Is this the basic operation routine for a thermostat?

Old 04-28-19, 06:05 PM
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Question Is this the basic operation routine for a thermostat?

I have a packaged unit on the roof of my house. It's a Trane.

As I understand, the voltage provided to the thermostat is 24V.

For A/C:

When the temperature sensor reading exceeds the target temperature, two things happen:

1. A relay closes the circuit on the compressor wire at the thermostat.
2. A relay closes the circuit on the blower wire at the thermostat.

From there, I would presume that additional relays in the unit itself handle power to the actual motors on the compressor and the blower.


A. Is it this simple in theory?

B. Do programmable thermostats build in functions to offset the trigger for these switches, whereby a setting of 78 would need to either be matched for a length of time, or the sensor would have to exceed the temperature by a higher amount to prevent short cycling the components?

C. Does the compressor need to run for a specific period of time prior to the blower kicking in, or vice versa, or do they both fire up at the same time?

I understand each manufacturer may program their thermostats differently, but I'm wondering in general what's going on with it.

P.S., I'm building my own thermostat relay and I want to make sure I don't zorch my system, so I'm starting by learning the basics. Thanks!
Old 04-28-19, 08:26 PM
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Yes...... basically it is that simple.

There is a large relay in the condenser called a contactor. It switches the 240v 30A power.
There could be a relay or timer board/relay in the air handler section.

The stat will energize the blower immediately but delay the condenser for several minutes to stop short cycling. Thermostats have algorithms built in to control the switching operation. Some have a setting called "cycles per hour" and some have an adjustable differential which will allow the temperature to vary from +/- 1 degree up to several degrees in either direction and some just try and hold close to the setpoint.

The stat display is shown as a whole number but the stat itself operates in tenths of a degree. If it was registering 70....... the actual temperature could be 69.5, 69.6, 69.7, 69.8, 69.9, 70, 70.1, 70.2, 70.3, 70.4, 70.5.

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