Why is Cold Air Blowing Out of Vents When the Heat is On?

The furnace is one of the most useful household appliances. When cold winter temperatures begin to drop, your heating system in Sacramento goes into overdrive. It works tirelessly to cozy up your home. Unfortunately, furnaces can develop problems over time, like every other household appliance. A faulty furnace may heat unevenly or stop heating at all.

Some furnace problems may cause your furnace to blow cold air. When you come home from work on a chilly evening, the last thing you need is a furnace that blows freezing air even when the heat is on. However, before you reach out to an HVAC expert for help, try to get to the root cause of the problem. If you successfully diagnose the issue, you can provide your HVAC contractor with vital information, helping speed up the repair process.

Here are some common reasons why cold air blows out of vents when the heat is on.

Dirty Air Filter

Your air filter is designed to catch dust, dander, and dirt. Over time, furnace filters can become clogged with contaminants. A clogged filter can do more harm than good. It restricts the airflow over your furnace’s heat exchanger, causing it to overheat. Your heating system in Sacramento then trips a high-limit switch, which causes the furnace burners to shut off. The built-in mechanism is designed to prevent damage to furnace heat exchangers.

A dirty air filter can force your system to work harder and longer, causing premature deterioration. As a result, you may have to replace your furnace sooner than expected. Also, a furnace with a dirty filter consumes more energy than usual.

Furnace owners should make replacing furnace filters a priority. HVAC experts recommend replacing furnace filters every 30-90 days. How frequently you need to replace your filter depends on factors such as whether you have pets, kids, and elderly adults in your family, the local climate, and if someone in your home has a seasonal allergies.

Pilot Light Problems 

A pilot light is a tiny gas light that burns to ignite the furnace burner. Some common causes of pilot light problems include a draft, a dirty valve, or a failing thermocouple. If you have an old gas furnace with a pilot light, make sure it is lit. If not, your burners will not light either. When this happens, no heat is produced.

Fixing a bad pilot light on your heater can be tricky. Especially when working with an old unit, pilot lights might not be as straightforward in their replacement.  If the flame goes out, something’s amiss. Your thermocouple may be failing and should be adjusted or replaced. This is a specialized task best left to the experts. If you think the piolet light on your heater might be at fault for the cold conditions, it is best to call in an expert.

If your pilot light does not come on, ensure the gas valve is switched on. If it is, presumably, your pilot light is dirty. Follow these steps to clean your pilot light:

  • Unplug the gas nozzle. Using a can of compressed air, loosen and remove any dirt particles around the nozzle and vacuum the surrounding areas.
  • Lightly clean the thermocouple with fine steel wool or emery cloth. Remove any loosened dirt particles that are still sticking to the thermocouple with a clean, soft cloth.
  • Turn the gas valve back on and follow the manufacturer’s instructions to relight the pilot light.

Clogged Condensate Line

If you notice water pooling around your furnace, probably your condensate line is blocked. When the condensate line of a furnace becomes clogged with dust, dirt, or mold, water backs up into it, tripping the overkill switch, which prevents the furnace from turning on.

A clogged condensate line can cause your system to freeze or overflow the drain pan. An overflowing drain pan can encourage microbial growth or damage your drywall or electrical wiring. The sooner you reach out to your contractor for help, the better.

Damaged Ductwork

Holes and cracks in ductwork allow cold air from the attic to enter the ducts. Check your ductwork for holes and cracks. If your ductwork is damaged, there is a possibility that it is working properly, but warm air is leaking out through holes and cracks, and as a result, your system is blowing cold air. Faulty or damaged ductwork can affect your system’s efficiency. Have a professional inspect and seal your ducts as soon as possible.

Faulty Flame Sensor

The flame sensor of a furnace is an important safety device that protects the furnace and helps prevent accidents. When a flame is ignited within a furnace, its flame sensor burns the natural gas to continue the heating cycle. If no flame is detected, the flame sensor automatically shuts off the furnace to prevent unburned natural gas from escaping into the system. Unfortunately, over time, your flame sensor can become damaged. If so, your contractor will clean or replace your flame sensor.

Incorrect Thermostat Setting

Check your thermostat to make sure it is set to “heat.” You will also want to switch your thermostat to “auto” to ensure that its fan only blows air when the system provides heat.

Bad Limit Switch

A malfunctioning limit switch can cause your blower to run continuously, preventing your furnace from completing a heat cycle. Check your filter and fan blades and give them a deep clean if they are dirty. Also, make sure your ducts are the correct size. If the problem persists after you clean your filter, in all probability, your limit switch has sustained irreversible damage and needs to be replaced.

South Placer Heating and Air is a leading furnace contractor in Sacramento. Our technicians have years of experience installing, maintaining, and repairing different heating systems. To make an appointment, call (916) 246-2089.