operate at high temperatures, since it damages the varnish insulating the motor windings, causing compressor burnout. Thus, the suction line superheat must be kept within limits to prevent these problems from occurring. Also, the evaporator coil should be warm enough to avoid water freezing on it.

Breuker and Braun (“Common Faults and Their Impacts for Roof-top Air Conditioners,” ASHRAE International Journal of HVAC&R Research, vol. 4, no. 3 [July 1998] pages 303-318) analyzed a database of more than 6,000 cases from a service company that focuses on rooftop units. Table 1 shows the percentage of service costs associated with different parts of the system.

Note that replacing compressors is the largest cost when fixing faults. Many compressors fail because units are not serviced or maintained well enough.


The owners of rooftop units are almost always in commercial facilities. They are probably aware, to some degree, that system efficiency affects their bottom line. Make sure they are aware of potential efficiency problems.

The system should be serviced to achieve the design capacity without excessive energy consumption. Although many aspects of the air conditioning operation affect power consumption, the system should operate with the minimum allowable pressure difference and with a liquid-vapor mixture in most of the heat exchangers.

The efficiency of air conditioners is traditionally described by a coefficient of performance (COP) or an energy efficiency ratio (EER). A measure of the EER under the current load would be useful, particularly if the highest possible EER at the current load were used as a benchmark. An efficiency index (EI) is:

EI = (Current EER)/(Ideal EER at current operating conditions)


While discomfort is understood by the customer, system protection and energy consumption may not be fully appreciated.

Cars may run without an oil change for a long period of time; however, it may increase the risk of destroying an engine. Car owners have been well educated that they should change the oil every three months or 3,000 miles. However, no similar campaign exists for air conditioning maintenance.

Show your customer data that clearly and simply reports the operating condition of the unit. It will help the customer see that a small service expenditure will help avoid an expensive compressor replacement.

The customer is aware of energy costs through the utility bills, but probably does not know how much energy the rooftop unit should consume. Therefore, as long as the unit is cooling, a customer has a difficult time making proactive decisions.

In both cases, you need to communicate the following to the customer:

  • Consistent data collection and documentation;

  • Accurate diagnostics with understandable explanations;

  • Capacity and efficiency estimates; and

  • Potential energy savings from servicing the unit to meet the goal.

    Table 2 shows the scenario for two units, each with two stages, under the following assumptions:

  • Estimated percentage of power used for purposes other than compressing the refrigerant (e.g., fans and controls) is nominally 20%;

  • 2,000 hours of annual runtime for stage 1 and 500 hours for stage 2;

  • Nominal EER is 10;

  • Electricity price is $0.10 per kWh.

    After servicing the unit, the contractor can provide a before-and-after data report, highlighting the improved performance and clearing the faults of the unit. Table 3 shows a before-and-after report for the units shown in Table 2, but with the actual performances after service. The units are not fault free, since only cleaning condensers and adjusting charge and airflow was approved.

    The facility studied has 14 units, 11 with two stages. For the 25 stages of cooling, the estimated energy savings are about $21,000; the services performed by the contractor for cleaning the condensers and adjusting the charge and airflow cost $4,300.

    Note: This site was one of the worst from the 100 facilities considered. An important benefit from the present approach is the ability to compare sites and units, and service only the ones with poor performance.

    In summary, by showing that the current operating conditions could cause a premature compressor failure or that the energy savings in dollars from servicing the unit are much higher than the service costs, the customer may be more inclined to approve proper service work.

    This can be accomplished with an accurate and consistent data collection system and reports that effectively communicate the cost and benefit to customers. That part is up to you.

    The authors are engineers with Field Diagnostic Services, Inc., Langhorne, PA. They are the developers of the Honeywell HVAC Service Assistant, a data collection and reporting tool that provides fault detection and diagnostics in the field. For further information, contact the company at 215-741-4959; (e-mail); (website).

    Publication date: 06/24/2002