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Is your furnace blowing cold air

In the dead of winter, there’s nothing quite like the instant relief you get when holding a cold hand over a warm heat vent. There’s also nothing like the disappointment of discovering your furnace blowing cold air inside when it’s turning cold outside. Before you hit the panic button thinking your trusty old gas furnace has picked a terrible time to go into hibernation, there are some legitimate reasons why normally functioning furnaces can give you the cold shoulder.

Incorrect thermostat settings... a wave of cool air from your heat vent before the warm air arrives... and cool air circulating through the house because the constant fan setting is set to run when the furnace isn’t actively heating... these are all common reasons for a heating system blowing cold air. Less commonly, if you have a duel-fuel system (heat pump/gas furnace combination) the heat pump will warm your home with a lower air temperature than provided by the gas furnace, which could make it seem like your system is blowing cold. Or, if you have a high efficiency, multi-stage furnace, it might be operating on low stage heating and you just aren’t getting that full-on blast of heat you normally expect.

Of course, for every reason a perfectly operating furnace is blowing cold air, there could also be a more serious issue lurking. If you’ve checked all of the easy-to-remedy reasons that can result in your furnace blowing cold air and you still haven’t solved the problem, it may be time to contact your local Carrier® indoor comfort expert to see if your furnace needs to be serviced, adjusted or replaced.

The Thermostat is Set Incorrectly

Symptoms: If a thermostat is set incorrectly, you might notice your furnace blowing cold air from the heat registers, a lower than expected temperature reading on the thermostat, or you may even hear your air conditioner running outside.

Details: There are a lot of options when it comes to thermostats, but they all control both heating and cooling. Programmable models take it a step further, allowing you to schedule temperature changes throughout the day, up or down, based on times you are typically home, at work, or in bed asleep. Wi-Fi® models offer remote access to change your settings from a computer or mobile device anywhere you have access to the internet. With all of the options available, there are a number of scenarios involving your thermostat that can result in your furnace blowing cold air.

Solution: If you experience any of these symptoms in your home, the first step is the check the thermostat and look for the obvious answers.

- Is it set for air conditioning instead of heating the air? If it is, simply re-set the thermostat for heating, or change the setting to auto so it will automatically change between heating and cooling as temperatures change.
- Is the fan set to run constantly, even when the system isn’t heating the air? If so, your system may be circulating cooler air between heating cycles. Changing the fan setting to auto will turn off the blower when the system is not actively heating.
- Check the temperature setting to make sure somebody else didn’t change it to a lower setting.
- If you have a programmable thermostat, review your “comfort schedule” for any issues, and make sure your programmed settings haven’t shifted times due to daylight savings time.

If you have made some of the described adjustments and your system does not respond, it may be time to contact your local Carrier HVAC expert for a professional analysis of your system’s operation.

The Heater Hasn’t Warmed Up Yet

Symptoms: You hear your furnace kick on... but there is no airflow or you find the furnace blowing cold air.

Details: While your first instinct might be to call your HVAC contractor, your furnace might be operating normally. Many furnace models include a fan limit switch with a function called “blower on delay”. This function allows the furnace to warm up the air before the blower pushes it out through the ductwork and into your living areas. This short delay is intended to keep you more comfortable by eliminating an initial blast of cold air from your heat vents.

Solution: Check the owner’s information for your furnace to see if there should be a blower delay after the burner kicks on. If, after a few minutes you still experience the furnace blowing cold air, you may need to contact your HVAC contractor.

The Pilot Light is Out

Symptoms: The furnace blowing cold air, or there is no heat in the house.

Details: Pilot lights used to be a standard component on any gas furnace before the 1990s. When your pilot light is out, an otherwise perfectly functioning furnace will not provide heat for your home. Because a pilot light is designed to burn continuously, it also uses more of your gas supply, which can be reflected in higher utility bills. Since the 1990s, the industry has shifted to using pilotless ignition systems such as hot surface igniters, so pilot-related issues are becoming less common. But if you have an older model furnace, your home is too cold and the furnace hasn’t turned on, your pilot light may be out.

Solution: Start by checking that your thermostat is set properly. Next, locate the pilot light assembly on your furnace and see if there is a flame. If the pilot light is out, follow the manufacturer’s directions on how to relight the pilot. If it won’t stay lit, it’s time to call a professional. And, if your furnace does have a pilot light ignition, it may be time to consider upgrading to a new, pilotless gas furnace. A new furnace can offer higher energy efficiency, new product reliability and warranties, and no issues with pilot lights blowing out.

The Fan Limit Switch Has Malfunctioned

Symptoms: If your furnace is blowing cold air, or is not operating at all, a component called the fan limit switch may have malfunctioned. You may also see an error message on your thermostat, or a blinking LED light on the furnace control board.

