Walking Into a No Heat Service Call on a Forced Air Furnace

by Peter Kirtschej

There are hundreds of different reasons for “no heats”.  I just wanted to share some of the most common that come to mind and possible resolutions.       

Upon arrival, listen to the homeowner.  They are paying for you to be there and trusting you in their home with the HVAC system. In some cases, the homeowner can give you some valuable clues in tracing the problem with the furnace.  They may have heard a noise or smelled something burning.  Maybe the furnace was short cycling.  After listening to the homeowner, I recommended inspecting the thermostat.  Does it take batteries? Is it flashing to indicate low batteries? Is there a call for heat?     

I always like to get a good visual before doing anything.  Inspect the furnace, look for water on the blower deck.  Is there any water under the furnace?  Pull the top door off and see if there is something out of the ordinary such as any wires disconnected.  Do you smell electrical burning?  Is there water staining?  Maybe a critter made its way inside.

If nothing obvious is found, I would then look at the sequence of operation. We know we had a call for heat at the thermostat.  If our inducer motor is running, then we know that the control board has a call for heat.  But let's consider, for example, if nothing was happening.

Verify that you have 24 volts between R and W terminals on the control board.  If you do not, locate the transformer and see if you have 120 volts coming in to it. Using your multimeter, look for 120 volts AC between the black and white wires.

If you do not have voltage, then you need to locate were the line voltage has been lost, working your way back to the panel box.  If you do have voltage on the primary side of the transformer, then check and see if you have voltage on the secondary side (you will be looking for 24 volts).  Normally, I see between 24 and 27 volts AC. If you do not, then the transformer has failed and needs to be replaced.

If you have 24 volts and the board is not responding, then look for a blown fuse.  Some control boards have a 3 amp fuse on the low voltage side protecting the board from shorts.  If you find a blown fuse, something has definitely shorted out or overdrawn the circuit.  Disconnect the thermostat wires from the control board.  In place of the fuse, install a resettable fuse (mini breaker).  Turn power on to the furnace and see if the fuse trips.  If it does not, then the short is in the wire or thermostat. If the fuse trips, then we need to look at the low voltage side of the furnace circuit. 

Most furnaces use low voltage safeties.  Locate that plug on the control board and unplug that harness.  Using your multimeter, see if you have a connection between cabinet ground and one of those wires.  If you do, trace the wire up and locate the short.  Replace or repair the wire.  There is also the chance that one of the safety switches could be shorted out.  I have seen the main limits short against the metal where they get mounted.  Sometimes, the insulation wears away on the legs causing a direct short to ground.  At this point, the limit needs to be replaced.

What if you checked the low voltage safety circuit and found no shorts?  With the harness unplugged, power the control board.  You may find that the board itself has a short in it. If that is the case, replace the control board.

If the inducer was running and the burners are not coming on, using a manometer hooked up between the pressure switch port on the inducer and the pressure switch, we need to see what the negative pressure is.  We check this using Inches of Water Column (inwc). Let's say the pressure switch rating is .60 inwc and the manometer is reading 1.10 inwc.  We know we have more than enough to close the switch, therefore, we have a bad switch that needs to be replaced.  Or let's say we are only pulling in .50 inwc, this means we could have a plugged vent, or a compromised heat exchanger, bad seal on the inducer, weak inducer, maybe a weak capacitor. At this point, I would try and remove the vent off the furnace and see if the pressure switch closes.  If it does, then we most likely have a plugged vent or possibly a sagging pipe that has water pocketing. 

Inspect the vent for proper pitch back to the furnace. We are looking for 1/4 inch per foot on the vent pipe. If you pulled the vent off, and the manometer reading did not change, then I would check what the voltage to the inducer was, making sure you had 120 volts AC.  If the inducer has a capacitor wired to it, check the capacitor and make sure that it has the right amount of microfarads. This can be done with a multimeter.  Make sure you pull the wires off the capacitor.  Next, set your meter to MFD and hold one lead from the meter to one side of the capacitor wiretap and the other to the other side of the capacitor wiretap. The reading you have should be plus or minus 6% of the rating of the cap. If it is below that, then we need to replace the capacitor.  If the capacitor looks good, then I would inspect the seal on the inducer. 

Look for moisture leaking out between the inducer and collector box.  Also look at the seal of the collector box. Most furnaces use a restrictor plate or orifice on the faceplate of the heat exchanger or collector box where the inducer motor opening meets the exchanger opening.  If this has deteriorated, the velocity has been slowed down, so now the inducer motor cannot pull in enough inches of water column.  If this is the case, then the collector box needs to be replaced or the heat exchanger if it is all one piece.  If the heat exchanger is cracked or comprised in any way, this would also cause the negative pressure being pulled in through. If the heat exchanger is compromised in any way it needs to be replaced.

Let's say you arrive and have a high limit code.  This is an indication that the furnace heat exchanger reached a high enough temperature that tripped the main limit. The main limit is a switch that senses the temperature of the heat exchanger.  When it is tripped, voltage to the gas valve gets interrupted and the blower motor circulates air across the heat exchanger cooling it enough to reset the switch.  If the furnace filter is plugged, it will cause the furnace to overheat.  Maybe someone rearranged the living room and covered a main return vent or maybe the gas pressure is set to high.  

