Cold air blowing from your HVAC vents is nice on a hot, summer day, but in the winter, it is a clear sign that something is wrong.

If your furnace is on but the temperature in your home is steadily dropping, check the following before calling a pro to diagnose and fix the situation.

Furnace Humming, But Not Warming?

It sounds obvious, but the first thing you want to check is that your thermostat is set to “heat.” Someone may have accidentally set it to cool.

Check that the thermostat’s fan setting is on “auto,” not “on.” If the fan is set to “on” the blower will run nonstop, even when the burner is off. Make sure the fan setting is on “auto.”

The thermostat is also a good way to tell if the air is in fact warming up the house or not. If you just came in from a jog or your body temperature is significantly higher than the air coming from your vents, it may feel cold. Check your thermostat to determine whether or not the temperature is actually rising or falling.

Do You Have a Heat Pump?

If you have a heat pump, you may simply be experiencing the heat pump’s defrost cycle. In the winter, your heat pump pulls heat from the outdoor environment and brings it inside. In order to draw in heat energy, the coils have to be colder than the temperature outside. This can produce frost and ice on the unit. In order to defrost, the heat pump reverses its refrigeration cycle to warm up the coils and thaw the unit.

During winter, your heat pump may enter defrost mode every 30, 60, or 90 minutes. This is completely normal and you may feel temporary cold air coming from your vents as result. If, however, the defrost cycle lasts more than 15 minutes or the heat pump remains iced over, you may have a problem. Contact your local HVAC technician for more information.

Vents and Registers

If your furnace is working and blowing warm air, but you just aren’t feeling it, you may have closed or obstructed HVAC vents/registers.  Check your home for any blocked or obstructed vents and registers.

You may also have excessively leaky or damaged ductwork. You can check if your ductwork is leaking by holding up a lit piece of incense or thin piece of toilet paper to your duct connections (when the HVAC system is off). If you see any erratic movement, you have a leak.

Contact your local HVAC company to seal (and clean) your ductwork. We highly recommend scheduling professional duct cleanings every 7-10 years or so.


While unlikely, a dirty filter could cause your furnace to overheat. When the furnace overheats, a safety mechanism will turn off the burners. This may cause air to flow through your system even if the burners are off. Replace the air filter if it is dirty. Dirty filters are also very bad for your indoor air quality, which is even worse during winter months.

Pilot Light and Burners

The air is heated in the heat exchanger, which gets its heat from the hot flames in your burners. If your pilot light is not working, the pilot sensor safety mechanisms turns off the gas flow.

Check your furnace to make sure the pilot light is on. In order to light the pilot light, carefully follow the instructions in your owner’s manual. It is likely hanging on the side of your furnace housing.

If you don’t feel confident relighting the pilot, contact a professional. This isn’t something you want to take a guess at.

Gas Valve

Make sure the gas valve hasn’t accidentally been turned off. If you have a handle valve, turn it parallel to the pipe to open the valve (perpendicular turns it off).

Extreme Weather Conditions

Sometimes, if the weather is cold or bad enough, problems can develop with your outdoor gas meter. If the gas meter starts to malfunction, gas flow into your home can be cut off or impeded.

Contact your local HVAC or gas company or if you suspect a problem with your gas meter.

One last thing you can do is reset the furnace. Follow the manufacturer’s instructions (usually located on the side of the furnace housing). This usually entails turning off the power switch and waiting around 10 minutes before turning it on again.

If your furnace still won’t blow warm air, contact the experts at On-Time Service.


Give OnTime Service a call to schedule your next electrical, plumbing, and HVAC appointment: (205) 942-1405

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