What Size AC Do You Need? Tonnage and Load Calculation

Tonnage in plain language

Twelve thousand BTU per hour is one ton of cooling, and typical homes need two to five tons, with wide variation. Too large a system cools quickly then shuts off before humidity comes out, while too small a system runs without a break and never catches up.

What goes into the calculation

• Duct location and condition
• Window size, glass type and orientation
• Insulation in the attic, walls and floors
• Design temperatures for Memphis
• House size, number of floors and ceiling heights
• Air leakage and the number of people

Why bigger is not better

• Noise and equipment cost go up
• Humidity is not removed, leaving a clammy feel
• Temperature swings between rooms are larger
• Short cycling wastes energy and wears parts

After sizing: ducts and equipment

Equipment selection and duct design both follow from the calculation. Request the load report, the capacity of the equipment chosen and the airflow figures from the contractor, and ask for static pressure to be checked after installation.

A checklist for this week

• Clear the outdoor unit of debris
• Inspect the drain line outlet and the area around the air handler for water
• Book a tune-up if it has been more than a year
• Test the thermostat and schedule
• Write down any noises or smells
• Check supply and return registers

Ducts, airflow and comfort

Comfort depends on airflow as much as on cooling capacity. Leaky ducts, undersized returns and blocked registers cause hot rooms even when the system is sound. A technician who measures static pressure and airflow, not just refrigerant pressures, will find these problems.

HVAC terms explained

• Static pressure: a measure of airflow resistance in the ducts
• Contactor: the switch that sends power to the outdoor unit
• Capacitor: stores charge to start and run the compressor and fan motors
• Evaporator coil: the indoor coil where refrigerant absorbs heat from the air
• EPA Section 608: the certification required to handle refrigerants
• Superheat and subcooling: readings that show whether the refrigerant charge is correct
• Condenser coil: the outdoor coil where heat is released
• Condensate drain: the line that carries moisture out of the house

Myths and facts

• Myth: maintenance is optional. Fact: dirty coils and weak capacitors cause many summer failures.
• Myth: a bigger AC is always better. Fact: oversizing causes short cycling and humidity problems.
• Myth: set the thermostat very low to cool faster. Fact: the AC cools at a steady rate and just runs longer.
• Myth: refrigerant is used up. Fact: low refrigerant means a leak.
• Myth: closing vents saves energy. Fact: it raises duct pressure and can cause leaks and noise.

What shapes the written scope

Two AC calls that sound alike can be very different jobs. The written scope should say what the technician found and which of these factors applied.

Have the provider show the diagnostic and the repair as separate items, and list any parts, labor or follow-up that are left out.

• Whether the ductwork, return air or thermostat contributes to the problem
• The age, tonnage and refrigerant type of the system
• Access to the air handler in the attic or closet and to the outdoor unit
• Whether the fault is an electrical part, an airflow problem, a refrigerant leak or a drainage problem
• Warranty status of the parts and whether the equipment is registered with the manufacturer

Safety and rules to know

Three hazards share one machine in an air conditioner: high-voltage electricity, pressurized refrigerant and moving parts. Opening electrical panels, bridging contactors and adding refrigerant are not jobs for homeowners, and capacitors can keep a charge after the power is off.

In the United States, anyone who buys or handles refrigerant must be certified under EPA Section 608, and refrigerant must be recovered and not vented. Ask the technician for their certification and check the contractor's licence with Tennessee Board for Licensing Contractors (HVAC classifications).

Questions to ask before approving a repair

• Will you give me the findings and the scope in writing before work starts?
• What are my options: repair now, repair and monitor, or replace?
• What does the repair warranty cover and for how long?
• How old is the system and which refrigerant does it use?
• What failed, how did you confirm it and can you show me the part or the reading?

Prepare your Memphis home before the technician arrives

• Check that the outdoor disconnect and the breaker are on, but do not open electrical panels
• Write down the sounds, smells, leaks or ice you have noticed, and when
• Set the thermostat to cool and note what it shows and what the house temperature is
• Clear weeds, leaves and debris from around the outdoor unit
• Replace or check the filter, and clear furniture from the return grille and the air handler

Mistakes that make AC problems worse

• Running a frozen system. Ice means airflow or refrigerant is wrong, and running it can damage the compressor.
• Putting off a small repair. A weak capacitor can take out a compressor if left alone.
• Adding refrigerant without finding the leak. The charge will drop again and the leak may grow.
• Ignoring a dirty filter. It is the most common cause of poor airflow and frozen coils.
• Setting the thermostat much lower to cool faster. An AC cools at a steady rate, so it only runs longer.

When to call a professional

Several situations call for a contractor: a system that will not start, ice or water, a problem that returns after your own checks, grinding or banging noises, or anyone in the house who is vulnerable to heat or cold. Use Call Now at the top of the page, describe what the system is doing, and you will reach an independent HVAC contractor who covers your ZIP code, confirms the address and diagnoses the system in person.

What this means for a home in Memphis

Hot, humid Mid-South summers with heat indexes often above 100 degrees; winters can bring ice storms. Humidity drives condensate drain clogs, sweating ducts, and mold in air handlers.

Memphis Light, Gas and Water (MLGW) is the local utility. Older homes in Midtown, Cooper-Young, Binghampton, and Whitehaven; newer homes in Cordova, Collierville, Germantown, and Bartlett. Bring these local details into any comparison of advice, and ask each technician how they apply to your system and your house.

Getting help in Memphis and nearby cities

Independent HVAC contractors covering 59 ZIP codes in Memphis, Tennessee are reached through Air Conditioning Repair Memphis: 37501, 37544, 38101, 38103, 38104, 38105, 38106, 38107, 38108, 38109. Providers who serve Ellendale, Germantown, Cordova and Collierville can also receive requests. Each address is confirmed separately.

Start with Call Now at the top of the page and say what you have found. The contractor checks your address, looks over the system and gives you the scope in writing.

Keep reading

Related pages for Memphis homeowners: Frozen AC Coil: Why It Happens and What to Do; AC Making Noise: What Each Sound Means; How to Lower Cooling Bills; Summer HVAC Checklist for Memphis. For the area list, see the service areas page.

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