Dr. Kool‘s

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AC & Heating—Advanced Basics

This article gives the basics of AC electrical circuits. It also gives guidance that will facilitate any contractor repair, or replacement job, that might become necessary.

Multiple Electrical Circuits

If your condenser is outdoors, and your furnace or air handler is indoors, you have what's called a split system. Your comfort system will have two or more electrical circuits—for the furnace or air handler, and for the outdoor condenser. All circuits must be energized for your comfort system to work. (See Electricity, Water, and Gas, for additional circuit information.)

Gas Furnace

If you have a gas furnace, it can be in a closet or in the attic, and it will plug into a 115 volt outlet. (Older furnaces may use a switch, rather than a receptacle.) This should be a dedicated circuit, which means it's not being shared with any other electrical load.

Inside the furnace, is a 115 volt blower, and a control transformer, which produces 24 volts. The 24 volt signal passes through the thermostat to control the various functions—both indoors and outdoors.

Air Handler

If you have electric strip heat or a heat pump, an air handler will take the place of the gas furnace. It can be in a closet or in the attic, and requires 240 volts. There may be more than one circuit to this unit. You will have a 240 volt blower, along with a control transformer.

A gas furnace is attached to a cooling coil. In contrast, most air handlers incorporate both the blower and the cooling coil in the same case. If you interrupt the circuit to either the furnace or the air handler, you also interrupt the 24 volt control circuit. Then, no matter what command you give the thermostat, nothing will happen.

Door Switch

A gas furnace has a door switch which controls 115 volt power. If the blower door is loose, the result is the same as if you had pulled the plug or tripped the breaker. Nothing will work. If nothing works, you should check to make sure the blower door is securely in place.

An air handler does not have a door switch.

Outdoor Condenser

Your outdoor unit has its own 240 volt electrical circuit. If the breaker trips for any reason, you can re-set it, and try again. But, often you will need to call a serviceman.

Sometimes people hear a buzzing sound at the condenser, and think that means the unit has power. No, the buzzing may be coming from the energized coil of the 24 volt contactor. That circuit has its origin in the furnace or the air handler.

For complete safety when working on a condenser, one should de-energize two or more circuits.

Access for the Repairman

When you call for repairs, the serviceman should not have to move stored items before he can go to work. Do not store things around your furnace, air handler, or condenser.

One of my customers built a fence around his condenser. I arrived late Friday, and told him I could not work on the unit. He should redesign his fence. I needed an access panel that could be opened—I would return Monday, and work on the AC.

Before I could get away, his wife tore the fence down, and called me back. Did they want kool or what!

Move Cars Out of The Way

Especially if the job is sizable, cars should be removed from the driveway—before the repairman arrives. The closer your serviceman can get to the house, the more efficient, and less time consuming, will be the job.

If someone is going to need his car while work is in progress, it helps if the repairman doesn't have to come out of the attic, and move his truck. More information is provided in Scheduling.

The Koolness Continues,

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