Furnace and Thermostat Compatibility
HVAC Services
Furnaces and thermostats are not mix and match appliances. There are numerous types of heating systems and thermostat systems and they need to be coordinated for safe and proper operation.
In this tutorial we’ll review the three types of thermostat systems used today:
-Millivoltage
-Low Voltage
-Line Voltage
We will then look at what system is used on what type of heating or cooling system in the home, including these systems:
-Gas Furnace System
-Boiler and Oil Fired Systems
-Electric Heat and Cooling Systems
-Heat Pump Systems
Thermostat
Identifying Your Furnace and Thermostat Types
When you look your existing thermostat and heating or cooling system over, you’ll want to look for a few things to help identify what systems you have. The following criteria profiles will be developed for each heating / cooling system type:
-Type of Heating or Cooling System
-Number of Wires to Thermostat
-Voltage
-System Attributes
Let’s look at the criteria profile for each of the following heating / cooling systems:
-Gas Furnace System
-Boiler and Oil Fired Systems
-Electric Heat and Cooling Systems
Heat Pump Systems
Type: Single Stage Heat Pump
-Number of Thermostat Wires: 4 or 5
-Voltage: 24 Volt AC
System Attributes:
Outdoor heat pump unit providing heating and cooling
Type: Multi-Stage Heat Pump
Number of Thermostat Wires: 7 or more
Voltage: 24 Volt AC
System Attributes: Outdoor heat pump unit providing heating and cooling
Electric Furnace
Type: Electric Furnace
Number of Thermostat Wires: 2; with air conditioning 4 or 5
Voltage: 24 Volts AC
System Attributes: Forced air furnace
Type: Electric Air Conditioning
Number of Thermostat Wires: 4 or 5
Voltage: 24 Volts AC
System Attributes: Outdoor condensing unit; cooling coils in furnace ductwork
Type: Electric Baseboard Heat
Number of Thermostat Wires: 2 or 4
Voltage: 120 Volts AC or 240 Volts AC
System Attributes: Localized heating with line voltage thermostat in each room
Boiler and Oil Fired Systems
Type: Gas Fired Boiler System – Low Voltage
Number of Thermostat Wires: 2 or 3
Voltage: 24 Volts AC
System Attributes: Hot water boiler for radiant heat or baseboard hydronic systems
Type: Oil Fired Boiler System – Low or High Voltage
Number of Thermostat Wires: 2 or 3
Voltage: 24 Volts AC or 120 Volts AC
System Attributes: Hot water boiler for radiant heat or baseboard hydronic systems
Type: Electric Boiler System – Low Voltage
Number of Thermostat Wires: 2 or 3
Voltage: 24 Volts AC
System Attributes: Hot water boiler for radiant heat or baseboard hydronic systems
Type: Oil Fired Furnace System – Low Voltage
Number of Thermostat Wires: 2; with air conditioning 4 or 5
Voltage: 24 Volts AC
System Attributes: Forced air furnace
Gas Furnace Systems – Low Voltage
Type: Gas Furnace – Standing Pilot
Number of Thermostat Wires: 2 or 3; with air conditioning 4 or 5
Voltage: 24 Volts AC
System Attributes: Standing pilot, forced air furnace
Type: Gas Furnace – Electronic Ignition
Number of Thermostat Wires: 2 or 3; with air conditioning 4 or 5
Voltage: 24 Volts AC
System Attributes: Intermittent pilot or hot surface ignition, forced air furnace
Gas Furnace System - Millivoltage
Type: Gas Furnace System – Millivoltage
Number of Thermostat Wires: 2
Voltage: 750mV
System Attributes: Gravity wall (room) furnace; direct or top vent wall furnace for small areas; mobile home direct vent wall furnace
Type of Systems – Line Voltage
Line Voltage System
Line voltage or high voltage thermostat systems run off your home’s 120 Volt or 240 volt AC power and are not particularly common as they are primarily used for electric resistance heating systems. These thermostats are not as sensitive as the low voltage types and may take up to 7°F of temperature change to register a change. As a result expect wider temperature fluctuations with these thermostats.
A way to check if you have a line voltage thermostat is to look at the wires. They will have thicker electrical wires like you would see going to a light switch or outlet, 14Gauge or 12Gauge as seen in the above photo. Low voltage wiring looks like old doorbell or telephone wiring or speaker wire.
Also you can check for voltage listing inside the cover of the thermostat or on the heat pump, furnace (inside the access door), or on the boiler or on the electrical baseboard unit itself.
Line voltage thermostats are typically hooked up to a 120 volt or 240 volt electric resistance heating system.
Type of Systems – Low Voltage
Low Voltage System
Low voltage thermostats are the most common and versatile type of thermostats found in the home. They are typically 24 Volt AC and powered by using a step-down transformer from the 120 Volt line voltage in the home. Some versions of this thermostat use a voltage as low as 6 volts and as high as 30 volts, but 24 volts is by far the most common. The transformer is usually mounted on or near the furnace.
Type of Systems - Millivoltage
Millivoltage System
These systems are not very common in a home as they are used often in direct /top vent wall furnaces. Thermostats that run on a millivoltage system rely on very low voltage, usually 750 millivolts (mV) or 0.75 volts. They do not require a step down transformer reducing line voltage of 120 volts to 24 volts as do low voltage systems. Instead they are self powered by a special thermocouple device called a powerpile or thermopile generator that produces direct current (DC). The thermopile is a device with thermocouples connected in series to increase the millivoltage output to 750mV.
These types of systems require a special thermostat and will not work with standard low voltage (24 V) thermostats.
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