Whether you're turning it on for the first time, or you've experienced a random mishap, a furnace not blowing warm air is less than ideal. This is especially true if you're in the middle of the winter months. If your furnace isn't blowing hot air, or the furnace is blowing warm air, but not hot — you're still in need of some troubleshooting and a fix. It's important to address this issue, not just for the comfort of your home but the safety of your family and longevity of your HVAC equipment! Here are 9 potential reasons why a furnace is not blowing hot air:
Thermostat Setting Is Wrong
To begin troubleshooting why your furnace is not blowing hot air, start with checking the thermostat setting. Start with making 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 pushing out 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.”
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 safety mechanism could be the reason a furnace is not blowing warm air.
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. The importance of changing your HVAC filters regularly can't be overstated.
There are several components of a forced air heating system that could be potentially damaged, but air ducts is 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.
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 the furnace isn't blowing hot air. 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 as a safety device. It might need to be replaced. If you don’t feel comfortable replacing it yourself then contact a furnace technician specialist.
Not Enough Gas
Remember the batteries in the thermostat that needed a second glance? If your furnace doesn't have enough gas to do its job, this scenario could result in the furnace blowing hot air then cold all of a sudden when it's hit its limit. Additionally, not having enough gas is a safety hazard, similar to the overheating we mentioned before. If your furnace doesn't have enough gas, the system may shut down as a safety measure.
A Too-Small or Too-Big Heater
Are you a new homeowner? It's possible that the furnace isn't the ideal size for your home. If it's too small (or too large), you can run into problems with effectiveness and efficiency. If your system is too small, you may experience cool air coming out because it can't get a good read on the internal temperature of your home. Understanding the right size furnace is an important step to take — especially before the colder months.
It's Just Been Turned On
This is the simplest of the potential reasons why a furnace is not blowing hot air: give it a minute. It takes time for the furnace to kick into gear, get a read on the internal temperature of your home and adjust accordingly! If you're heating your home with efficiency in mind, you don't have your fan constantly blowing. So give it a few extra minutes to get acclimated before sounding the alarm.
Understand the basics of your HVAC system: the burner is the space where the fuel mixes with air and subsequently heats your home. Some systems operate with one burner, while others have multiple. If your furnace is not blowing hot air, you could have a standard HVAC maintenance issue on your hands with clogged burners subject to dirt and debris. Like many components of your HVAC system, routine maintenance and cleaning improve effectiveness, efficiency and longevity of your equipment.
Blower Motor Panel Isn't Closed
Another great safety mechanism your furnace may have: not operating if the blower motor panel door isn't closed properly. If some maintenance has been done recently, or maybe you've inspected your HVAC equipment for another reason, make sure the furnace blower panel is securely shut.
Regular furnace maintenance keeps you ahead of potential issues
A furnace not blowing hot air can be an easy fix — or a more complex one. When more intense issues go unchecked, they can be damaging to your system and expensive. 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. Make sure to know what to expect with an HVAC inspection.