2.3: Low-Voltage Thermostats
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Low-voltage thermostats are smaller and cheaper to install and maintain, and the wiring does not need to have as high a voltage and power rating as their line-voltage counterparts. This is why low-voltage control circuits are favoured for central heating units. Low voltage thermostats are also more sensitive to changes in heat than line-voltage thermostats, and so can provide more precise control.
When controlling a single large heating unit, such as a central gas furnace or electric furnace , there is no zone control as with baseboard heaters; either the whole house is getting heated, or none of it is. A low-voltage thermostat installed in a central location, and powered by a 120 - 24V transformer , can senses the ambient temperature and be used to control a relay to deliver power to the heating load.
By using an extra-low-voltage source for the control circuit we get the benefit of lower cost and easier installation along with increased safety, allowing a simple two-wire low-voltage cable to be installed in an ideal location in the house. Because it is only meant to handle the control current and not the load current, low-voltage thermostats are highly responsive and can provide more accurate control than line-line voltage thermostats.
Some models of low-voltage thermostats used to control central gas furnace or electric furnace loads incorporating heat exchanges and blowers, will have a small, series connected, resistive device called an anticipator resistor . The purpose of this device is to fine tune the sensitivity of the thermostat and prevent overshoot of the desired room temperature.
Low-voltage thermostats, fall into two general categories, some are simple temperature activated switches while others are more modern electronic thermostats incorporating solid-state circuitry and allowing for programmable behaviours such as 24-hour cycles, and weekday vs weekend scheduling.
Electronic thermostats use semiconductor components for both temperature sensing and control-circuit switching. If installed to replace an existing analogue thermostat, the electronic thermostat must have a separate power source, which is usually a battery.