Sometimes Doesn’t Heat
FURNACE SOMETIMES DOESN’T HEAT – In this situation, a gas furnace will heat most of the time, but every once in a while will stop operating. There are several issues that can cause this – most require the services of a qualified service technician.
This situation is often caused by a furnace control board that recognizes a fault condition and puts the furnace into a "lockout" mode, which keeps the furnace from operating.
A "lockout" should be indicated by some type of flashing (usually red) light on the control board itself.
Most lockouts can be reset by moving the thermostat system switch to the "Off" position for several seconds and then back to "Heat" or "Auto".
POSSIBLE PROBLEM & DESCRIPTION
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 will allow the furnace to fire and function properly the majority of the time.
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.
It is not uncommon for a failing motor to start most of the time and only sometimes not start.
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.
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 slow drain may show no symptoms during mild weather conditions and only cause a problem during periods of long run-cycles.
A common cause of blocked furnace drains during very 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.
It is not uncommon for a failing motor to start the majority of the time and only sometimes fail to start. If the blower motor hums but does not start, it is most likely bad or has 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.
Sometimes a motor will still be able to start (with difficulty) with a bad capacitor.
A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor.
Clogged Air Filter
A dirty or clogged air filter will prevent the furnace from delivering the proper amount of heated air to the space and will often cause the furnace to cycle on and off on its over-temperature device.
A dirty air filter will commonly not cause noticeable problems during mild weather conditions, but can cause heating failures during periods of long run-cycles.
A clogged air filter will generally be identified by a weak flow of air from the registers that is very warm or hot.
If these symptoms are identified, try removing or changing the filter to see if the conditions improve.
Restricted Air Flow
Similar to a clogged air filter, restricted airflow will also prevent the furnace from delivering the proper amount of heated air to the space and will often cause the furnace to cycle on and off on its over-temperature device.
Restricted airflow will commonly not cause noticeable problems during mild weather conditions, but can cause frequent heating failures during periods of long run-cycles.
Restricted airflow will generally be identified by air from the registers that is very warm or hot.
Common causes of restricted airflow are dirty or clogged air conditioner coils, too many closed or covered registers, or bad / mis-sized ductwork.