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AFUE DEFINITION, RATINGS
AGE of AIR CONDITIONERS & HEAT PUMPS
AGE of HEATERS, BOILERS, FURNACES
AIR CONDITIONING & HEAT PUMP SYSTEMS
AIR FILTERS for HVAC SYSTEMS
BACKDRAFTING HEATING EQUIPMENT
BLOWER FAN OPERATION & TESTING
BLUE vs YELLOW COMBUSTION FLAMES
CAPACITORS for HARD STARTING MOTORS
CARBON MONOXIDE - CO
CHIMNEY INSPECTION DIAGNOSIS REPAIR
COOL OFF HEAT, THERMOSTAT SWITCH
CONDENSING BOILERS/FURNACES DAMAGE
CONVECTOR HEATERS - HYDRONIC COILS
DEFINITION of HEATING & COOLING TERMS
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DRAFT HOOD, GAS HEATER
DRAFT REGULATOR, DAMPER, BOOSTER
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON THERMOSTAT SWITCH
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FIRE SAFETY CONTROLS
FLOODED HEATING EQUIPMENT REPAIR
FLUE VENT CONNECTORS
FURNACE CONTROLS & SWITCHES
FURNACE EFFICIENCY, HIGH vs MID
FURNACE OPERATING TEMPERATURES
GAS BURNER FLAME & NOISE DEFECTS
GAS PIPING, VALVES, CONTROLS
GEOTHERMAL HEATING SYSTEMS
HEAT EXCHANGER LEAK TEST
HEAT PUMPS, DIAGNOSIS, REPAIR
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SMALL LOADS
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
HIGH EFFICIENCY BOILERS/FURNACES
GAS LP & NATURAL GAS SAFETY HAZARDS
MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH
NO HEAT - FURNACE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, DUCT VIBRATION DAMPENERS
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
PULSE COMBUSTION HEATERS
RESET SWITCH - HEAT CONTROL
RESET SWITCH, ELECTRIC MOTOR
RESET SWITCH, STACK RELAY
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SPILL SWITCH, FLUE GAS DETECTOR
SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS
STAIN DIAGNOSIS on BUILDING INTERIORS
THERMOSTATS, HEATING / COOLING
Furnace blows cold air: this article describes what to check if the warm air heating blower fan delivers intermittent cold air into the occupied space, or if the furnace cycles between blowing warm air & cool or even cold air into the building.
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Question: forced air heating furnace blows COLD, HOT, COLD, then OFF - what's up with its fan limit control?
Two story 75 year old classic cape cod home. Gas forced air furnace is Lennox G16 24 years old, in good condition. With Honeywell fan limit switch, outside fresh air intake and draft motor. We have owned home 12 years. (has AC). Owner not a technician but understands mechanical/heating concepts.
Furnace operating problem
When heat called for and fan comes on it blows cold air, then heat, then cold, then off. It has always done this. Limit switch is at off 100, on 125 off 150.
Changing set points on limit switch does not seem to change pattern. I do not dare force it. This winter local utility provider replaced burned out fan motor with same HP, number of speeds, etc. Since the draft motor was noisy we replaced that as well. Upon dissection of draft motor is was covered with white crusty minerals probably from the water from the humidifier mounted on the nearby cold air return. When we first owned this house we have removed the humidifier (and installed a whole house water filter).
The fan limit switch sensor which protrudes into the burn area is crusted and therefore "locked" into a former position and now un adjustable. What do you think? - J.C. 4/15/2013
Reply: stuff to check when the fan is not turning on and off at reasonable temperatures
Thanks for the interesting question and the excellent photo. Unfortunately the photo you sent had a virus - we had to clean it up so what we display here is a bit different and at at lower resolution that your version. I don't know what's happening with your heating system but I can, from your note make a few diagnostic suggestions:
Watch out: you are quite right not to force the set point adjustments on a fan limit switch. Doing so can make the heating system unsafe, even risking overheating, heat exchanger damage, and fatal carbon monoxide poisoning of building occupants.
I like Dan Holohan's expression "Keep your hands in your pockets" - my mom used to make me sit on my own hands. (No not recently.)
Take a look at the duct system itself for things that can make it cool down quickly blowing cold air sooner than one might expect, such as a long uninsulated run, missing insulation, air leaks at the return side.
Check that the air filter is clean and that the ductwork is not obstructed. (This is just on general principle).
Now about that pesky fan limit control, you could
Keep us posted - what you learn will help other readers diagnosing a fan limit switch problem with their own furnace.
Other Causes for a Heating Furnace to Blow Cool or Cold Air into the Occupied Spaces
Check for air duct leaks
Check for an air duct leak, particularly, leaks from cold areas such as an un-heated crawl space or basement into the return air duct system. Such leaks not only increase heating costs unnecessarily, they also are unsafe.
Check for deliberate introduction of cold air into the HVAC system
In some buildings we like to run the blower fan continuously for more even heat distribution and improved air filtration - improvements to building indoor air quality. But in some buildings where heating ducts run through cold spaces and are not well insulated, or in installations in which the heating system is inducing cold outdoor air into the duct system, when the furnace burners are off the system will indeed blow cold air onto building occupants.
Check the following two switches to make sure that the furnace blower fan is not simply set to run continuously:
If you need to introduce outdoor air into a building for air quality reasons, consider an air to air heat exchange system to save on heating costs. Check out
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