When making air conditioning recommendations to homeowners, having a firm grasp on the best options available and communicating which system will best suit their specific needs is essential. What is most energy-efficient? Which systems operate at optimal speeds? What will deliver ideal home comfort levels? These are all questions that homeowners will be seeking answers to when shopping for a new system.

While many homes still use the standard single-stage air conditioner, two-stage and variable speed systems provide better load matching, efficiency, and dehumidification. Learning more about their differences and benefits can help you guide customers toward making the best choice for their homes.

Two-Stage AC System

A two-stage compressor (sometimes labeled as “dual-stage”) works at two different capacities. The system varies its output as compared to a single-stage compressor, which only offers one level. Unlike single-stage, which either operates at 100% capacity or not at all, two-stage compressors can operate at two levels: high and low.

Rather than running continuously, two-stage systems either run at full capacity or kick to a lower setting – usually at 60-70% capacity – which uses significantly less energy than constantly turning on and off. As a result of the energy savings, a two-stage compressor typically has a longer lifespan and requires less repairs.

Variable Speed AC System

A variable speed system operates across a capacity range, which provides maximum energy efficiency and ideal home comfort levels. This system will make slight adjustments to its speed as needed to circulate even cooling throughout the house, resulting in precise temperatures, desired comfort levels, and effective dehumidification.

Unlike single-stage or two-stage systems, variable speed compressor systems rarely turn off. Instead, they typically operate below 100% capacity while varying speed to best maintain indoor setpoints. Since this system does not frequently power on and off, it provides substantial savings on electricity costs and generally allows for a longer lifespan.

Benefits of Each System

The two-stage air conditioning system is a good middle ground between the lower-end of single-stage and the higher upfront cost of a variable speed system. Compared to a single-stage system, it will save on energy costs while usually costing less than a variable speed system to initially install.

Variable speed systems run the most efficiently and are also quieter than most other air conditioners since they typically run at reduced speeds. These systems tend to offer the most savings on energy costs and provide better control over home temperature and humidity levels.

How to Choose the Best System

When helping homeowners come to the best decision for their home and lifestyle, find out what their long-term plans are. If they plan to stay in their home for several years, a variable speed system will save money and energy in the long run despite a higher installation cost.

On the other hand, if they are planning on staying in one place for a short amount of time, considering a two-stage system might make the most sense. Keep in mind, however, a higher-end air conditioning system could help improve the home’s resale value.

The size of the house also plays a factor in determining the best option. Newer systems like variable speed are often larger than older systems, such as single- or two-stage. It is important that there is enough space to house the equipment properly.

There are always more considerations, but they will likely be specific to the individual homeowner. In the end, it comes down to what will best fit the home, preferred lifestyle, and budget.

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Comfort Basics

Humidity is a key factor when it comes to interior comfort. Low or high RH can cause physical discomfort for the occupants and can damage products inside the building such as instruments or hardwood flooring. High humidity over time can become a health hazard, creating an environment for mold and bacteria growth, especially in levels above 50-70%. Interiors with high humidity are an ideal space for odors to develop, asthma to worsen, and condensation that can damage buildings.

Comfortable interior humidity levels vary based on location and climate. ASHRAE Standards state that the ideal ranges for comfort in the winter are 68 °F – 74 °F at 40-60% relative humidity. In the summer, this range is 73 °F – 79 °F at 40-60% relative humidity. These levels provide the best levels for comfort and for preventing unwanted microorganism growth and changes to building materials and fixtures.

Challenges with AC Sizing

As homes have become more energy efficient with better insulation and ventilation, contractors have been able to rethink AC sizing since cooling loads have shrunk over the past several years. However, although cooling loads have shrunk, latent loads (which involve moisture and humidity) are not as readily reduced. Because AC sizes have been optimized to better meet cooling loads, they may not be optimized for latent heat (humidity in the air) removal. More than ever, it is important that HVAC systems are correctly set up to remove moisture. If they are not properly installed to maximize humidity control, a possible result is cold and humid homes where the moisture in the air can develop into mold and cause structural damage.

Naturally, if the air conditioner is too small for the building, it will not be able to remove all of the humidity in the space or provide the proper cooling at times. On the other hand, when an AC unit is oversized, the system does not run as frequently due to its larger size. It quickly brings down the temperature, but the humidity itself is not removed in such a short time.

Now, it is more important than ever to perform a load calculation and determine the heat loss and gain of every building where an HVAC system is going to be replaced.

Fan Speed Adjustments

To ensure that the building’s HVAC system is working at maximum efficiency for controlling humidity levels, the fan speed needs to be checked and adjusted if needed. Any changes will be based on the size of the AC paired with the furnace and ductwork capabilities to make sure that the setups and sizes match and run together efficiently.

Lowering the fan speed is one way to control a large amount of interior humidity. A colder evaporator coil can remove more moisture from the air, but it is important that the evaporator coil doesn’t freeze. This would mean that it could no longer absorb sufficient heat and would block air from blowing across the evaporator coil; a condition like this can damage the compressor. Make sure to first understand what the thermostat, blower board, and motor are capable of and are already doing before making adjustments. Another precaution to keep in mind is that if the fan speed is overly low and is combined with an Electronic Air Cleaner, ozone may be produced, causing harm to the occupants’ health.

Unfortunately, when the fan remains running at a slower speed, this can lessen the overall efficiency of the system. However, when the AC works continuously at a low speed, interior humidity is reduced, and temperatures are more balanced.

Summary

In order for your HVAC system to run correctly and dehumidify the interior space efficiently, your equipment needs to be sized correctly before installation or replacement installation needs to be done properly with fan speeds set to the desired speed for a balance of efficiency, even temperatures, and moisture reduction. Equipment controls should also be set to a point that will best optimize the system.

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