Are you looking for help with projects around the house? Our blog offers helpful tips and DIY videos, or schedule a service appointment today.

Why Is My Gas Furnace Not Blowing Hot Air?

Gas furnace

With winter coming in hot — or cold for that matter — any issue with your gas furnace is a major concern. One problem homeowners encounter with their furnace heating system is cold air coming from vents when the heat is on. If temperatures are below freezing, it’s a dangerous situation if your furnace is not blowing hot air.

1. Thermostat Setting Is Wrong

To begin troubleshooting why your furnace is not blowing hot air, start with checking the thermostat setting. Make sure your thermostat is set higher than the temperature in your home. If your thermostat is powered by batteries, make sure your batteries aren’t dead.

Additionally, if your thermostat fan setting is switched to “ON,” then your furnace blower will constantly be operating even when it is not heating warm air for your house. The cold air blowing out of your vents can be attributed to the fan blowing because it is always on. This problem has an easy fix — simply switch the fan from “ON” to “AUTO.”

2. Clogged Air Filter

A clogged air filter blocks airflow from the furnace’s heat exchanger, causing it to overheat. When your furnace overheats, it can trip a high limit switch (similar to the way an overloaded electrical outlet trips the circuit breaker). This shuts off the furnace burners, so the heat exchanger doesn’t crack.

This could be the source of why there is cold air coming from vents when heat is on. To fix this issue, turn off your furnace at the thermostat, and open up the furnace filter. If the filter looks dirty, change it. At a minimum, you should change the furnace filter every three months. Consider calling a trained HVAC technician to help you replace the dirty furnace filter and reset the furnace.

3. Damaged Ducts

There are several components of a forced air heating system that could be potentially damaged, but air ducts are one aspect that could be leaking. If your furnace is functioning properly and some rooms in your home are warm, it might be a sign you need to replace your ductwork.

Ducts are used to distribute air throughout the house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts. This translates to hefty utility bills and dust being sucked up into the ducts. This leads to poor air quality in your home and being perpetually cold.

First check for handles protruding from the ducts and make sure the vents are fully open. If some rooms are still cold, then your ducts may have gaps, cracks, or holes in them. This would allow hot air to escape. Find where the leaks are in the ducts and seal them with a mastic sealant or call an HVAC professional.

4. Pilot Light Is Out

If your furnace is an older gas furnace, then it might have a pilot light. It’s easy to diagnose your pilot light problems. If the pilot light isn’t lit, then the furnace burners won’t ignite, which is why heat won’t be flowing from your furnace. If the pilot light won’t remain lit, check if gas is flowing into the furnace. Make sure the gas valve is parallel with the gas supply pipe, so gas can flow through. If the pilot light is dirty, clean it to allow gas to come through.

Alternatively, if the pilot light ignites, but won’t stay lit, then there might be an issue with the thermocouple. The thermocouple is the copper rod that acts a safety device. It might need to be replaced. If you don’t feel comfortable replacing it yourself then contact a furnace technician specialist.

Call John C. Flood for Furnace Maintenance & Repairs

Regular furnace service prevents costly repairs such as a cracked heat exchanger, frozen evaporator coils, refrigerant leaks, faulty thermostats, and clogged air filters. Paying a little bit for scheduled maintenance ensures you’ll have a healthy, smoothly operating furnace.

The warranties for modern furnaces often require an annual inspection and are more likely to break unexpectedly if you skip this maintenance check.

Schedule your annual furnace inspection with John C. Flood this year and experience our top-notch service and professionalism. We handle all sorts of home heating, cooling and plumbing issues and would love to turn your cold house into a warm home for the coming winter. Schedule service today!