Dr. Kool‘s

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AC & Heating—The Basics

I have given considerable advanced material on air conditioners. It would be a mistake if I didn't cover the basics.


A thermostat senses air temperature, and should be located near the air return. It's the control center for your comfort system.

Most thermostats are of the manual type, which means you have to move a switch to select whether you want cooling or heating. Another switch allows you to select whether you want the blower to run in automatic or continuous mode.

In the summer, most people select Cool and Auto. During normal operation, the indoor and outdoor units may start and stop at exactly the same time. But, some high efficiency systems have brief signal pauses that help harvest more of the cooling.

(Some thermostats incorporate fancy electronics, and require special instructions. If you have one, be sure and hang onto the owner's manual.)

Continuous Fan Blower Operation

There are times when a user will run his indoor blower continuously. Continuous Fan Blower Operation explains how this choice applies to different situations.

Compressor Time Delay

It's not a good idea to stop an AC compressor, and then immediately restart it. There's often a 3-5 minute time delay, which prevents an immediate compressor restart. If your indoor unit is running, and your outdoor unit isn't, please wait…before assuming something's wrong.

Fire Hazards

Do not use or store flammables in the vicinity of a furnace, hot water heater, condenser, appliance, light switch, cell phone, or any other spark emitting device. My article Fire Hazards, gives a more complete story.

Air Vents (Registers)

For your comfort system to operate properly, you must have sufficient air flow (400 cfm/ton). A three ton installation requires 1200 cfm; a four ton, 1600 cfm etc. Usually your ductwork is sized accordingly.

So what does that mean? If you close more than one register, you may choke down the system, and cause it to freeze up. So be careful when closing registers.

Air Filter Locations (Upflow Furnace, in a Closet)

  • Inside the furnace, behind the blower door.
  • Under the furnace, in an EZ change filter base.
  • Wall mounted filter grille, below the closet door.

Air Filter Locations (Horizontal Furnace, in the Attic)

  • Wall mounted filter grille, near the floor.
  • Ceiling mounted filter grille.

Please observe directionality. An arrow, on the filter element, points in the direction of air flow.

Air Filters

The importance of air filter maintenance cannot be overstated. I have written an entire article on the subject—Air Filters. If you'd like a periodic filter change reminder, we'll be happy to e-mail you one. When you register, just check the appropriate box. If you've already registered, log in and then click on "Update My Info." Scroll to the bottom, and check the appropriate box.

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