Furnace Blower Runs Constantly but No Heat: Thermostat, Board or Motor?

The blower is running. The thermostat is calling for heat. The air coming out of the registers is room temperature or colder. On a gas furnace, that combination is rarely a blower motor problem, because the motor is doing exactly what it was told to do. The question is who told it, and why the burners are not lit.

This is the heating-season version of the split. For the opposite symptom, where the system runs and the blower stays dead, see the system runs but the blower doesn't.

Safety first: this guide is written for HVAC technicians. Gas valves, burners, flame rollout switches and line voltage carry real risk of fire, carbon monoxide and shock. If you are a homeowner, check the thermostat and the filter, then call a licensed technician. Never jumper a limit or rollout switch to keep a furnace running.

Is the Fan Simply Set to On?

Start at the wall. If the thermostat fan setting is ON instead of AUTO, the blower runs around the clock regardless of what the burners are doing. Between heat cycles that feels like cold air, and customers report it as "no heat."

Set the fan to AUTO and wait. If the blower stops and then starts again after the burners light, the system is working. If the blower keeps running with the fan on AUTO, a G call is still present, so check the thermostat wiring for a short between R and G before moving to the furnace.

What the Board Is Telling You

Most modern furnaces run the blower on purpose when something goes wrong on the heat side. An open high limit is the classic case: the control board energizes the blower to pull heat off the heat exchanger and keeps it running while the limit stays open. The burners stay off. From the hallway, that is exactly "blower runs constantly, no heat."

So read the status LED before you touch anything. Count the flashes, look up the code on the door label or in our HVAC fault code lookup, and for Carrier, Bryant and Payne equipment use Carrier furnace and AC error codes explained. Write the code down before you cycle power, because many boards clear the active code on a reset.

Why the High Limit Keeps Opening

A limit that opens is reporting a symptom, not causing one. The furnace overheated because air is not moving across the heat exchanger fast enough. In the order we see them:

• Dirty filter. The first heating call of the season after a summer of cooling is when this shows up most. Check it first, every time.
• Closed or blocked supply and return. Closed registers, furniture over returns, a collapsed flex duct or a return grille stuffed with insulation.
• Dirty blower wheel. A wheel caked with dust moves far less air at the same speed. Pull the assembly and look at the blades.
• Wrong blower speed for heat. A heat tap set too low after a previous repair, or a programmed ECM set for a different airflow than the furnace needs.
• A weak motor. A motor that turns but cannot reach its programmed speed or torque. This is the one case on the list where the blower motor is the actual fault.

Measure temperature rise once the furnace is firing again and compare it against the range on the furnace rating plate. A rise above the range confirms an airflow problem even when the limit has not tripped yet.

Lockout, Rollout and Flame Problems

If the code points to ignition rather than the limit, the blower behavior depends on the board. Some boards run the blower through a post-purge after failed ignition attempts, others leave it off during lockout. Either way, the fault is on the burner side:

• Flame sensor. A dirty sensor lets the burners light for a few seconds and then shuts the gas valve. Customers describe this as the heat "trying" and the fan running.
• Pressure switch and inducer. On condensing furnaces, a blocked condensate trap, a cracked hose or a blocked vent keeps the pressure switch from closing, so ignition never starts.
• Gas supply. A closed manual valve after summer work, or a gas valve that is not opening on command.
• Flame rollout switch. An open rollout points to a venting or heat exchanger issue. Find the cause before you reset it, and do not return the furnace to service until you have.

When the Board Itself Is the Problem

If the blower runs with the thermostat disconnected and no fault code is flashing, look at the board. A blower relay with welded contacts keeps the motor energized whatever the inputs say. On an X13 constant torque motor, check whether 24 volts sits on a tap input with no call present. On a PSC motor, check for line voltage on the speed lead. Voltage with no call from the board logic means the board, not the motor.

Where the Blower Motor Fits In

In this symptom, the motor is usually the last suspect, and that is the expensive mistake to avoid: replacing a working motor because it was the part that was running. The motor becomes the fault when it is the reason airflow is low, which shows up as repeated limit trips with a clean filter, open ducts and a clean wheel.

Confirm it before you order. For ECM and X13 motors, follow the full ECM diagnostic sequence. For PSC motors, use how to test a blower motor with a multimeter. For the wider list of blower symptoms, start at blower motor troubleshooting.

If the motor is the problem, match the replacement to the part number on the motor label, not just the furnace model. Our HVAC serial number lookup helps when the label is worn or missing.

Quick Sequence

1. Thermostat fan on AUTO, heat call present, R to G not shorted.
2. Read and record the status LED code before resetting power.
3. Limit code: filter, registers and returns, blower wheel, heat speed, then temperature rise.
4. Ignition code: flame sensor, pressure switch and condensate path, gas supply, rollout.
5. No code and blower runs with no call: board relay or tap input voltage.
6. Motor last, confirmed by test, matched by part number.