Heater Not Working? Furnace Blowing Cold Air?

Why Your Heater Isn’t Working (And How to Troubleshoot – Step by Step)

A broken heater in winter is more than just cold — it can cause frozen pipes, water damage, or even harm your house. Here’s an easy-to-follow guide to get your heat working again. Try these steps in order before calling a pro.

Step 1: Check Your Thermostat

Most “broken” heaters are just forgotten thermostats!

• Is the thermostat turned ON and set to “HEAT”?
• Is the screen lit up? (If it’s blank, keep reading.)
• Turn the temperature 5–10 degrees higher than the current room temperature.
• Wait 1–2 minutes. Still nothing? Turn it way up (like 80–90°F) to test.

Step 2: Fix a Blank/Dark Thermostat Screen

If the display is completely off:

A. Battery-powered thermostat? → Replace the batteries (usually AA or AAA).

B. Still dead? (Common in Texas & other swing-weather areas) Your AC drain line might be clogged. When it clogs, a pan under your AC unit fills up and there’s a safety switch kills power to the thermostat to prevent water from overflowing and causing water damage. → This usually needs a professional HVAC tech to unclog the drain with professional equipment.

Step 3: Gas Furnace – Is Gas Actually On?

Look for the gas valve within 6 feet of the furnace. It could look like:

• A small lever (often red or yellow)
• A little handle on a disk
• A small box

Rule of thumb:

• Handle parallel (║) with the pipe = ON (gas flowing)
• Handle perpendicular (⊥) to the pipe = OFF

→ Turn it parallel to open the valve.

Still no heat?

• Test another gas appliance (stove, water heater, fireplace).
• If nothing works, gas may be off at the meter or there’s an outage.
• Check the gas meter outside — the valve should also be parallel.
• Call your gas company (in San Antonio: CPS Energy) or check if you accidentally missed a bill.

Step 4: Find and Flip the Furnace Power Switch

There’s often a plain light-switch on or near the furnace (on the wall, ceiling, or even the furnace itself).

• Flip it to the opposite position, wait 3–5 minutes.
• People bump this switch all the time when moving boxes in the attic!

Step 5: Replace the Dirty Air Filter (Super Common Fix!)

A clogged filter can make the furnace shut off for safety.

1. Find the filter (usually where air gets sucked into the furnace — big slot with a cover).
2. Slide the old filter out.
3. Look at the arrow on the side of the filter — it shows airflow direction.
4. Buy the exact same size (written on the side).
5. Put the new filter in with the arrow pointing toward the furnace. → Change filters every 1–3 months in winter.

Step 6: Still No Heat? It’s time to CALL A PRO. What a Pro Will Check:

One of the most common pro fixes is cleaning the flame sensor (a small metal rod inside the furnace).

Warning: Only do this if you’re comfortable — otherwise call a tech.

Basic idea:

1. Turn off power and gas to the furnace.
2. Remove the sensor (usually one screw).
3. Gently clean the metal rod with steel wool or a knife edge.
4. Reinstall and turn everything back on.

If you’ve tried all the steps above and still have no heat, it’s time to call a professional.

In San Antonio, you can call Air & Plumbing Today at 210-519-5959 in San Antonio or (512) 859-5959 or book online.

Stay warm — most heater problems are simple fixes!