Some heating-only thermostats are powered by the thermocouple that senses that the pilot is still lit. These generate very small voltages, in the millivolt range, and this is called a "millivolt system".
A cooling system will always use a transformer.
A combination heating-cooling unit, or a heat pump will usually have one transformer, but separate units may have two, which the appropriate thermostat will treat separately.
Are all thermostat circuits powered by a 24 vac transformer? For both the heating circuit and the cooling side?
Generally, yes ... if you are referring to a wall mounted thermostat in a modern home. But you've not given enough specific information to guarantee that answer in every possible situation.
Better to get the model numbers of the heating/cooling equipment involved, get the schematic/wiring diagrams for them and make sure.
Jeff
Hi William, hope you are having a nice day
On 26-Mar-05 At About 07:34:36, William W. Plummer wrote to All Subject: Thermostat circuit
WWP> From: "William W. Plummer"
WWP> Are all thermostat circuits powered by a 24 vac transformer? For WWP> both the heating circuit and the cooling side?
Not all. some are 24 volt, some are 110 volt and some are even less than one volt. some 24 volt systems use 1 transformer and some use one for both heat and cool. this is why it is really important to know what you have and how to wire it.
-=> HvacTech2
No. Some are 110 volts or 220 volts.
No, they are not. There are several different systems in use that you may see. The most common are; Milivolt control, Low Voltage (i.e. 24 volts), Line voltage (115 volts). There can be one or two transformers depending on unit configuration. That said, the most common control system for heating and cooling today is a single transformer 24 volt system.
Joseph
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