Component
Thermostat
The thermostat is the user's half of the furnace: it calls for heat, cooling or fan, and on communicating systems it is the control interface. A surprising share of no-heat calls start at the thermostat: dead batteries, wrong mode, or a set point below room temperature.
Short answer
Before anything else: set the thermostat to Heat, raise the set point above room temperature, and replace dead batteries. Check for a blank screen, a low-battery icon, or a schedule that is not calling. If the thermostat is calling (a flame or heat icon shows) and the furnace still does nothing, the fault is in the furnace, and the code on the control will say so.
Thermostat basics
Standard thermostats switch 24 V signals (on many furnaces the terminals are R, W, G, Y and C; two-stage systems add W2 and Y2). Communicating furnaces such as Carrier's Infinity line are designed for their own communicating controls, not generic thermostats, and the correct control is part of the system.
Thermostat-caused furnace behaviour
A thermostat that never releases the heat call makes the furnace run until its limits stop it. A thermostat that cycles aggressively shortens furnace cycles. A thermostat on a warm wall or in sun reads the wrong temperature. These are control problems, not furnace faults, and the furnace's code history usually shows nothing wrong.
Upgrading thermostats
Upgrading to a smart thermostat is straightforward on non-communicating furnaces when the wiring has a C (common) wire or the furnace can supply one; the terminal lists on this site's model pages show what each furnace actually has. Matching the thermostat to the furnace's staging is the part most installs get wrong.
Related
- ServiceThermostat and Control IntegrationThermostat and control set-up that matches the furnace, not the box
- ServiceGas Furnace RepairFurnace repair in Greater Vancouver, diagnosed before parts are replaced
- ServiceFurnace ConsultationA consultation before you buy: sizing, staging, efficiency and whether a heat pump belongs in the plan
- SymptomFurnace not heating
- SymptomUneven heating between rooms
Questions people ask
My thermostat screen is blank. Is the furnace broken?
Probably not: check batteries first, then the breaker for the furnace, since many thermostats draw their power from the furnace's 24 V supply. If the furnace has power and the thermostat is still blank, the low-voltage wiring or transformer needs a check.
Can any smart thermostat control my two-stage furnace?
Most two-stage furnaces work with smart thermostats that support two-stage heating, wired to the W2 terminal. Modulating and communicating furnaces need their own controls; check the furnace's documentation.
Why does the house heat unevenly with a smart thermostat?
The thermostat reads one location. If that location is warm and other rooms are cold, the furnace satisfies early; the fix is airflow, zoning or sensor placement, not another thermostat feature.
Need this looked at?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.