---
title: "Furnace Not Turning On or Blowing Cold Air? 7 Checks Before You Call"
description: "Furnace not turning on or blowing cold air? Try these 7 safe checks first: thermostat, power, door switch, filter, gas, error code and condensate drain."
url: https://des-moines-plumbing-hvac.b-daniels.workers.dev/blog/furnace-not-turning-on-blowing-cold-air/
published: 2026-10-23
author: Nate
category: HVAC
---

# Furnace Not Turning On or Blowing Cold Air? 7 Checks Before You Call

By Nate, Lead Technician, Des Moines Plumbing & HVAC · Published October 23, 2026

**Quick answer:** If your furnace won't turn on or is blowing cold air, check seven things in order: the thermostat's mode, setpoint and batteries; the breaker and furnace switch; the blower door safety switch; the filter; the gas valve; the blinking error code on the control board; and, on 90%+ furnaces, the condensate drain. If you smell gas, get out and call 911 or the gas utility from outside. Call us if it keeps locking out or the air is still cold after 10 minutes.

The house is getting colder, the thermostat says it's calling for heat, and the furnace is either doing nothing or pushing air that feels like it came off the basement floor. Plenty of the no-heat calls we get in the first cold week of fall turn out to be a setting, a switch or a filter. Here are the seven checks a tech starts with, safest and most likely first, and exactly where to stop.

## Is it normal for a furnace to blow cold air at first?

A little, yes. On most modern gas furnaces, a normal start goes like this: the inducer motor (a small fan) starts, the ignitor glows bright orange for a few seconds up to half a minute, the burners light, and the main blower kicks on within a minute or so.

If the thermostat's fan is set to On, the blower runs nonstop and you'll feel room-temperature air between cycles. After the burners shut off, the blower also runs briefly to pull the last heat out, so the air cools at the end of a cycle. None of that is a problem. Cold air that keeps coming well after the furnace should be warm is.

Match what you're seeing to where to start:

| What's happening | Start with |
|---|---|
| Nothing at all: no sounds, no fan | Checks 1, 2 and 3 |
| Fan runs, but the air is cold the whole time | Check 1 (fan setting), then 4 and 5 |
| Burners light, then shut off within seconds | Checks 4 and 6, then call |
| Starts, then shuts down after a few minutes | Checks 4 and 7 |
| A light is blinking on the furnace | Check 6 |

## Check 1: Is the thermostat actually asking for heat?

A surprising number of no-heat calls end right here.

- **Mode:** It should say Heat, not Cool, Off or a mode you don't recognize. After a summer of AC, it's easy to miss.
- **Setpoint:** Set it a few degrees above the current room temperature so it has a reason to call.
- **Fan:** Set it to Auto, not On, if your complaint is cold air between cycles.
- **Batteries:** Some thermostats keep the display lit on weak batteries but stop sending the call for heat. Replace them anyway; it's cheap.
- **Smart thermostats:** Check the app for a forgotten schedule, hold or eco setting. On a new install, a missing common wire or the wrong system-type setting can keep it from running the furnace.

## Checks 2 and 3: Does the furnace have power?

### Check 2: The breaker and the furnace switch

Find the furnace breaker in your electrical panel. A tripped breaker sits between On and Off; push it all the way to Off, then back On. If it trips again right away, leave it off and call.

Then find the furnace service switch. It looks exactly like a light switch, usually on the wall or a joist near the furnace, and people flip it off thinking it runs the basement lights. It should be On.

### Check 3: The blower door safety switch

The lower front panel presses a small safety switch when it's in place. If the panel is off, loose or crooked, the furnace won't run. This happens a lot right after a filter change. Set the panel back firmly, bottom tabs first. Never tape down or bypass that switch.

## Check 4: Is a clogged filter shutting the furnace down?

A filter packed with dust starves the furnace for air. The heat exchanger runs hot, the high-limit switch shuts the burners off, and the blower keeps running to cool things down. You feel lukewarm or cold air from a furnace that keeps short-cycling.

Pull the filter and hold it up to a light. If you can't see light through it, replace it, with the arrow pointing toward the furnace. Pets, remodel dust and older houses clog filters faster than the package says.

**Pro tip:** Write the filter size on the side of the furnace with a marker and buy three at a time, so a fresh one is always in the basement on a cold night.

## Check 5: Is gas getting to the furnace?

**If you smell gas, a rotten-egg odor, skip every other check.** Don't flip switches or try to relight anything. Get everyone out, then call 911 or MidAmerican Energy's gas emergency line from outside.

Otherwise, look but don't adjust. On the gas pipe into the furnace, a shutoff handle parallel to the pipe is open and crosswise is closed; the knob on the furnace's own gas valve should read On. If either is off and you don't know why, don't turn it back on. Call us, and we'll find out why before relighting.

Then try another gas appliance. If the stove won't light either, the problem is upstream of the furnace, so call MidAmerican Energy, not us. On propane, check the tank gauge.

