A Picture of Perfection
In first grade, I remember enviously watching as two classmates would ride the seesaw with such perfect ease. Effortlessly, they’d go up and down, carrying on a conversation as if it were motorized. They must have been the exact same weight. They would just slightly push with their toes. It was perfection. Even their dismount was perfect. They would hold the plank perfectly level and they would each swing their leg off with the synchronicity of ballet dancers. It was poetry in motion.
The Cost of Unbalance
Your air conditioner doesn’t cool your house. It removes heat. It is absorbing the heat in your house through the cooling coils and transferring it to the condenser outside. When the balance is perfect, your air conditioning system will effortlessly remove the heat from your home. A system properly sized with clean refrigerant and clean coils is quite capable of achieving a near-perfect balance. A well-maintained system will last a really long time. When it isn’t well maintained, the system balance is affected, the system is unable to absorb and transfer heat. The system must overexert itself to accomplish the transfer. It has to run harder for longer. This exertion leads to higher electric bills and shortens the life of your system.
Common Causes of the Unbalance
A simplified expression of heat transfer is:
Q = m(Tin – Tout)
Where Q = heat transfer, m = airflow, Tin = Air temp in your home, Tout = Air temp coming out of your vents
Thermostats are placed near returns to monitor the temperature of the air going into the cooler or Tin. If the system is calling for a lower inlet temperature or air temperature, the system responds by providing a lower outlet temperature (Tout) using the refrigeration process.
Q or your heat transfer capacity is decided by the rating of your system expressed in Tons. 12,000 BTU/hr is one Ton. Your 3-ton system is capable of 36,000 BTU/hr. Determining what capacity is needed is done by evaluating the heat load of your home.
Cause #1 – Improperly sized system
If the system is undersized, it simply will not transfer enough heat to lower the temperature coming out of your vents enough to lower the temperature in your house. The thermostat will never be satisfied and your system will constantly run. Imagine that seesaw with an adult on one end and first-grader gets on the other. It’s just not physically possible for the child to lift the adult.
Oversized? The opposite is true. You have the first-grader on the seesaw. An adult gets on and goes crashing to the ground. Your thermostat calls for the system. The system comes on and moments later goes back off. This results in more frequent cycling. Another consequence is humidity in your home is higher. The process of cooling your home removes the humidity from the air. For that to be effective, you need plenty of air to pass through the coils. Shorter run times mean less air passing through and thus less humidity is removed. This means you have to cool your house to a lower temperature for it to “feel” comfortable.
Cause #2 – Poor airflow
Our service manager believes poor airflow accounts for 75% of poor system performance issues. It is a very common unaddressed issue. The source of which typically dates back to duct design and installation. Going back to our analogy, our two riders are the same size but one side of the seesaw board is a little longer than the other. The kid on the short end is having to push a little harder to keep them going and is getting tired more quickly. The system will be fine during the low load periods of Spring and Fall when there isn’t much need for the system to run. But in the heat of the summer, that kid is going to get tired and he just isn’t going to be able to remove enough heat to keep up. Your upstairs rooms will always be a little too warm and the system will run for long periods of time. The most common controllable cause of poor air flow is a dirty filter followed closely by too many registers completely closed or obstructions in front of returns or outlets. Design issues are sometimes a little more difficult to identify and more expensive to fix. Some easy things to check: airflow noise at your return or just walking around feeling the flow with your hand.
Cause # 3 – Dirty coils
I know the analogy is getting tired but one last time… If you have dirty coils, it’s like the fulcrum of that seesaw is rusty and warped. The loads are equal and the ends of the board are the same length but the seesaw rubs going up and down and is slow. It’s causing much more effort to play. Slowing down the heat transfer has obvious consequences. Your system has to run for longer to cool down your house. This problem surprisingly goes unnoticed for a while. The reason being, inspecting the inlet side of your evaporator coil can be difficult. Technicians really need an inspection camera. Otherwise, a technician will have to do a significant amount of work to inspect it. Similarly, homeowners very often decide to not clean it because it could become costly depending on the arrangement. But a dirty evaporator coil can have significant 2nd and 3rd order effects. From a pressure-temperature perspective, it can be misdiagnosed as a problem with refrigerant charge. Improperly adjusting refrigerant charge leads to other more serious issues. Dirty coils are going to be wet which are the conditions needed for the growth of mold and bacteria. They often cause bad odors as well. This like in most cases, the prevention is much better than the cure. Keep filters changed and do regular maintenance.
These are causes primarily from an indoor perspective. You can apply the same principles to the outdoor unit and come up with similar causes and cures. Dirty condenser coils, bad airflow due to vegetation or something placed near the unit, and so on. But to get the maximum life and full rated capacity out of your heating and air system, you have to stay focused on the balance and what can affect it. Many of these things are well within a homeowner’s control. For the things that aren’t, maybe this information will help you ask the right questions when speaking to your HVAC contractor.