Details: The fan limit switch measures the air temperature inside the furnace or hot air supply plenum and controls blower motor operation. When the furnace starts heating, the fan limit switch waits until the air reaches a predetermined temperature before turning on the fan to blow warm air into the home. At the end of the heating process, it waits until the internal air temperature drops below its predetermined level, then shuts down the fan until the next heating cycle begins. If the fan limit switch malfunctions, the blower might continue to operate even when the furnace is not heating the air resulting in your furnace blowing cold air from your registers. Or, it may prevent the furnace from operating at all.

Solution: Check your air filter, and clean or replace if needed. A dirty air filter can restrict airflow, causing the heat exchanger to get too hot, and in turn causing the fan limit switch to shut down the furnace. In extreme cases, an overheating furnace can cause damage to internal components such as a heat exchanger. If cleaning or replacing the filter doesn’t solve the problem, it’s probably best to contact your local HVAC professional to diagnose the issue.

The Flame Sensor Has Malfunctioned

Symptoms: If the temperature in your home is cooler than the temperature you set on your thermostat, or if you hear your furnace start up and shut down quickly over and over, you might suspect a flame sensor has malfunctioned. If this is the case, you may also see an error message on your thermostat, or a flashing LED light on the furnace control board.

Details: The flame sensor’s job, as its name suggests, is to detect a flame from the burner. A properly working flame sensor will shut down the furnace if it does not detect a flame while the gas valve is operating. This helps to prevent natural gas (or propane) from continuing to flow into the home’s air supply. A malfunctioning flame sensor shuts down the furnace when the burner is working correctly (flames are present) because it does not sense the heat.

Solution: If you can see the burner in your furnace, you may be able to visually inspect the flame sensor for cracks in its porcelain base, or soot build up on its tip. Either of these would be a clue that a malfunctioning flame sensor could be causing your furnace to shut down. A dirty sensor can be simply cleaned by your local Carrier dealer for an inexpensive repair. Replacing the sensor should be a fairly easy fix for your contractor as well. Because a functioning flame sensor and a malfunctioning flame sensor can both result in a furnace shutting down, it may be best to contact an expert to determine why your system isn’t functioning properly.

A Dirty Filter Caused The Furnace to Overheat

Symptoms: If you notice the air temperature in your home is cooler than normal... if the airflow from your heat vents seems to be weaker than usual... and/or if you hear the furnace running for short periods of time before shutting down (short cycling), you may have a dirty air filter to blame.

Details: Whether you have a basic, 1-inch furnace filter or a more sophisticated electronic air cleaner or purifier, maintenance is the key to effective air filtration and proper furnace operation. Lack of maintenance can lead to build-up of dirt, dust and other airborne pollutants that can clog your filter, restrict airflow and lead to both discomfort or in more extreme cases, system failure.

Solution: Before checking your furnace filter, be sure the thermostat is set for heating and that the temperature setting is correct. If you still suspect the problem is a dirty air filter, consult your owner’s manual for instructions on removing and cleaning or replacing your air filter. Basic filters are often inside the furnace cabinet, but larger, 4-inch media filters, electronic air cleaners, and whole-home air purifiers are likely mounted outside the furnace between the cabinet and the return air duct. If cleaning or replacing the filter doesn’t solve the problem, contact your local Carrier dealer for assistance.

Too Many Closed Supply Vents Caused the Furnace to Overheat

Symptoms: If you notice a room is too cold or hot... inconsistent temperatures in different areas of your home... the temperature reading on your thermostat doesn’t match the temperature you set... or if your furnace is constantly turning on and then back off after a short time (short cycling), you may have too many closed vents.

Details: To force heated air into certain rooms, or to save a little energy, some homeowners may close the heat vents in rooms that go unused in the winter. However, furnaces are “sized” to efficiently deliver comfort to the entire home. Closing too many vents may cause the furnace to adjust by cycling on and off more frequently, which can reduce overall comfort in the house and can shorten the life of the furnace.

Solution: Try opening all of your heat vents and see if that solves the problem. If it does, and you still want to close a few vents, try closing fewer vents this time. If opening all of the vents doesn’t solve the problem, you may want to contact a local HVAC professional to evaluate your heating system and ductwork.

Air Ducts are Leaking Warm Air

Symptoms: If you notice your furnace blowing cold air... an increase in your utility bills... or can actually feel warm air coming from seals or joints in your ductwork... it’s time to investigate.

Details: Depending upon the age, condition and quality of the construction, your home’s ductwork may have some significant leakage. This is a common problem with homes in the United States. Leaky ductwork not only leads to reduced comfort, it can result in higher utility bills because of the amount of heat lost due to the leakage.

Solution: If you have exposed ductwork, typically in a basement or crawlspace, checking for leaks can be relatively easy. Simply turn your system on, and hold your hand over joints, connections, or anywhere you can visibly see a seal in the ductwork. The average homeowner can often make simple repairs by straightening sheet metal and securing sheet metal joints with screws, or by using an appropriately rated tape product or sealant. If you continue to have problems associated with duct leakage or your furnace blowing cold air, an HVAC professional can provide a more sophisticated analysis and duct sealing options for ducts that are not easily accessible.

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