Why one size does not fit all when you replace your AC

When an air conditioner quits in July, the first instinct is to replace it with whatever matches the label on the old one. It feels safe. The old unit cooled the house, so the same size should too. Air Oracle's team in Cypress sees the flaw in that logic often enough that it sits on their own list of principles: one size does not fit all.

Their line on it is blunt. "Best Sizing & Installation Practices are critical when replacing your AC system. Improper system equipment may produce mold, embarrassing smells and uncomfortable living conditions." Here is what that means in a house in Cypress, Katy or Tomball, and what to ask before you sign anything.

Size starts with the house, not the old nameplate

Air conditioners are sized in tons, and one ton is 12,000 BTU of heat removed per hour. A three-ton system and a five-ton system are different machines moving different amounts of air and heat. Picking between them by guessing, or by copying the number on the unit you are throwing away, skips the only step that matters.

The right number comes from a load calculation. That is a measurement of how much heat your house gains on a hot afternoon and how much moisture the system has to pull out of the air. Square footage is one input, and not even the biggest one. A tight, well-insulated two-story can need less cooling than a leaky single-story of the same size.

• How much insulation is in the attic, and whether it is blown-in or spray foam
• Window size, window type and which direction they face
• Shade from trees, or none at all on a west wall
• Ceiling height and how open the floor plan is
• Where the ductwork runs and what shape it is in
• Changes since the old unit went in, such as new windows, an addition or a converted garage

That last item is the trap. Houses change, and the old system may have been wrong from the day it was installed. Matching its size just copies the original mistake into a new machine.

What an oversized unit does in Gulf Coast humidity

Bigger sounds better. A large unit drops the air temperature fast, the thermostat is satisfied, and the system shuts off. Then the room warms a little and it starts again. That pattern is called short cycling, and it is the signature problem of an oversized system.

The reason it hurts here is moisture. An air conditioner dries your air by condensing water on the cold indoor coil, and that takes run time. A unit that runs for five minutes can cool a room without drying it. You end up with air that reads 72 degrees on the thermostat and still feels damp on your skin. Gulf Coast air carries a lot of water, so the gap between cool and comfortable is wider here than in a dry climate.

• The system starts and stops again and again instead of running in long steady cycles
• Rooms feel cool but sticky, especially in the evening
• A musty smell when the system kicks on
• Noticeable hot and cold swings between rooms
• Condensation on windows, vents or ceilings in humid weather

Damp conditions are also where mold and stale smells take hold, which is exactly what Air Oracle's warning about improper equipment is getting at. The unit you bought to make the house comfortable ends up working against it.

What an undersized unit does

The opposite mistake is just as real. An undersized system runs and runs. On a normal evening it may keep up, but on the hottest afternoons of August it never catches the house, and the indoor temperature drifts a few degrees above where you set it. Long run times also mean more wear on the compressor and blower over the years.

Undersizing usually happens one of two ways. Either the house grew after the system was installed, or someone sized down on purpose to hit a lower number on a quote. Neither shows up until the first real heat wave.

Tonnage is only one piece of a matched system

Even a perfect tonnage choice can go wrong if the pieces do not work together. The outdoor condenser, the indoor coil and the air handler or furnace need to be a matched set, and the ductwork has to move the air that set was designed to deliver. Duct that was sized for an old, smaller blower can choke a new one. If rooms were uneven before the replacement, they will probably be uneven after unless the duct is addressed.

Questions to ask any contractor before you sign

• Did you look at my house and attic, or are you reading the old nameplate?
• How did you arrive at this tonnage, and can you walk me through it?
• Are my ducts and returns able to carry the airflow of the new system?
• Are the indoor and outdoor components a matched set?
• What does this system do about humidity, and not only temperature?
• Will the installation include an emergency float switch to protect my ceiling?

Air Oracle is a family-owned HVAC contractor on Fry Road in Cypress, an authorized dealer for Trane, Goodman and Ruud, and a Trane Comfort Specialist. A complete system changeout comes with a free estimate, so you can talk through sizing before you commit to anything. Their system installation page explains how a changeout works, and you can reach them at (888) 524-7247.