The natural gas furnace is looking for 3.5 inches of water column on the manifold side of the gas valve.  We check this using a manometer.  Propane furnaces are looking for 10.5 inches of water column on the manifold side.  Is there an evaporator coil on top of the furnace?  Is it possible that it is plugged?  If the filter does not have a good seal around the cabinet or the furnace and/or the buffalo boot, it will allow dirt and dust to pass by and dirty the coil.

It is also possible that the furnace may be too large for the home.  If the duct work is too small on either the return or supply side, the furnace will run too hot and short cycle.  If this is suspected, then we can check the temperature rise of the furnace.  This is done by obtaining the temperature of the supply about a foot above the furnace and subtracting the return air temperature entering the furnace.  This reading can be obtained on the return air drop. The difference between these temperatures is called “temperature rise”.  Most furnaces have a temperature rise listed on the name plate of that furnace in which it was designed to operate in.

Or, let's say you arrive and the furnace has failed flashing, the main limit code and the blower motor is not running.  I would check and see if the blower motor is getting voltage from the control board, using your multimeter test, and see if the blower motor leads are receiving 120 volts.  If you find that you have voltage and the motor does not turn, it has most likely failed.  Carefully check and see if the motor is hot to the touch.  Most likely you will be able to fry an egg on it. There is also the chance that the capacitor failed, but if the motor has been trying to start for a while and the cap failed, the motor windings have cooked and both will need to be replaced.  If the control board did not send voltage to the blower motor, then the board has failed and needs to be replaced.

A roll out is a safety switch that is designed to break voltage in the low voltage circuit.  It is based on temperature. The larger the furnace, the more roll outs it has.  These switches are usually found in the burner compartment.  If the flame rolls out of the heat exchanger the switch should trip.  Or, if the area around the burner is too hot, this will also trip a roll out. If you find that a rollout has tripped, I would get a temperature reading next to the roll out that is suspect. See if the temperature of that area is higher than the limit.  If it is not, then the limit could simply be failing.  I would also start the furnace numerous times making sure the flame is not rolling out.  

One possible cause for a roll out is if the draft inducer is not working properly.  In other words, if the draft inducer is supposed to run 1500 rpm but only runs 900 rpm, the flame could bounce back.  Because we have no tool in our industry that tells us what the rpm of the inducer is, we check amp draws and use a manometer to see what the inducer is pulling in.

Another cause for a roll out is delayed ignition.  If the gas valve opens and fills the burner compartment with fuel before igniter is energized, we will definitely have flame roll out.  If this is the case, the control board timing is off and needs to be replaced.  

Lazy spark could also cause flame roll out. Maybe the electrode needs to be cleaned or replaced.  If the gap is wrong, this could cause improper ignition. Rheem furnaces are looking for 3/16 gap on the igniter.  If the gas is not being spread out evenly across the burners, once the fuel makes its way in front of the ignition source it will most likely roll out.  If this is the case, then the burners need to be cleaned or they could be rusted out.  Inside the front of the burner there is a small area that carries over fuel.  If this area is dirty or plugged, you will not have a good ignition.

A compromised heat exchanger can also create a flame roll out.  The inducer motor has two jobs; it pushes exhaust gasses out of the heat exchanger but it also pulls the flame through the heat exchanger.  If the heat exchanger has a crack and the vent switch closes because there is just enough negative pressure to close the pressure switch, the heat exchanger will gradually heat up and then crack open.   If you suspect the heat exchanger has failed, you need to use a combustion analyzer and check the co level in the flue and the air stream.  Cracks cannot be fixed.  The heat exchanger would need to be replaced or the furnace.  If the hot surface igniter is weak, this too could potentially cause rough ignition. Rheems HSI igniters are good between 70 ohms and 120 ohms.

What if you arrive and the burners light but do not stay on for longer than 20 seconds?  On Rheem furnaces, there is an amber indicator light that will illuminate if the control board senses flame.  If the light blinks, that is an indication that the control board senses low flame sense.  Rheem control boards are looking for at least 1.5 micro amps DC.  If the reading is between .5 and 1.5 micro amps DC signal, then the control board flashes the amber light to let us know.  If there is less than .5 micro amp DC, then the control board stops voltage to the gas valve and no flame signal code will be displayed.

So, let's say that you clean the flame sensor and the board still reads low flame sense.  I would start looking at grounds and neutrals.  Making sure that we have a good connection to chassis ground and panel box.  One may wonder what grounding has to do with this.  Flame rectification is when an AC voltage is converted to a micro amp DC signal through the path of flame to ground. Therefore, the flame sense wire will always have voltage on it when the control board has voltage.  Usually between 70 to 110 volts can be found on this wire.        

Or, let's say the igniter is lighting up and there is no flame.  Check and see if you have 24 volts on the gas valve.  If you do and you are sure that you have gas pressure, the gas valve needs to be replaced.  On most furnaces there is a safety switch that breaks voltage to the gas valve.  If you do not have voltage to the gas valve, start working your way back to where you lost voltage.