On an older furnace with a standing pilot, leave relighting to a tech; a pilot that goes out on its own usually means a worn thermocouple or a draft problem.

## Check 6: What is the blinking light on the furnace telling you?

Most furnaces from the last few decades have a small LED on the control board, often visible through a little window in the lower panel. Its blink pattern says what the furnace thinks is wrong, and the decoding chart is usually printed inside the furnace door.

**Pro tip:** Read the code before you turn off the power or pull the panel. Cutting power can clear it, and then you've lost your best clue. Count the blinks and take a phone photo of the light and the chart. Telling us the code when you call often means the tech brings the right part.

Many furnaces try to light three times, then lock out and wait about an hour before trying again. You can reset a lockout once by turning the furnace switch off for 30 seconds and back on. If it locks out again, something needs fixing, and resetting it over and over won't help.

## Check 7: Is a high-efficiency furnace's condensate drain backed up?

If your furnace vents through two white PVC pipes instead of a metal flue, it's a 90%+ condensing furnace. It makes water, which drains through a trap and tube to a floor drain or a small condensate pump. When that drain clogs or the pump fails, a float switch shuts the furnace off, or water backs up and the pressure switch won't let it run. Look for:

- Water in the pump's reservoir that isn't pumping out, or a pump that got unplugged
- Water on the floor around the furnace
- A gurgling sound from inside the furnace cabinet

Make sure the pump is plugged in and the drain tube isn't blocked. Lines through a cold garage or along an outside wall can freeze in an Iowa cold snap. Also check the PVC intake and exhaust outside for snow, ice, leaves or a nest; a blocked vent trips the pressure switch too.

## When should you stop troubleshooting and call?

Stop and call if:

- You smell gas, or a carbon monoxide alarm goes off. Get everyone out first, then call 911 (or the gas utility, for a gas smell) from outside.
- The breaker trips again, or the furnace locks out again after one reset.
- The burners keep lighting and shutting off within seconds, often a dirty flame sensor or failing ignitor.
- The air is still cold after the furnace has run for 10 minutes.
- You hear a boom when it lights, smell hot plastic or electrical burning, or find water you can't trace.

| Common cause | Who fixes it | Typical cost |
|---|---|---|
| Thermostat setting or batteries | You | A pack of batteries |
| Tripped breaker, furnace switch or loose panel | You | Nothing |
| Clogged filter | You | A new filter |
| Dirty flame sensor or failed ignitor | Tech | Lower end of the $150–$800 furnace repair range |
| Inducer motor, control board or blower motor | Tech | Middle to upper end of $150–$800 |
| Calling after hours | Us | $75–$125 dispatch premium on top of standard pricing |

*Planning ranges for the Des Moines metro in 2026; your written quote depends on the equipment, access and what we find on site.*

Call us at (515) 494-5483 any time. We answer 24/7, and if it's single digits outside with no heat, don't wait for morning. The after-hours premium is a flat add-on, not double or triple rates. Until we arrive, never heat the house with the oven or stove, and open the cabinet doors under sinks on outside walls; our [frozen pipes guide](https://des-moines-plumbing-hvac.b-daniels.workers.dev/blog/frozen-pipes-iowa-prevention-thawing/) has more.

See our [furnace](https://des-moines-plumbing-hvac.b-daniels.workers.dev/services/furnace/) and [emergency](https://des-moines-plumbing-hvac.b-daniels.workers.dev/services/emergency/) service pages. If problems like these keep coming back, read [7 signs your furnace is dying](https://des-moines-plumbing-hvac.b-daniels.workers.dev/blog/7-signs-your-furnace-is-dying/), and if nobody has looked at it in a year, our [fall tune-up guide](https://des-moines-plumbing-hvac.b-daniels.workers.dev/blog/furnace-tune-up-des-moines/) explains what a real checkup catches.

## Frequently asked questions

### Why does my furnace turn on and then shut off after a few seconds?

The most common reason is a dirty flame sensor: the burners light, the control board can't confirm the flame, and it shuts the gas off for safety. A clogged filter or a blocked vent pipe can cause short cycling too.

### How do I reset my furnace?

Turn off the furnace power switch for about 30 seconds, then turn it back on. That clears most lockouts. If it locks out again, don't keep resetting it; call for a repair.

### Is a burning smell from the furnace normal?

A dusty smell for the first few minutes on the first cold day is normal as dust burns off. A smell that lingers, smells electrical or like hot plastic, or comes with smoke means shut it off at the switch and call.

### Why does my furnace blow cold air when the fan is set to On?

On the On setting the blower runs nonstop, including between heating cycles, so you feel room-temperature air. Set the fan to Auto and it will only run once the burners have warmed the furnace.

**Des Moines Plumbing & HVAC** · (515) 494-5483 (24/7 emergency; standard hours Mon–Sat 7am–7pm) · Polk County & Central Iowa