There are 3 basic styles of operating features with forced air heating and air conditioning systems. These features themselves do not save money but rather improve indoor comfort. Typically the systems with the added features are more efficient but the efficiency is only part of the picture. Most of these features boil down to 3 main benefits as you go up the scale. More even air temperatures, lower humidity, and quieter operation. I will try to do my best to describe them by making a comparison to a car.
SINGLE STAGE– These are the most common type of system. The system comes on full blast with enough power to cool the home at 100 degrees or heat the home at 0 degrees whether it is needed or not. It is like having a car that is either stopped or doing 100 with no in between. While they will do an effective job at maintaining the temperature in the house they do not run very long cycles so they turn off before the air between rooms is mixed and dehumidified effectively. If these systems are paired with zoning you have to “bypass” or recirculate the excess air if only one zone is calling which hurts efficiency. The 95% efficient furnaces and XR16 outdoor units are in this type of system are at or near energy star level.
TWO STAGE– Two stage systems have a low speed the unit can run at on a mild day with the full capacity available when needed for the extreme days. It is like having a car that will either drive at 70 or 100 MPH. With the ability to run at a lower capacity these systems will run longer which helps to get more air mixture between rooms and gives the system a chance to do more dehumidification. The longer run times do not cost more they actually cost less as the system is using less electricity or gas on the lower setting and they cycle off and on less often which is when the systems draw the most energy at startup. Starting and stopping is also what wears out motors. When paired with zoning two-stage systems reduce the need for bypassing air as they will run at the lower speed if only one zone is calling. The 95-97% efficiency on the furnaces and 16 to 18 SEER on the outdoor units are at or near energy star level and typically use 8 to 12% less electricity for air conditioning when compared to single-stage units. With a full two-stage system (indoor and outdoor) the furnace will have a variable speed fan motor that uses less electricity and ensures proper airflow through the duct system as it has the ability to ramp up to overcome restrictions in the duct system and deliver air to harder to reach rooms.
MODULATING/VARIABLE SPEED- Variable speed systems have the ability to ramp up or down depending on demand from 30% to 100% on the outdoor units and 40 to 100% on the furnaces. This is the same technology that made ductless and VRF systems all the rage for efficiency in a ducted unit so you don’t have the boxes hanging on the wall. They operate more like we are used to in our vehicles, set the cruise at 70 and it will accelerate when going uphill or let off the gas as it goes down. Another way to describe it would be that when it is 100 outside it will run full tilt like any other unit but as the day cools off instead of shutting off it simply slows down, again using less energy at the lower rate. As they slow down the noise is reduced and with the longest run times they can keep the evenest air temperatures, have fewer temperature swings between the on/off cycle and do the best job dehumidifying in the cooling season. If the outdoor unit is a heat pump it will have higher supply air temperatures to lessen the chance of the air feeling cold if a vent is located in a place where it is blowing on you and they extract more heat from the outdoor air reducing the amount of backup heat needed which helps with energy bills. When paired with zoning these systems eliminate the need for a bypass damper as they will only deliver the amount of air that is needed for the zone(s) that are calling. The 95-97%% efficiency on the furnaces and 18 to 20 SEER on the outdoor units are at or near energy star level and are the most efficient option for air source systems. The furnaces will a variable speed fan motor which uses less electricity and ensures proper airflow through the duct system as it has the ability to ramp up to overcome restrictions in the duct system and deliver air to harder to reach rooms.
– Jeremy Grisham