FURNACE DOESN'T HEAT – The gas furnace fails to provide any heat for the space.
POSSIBLE PROBLEM & DESCRIPTION
Thermostat Set Incorrectly
Many "no heat" calls, especially early in the heating season are the result of thermostats that are not set correctly. Make sure the thermostat system setting is set for either "Heat" or "Auto" and the desired temperature is set higher than the space temperature.
Loss of Power to the Furnace
Low voltage power for the thermostat and controls comes from the furnace. Make sure there is high voltage power to the furnace. Make sure breakers and any service disconnects are turned on.
Most newer furnaces are equipped with a blower door safety switch the "kills" power when the blower door is removed. Make sure the blower door is securely in place.
Only about 10% of the “No Heat” service calls we receive turn-out to be a problem with the thermostat. In most cases, the problem turns-out to be a malfunction somewhere in the furnace itself.
If you suspect a malfunctioning thermostat, we recommend having a qualified technician diagnose the problem.
No Pilot/Bad Thermocouple
Many older gas furnaces utilize a “standing pilot” (a small pilot flame that burns continuous inside the burner compartment).
Standing pilot furnaces require a thermocouple to ensure that the pilot is lit before allowing the main gas valve to open.
If a standing pilot furnace will not come on, check to see if the pilot is burning.
If the pilot flame goes out, you may try relighting it. If it will not stay lit, the most likely cause is a bad thermocouple.
Bad Vent Motor - Qualified Technician Recommended
Newer gas furnaces above 80% efficient use a vent motor (also known as a “combustion” or “inducer” motor) to force combustion air into the burners and promote proper combustion. The first step in the ignition sequence, after receiving a call for heat from the thermostat, is to start the vent motor.
If the vent motor hums but does not turn, it is most likely faulty.
If the vent motor fails to operate, a fan-proving pressure switch should keep the furnace from operating (see below).
Bad Pressure Switch - Qualified Technician Recommended
Before a gas furnace is allowed to run an ignition sequence, it must prove that the vent motor is operating. This is typically done by means of a negative-pressure-activated switch. This switch is designed to prove that the vent motor is operating and that the venting system is clear. Some furnaces use multiple pressure switches.
Normal reason for a pressure switch fault are a faulty vent motor, blocked or restricted vent or intake piping, or clogged drain lines (on 90% efficiency and higher models). However, some no-heat calls are the result of a faulty pressure switch that will sometimes stick closed or open.
A pressure switch fault will commonly generate either a code 2 or 3 on a furnace control board, but may vary depending on the make and model.
Bad Igniter/Glow Plug - Qualified Technician Recommended
Most newer gas furnaces utilize an automatic ignition system and a hot-surface igniter also referred to as a "glow plug".
On a call for heat, after the vent motor starts and the pressure switch proves proper venting, the furnace control board should power the hot-surface igniter.
After several seconds of being powered, the igniter should start to glow bright orange or yellow.
After an igniter warm-up period of 30 to 60 seconds, the control board should open the automatic gas valve to start gas flow to the burners.
If the gas valve opens without the igniter ever glowing, this typically indicates a bad igniter.
A bad igniter often results in a smell of natural gas from the furnace as the burners attempt to fire.
Dirty Flame Sensor - Qualified Technician Recommended
After a gas furnace opens the gas valve to allow gas into the combustion chamber, the furnace control board will monitor for proper ignition of the gas. This is accomplished by a flame sensor. If the flame sensor does not detect a flame within 6 seconds after opening the gas valve, the ignition cycle is terminated.
If the burners visibly ignite, but then go out after approximately 6 seconds, this may indicate a dirty flame sensor.
A dirty flame sensor will generally cause occasional ignition failures, but may allow the furnace to fire and function properly some of the time.
Blocked Vent Pipe
A blocked vent or intake pipe on a gas furnace should produce a pressure switch fault, but if only partially restricted, a furnace may run normally the majority of the time and only fail after long run cycles.
Common causes of a blocked vent pipe are external restrictions (such as material placed against the pipe outlets), internal restrictions (such as leaves, birds or bird nests inside the pipe), or water build-up caused by an inability to drain water from the vent (improperly sloped vent or clogged drain).
Blocked Drain Line
90% or higher efficient gas furnaces produce water when operating. This water must be drained out of and away from the furnace. If the drain line becomes clogged, the water will build-up in the furnace and eventually cause a pressure switch fault. A very common cause of blocked furnace drains during cold weather is freezing.
Bad Blower Motor - Qualified Technician Recommended
After the burners have ignited and proper ignition has been detected by the ignition control board, the next step in the heating cycle is starting of the blower motor.
This will be done either by a heat-activated switch or internal timing of a control board. In either instance, the blower motor should start within about 1 minute after ignition.
If the blower motor does not start (no air blowing out of the registers), it can be the result of the activating switch or the furnace control board, but often the problem is a bad blower motor.
If the blower motor hums but does not start, it is most likely a bad motor or a bad capacitor (below).
Bad Capacitor - Qualified Technician Recommended
Most direct-drive PSC fan motors require a capacitor which provides extra power for starting.
A fan motor with a bad capacitor will typically attempt to start but will fail while making a louder-than-normal humming noise.
After several seconds of humming, the motor will typically overheat and shut-off and then retry after about 30-60 seconds of cooling.
A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor.