| TROUBLESHOOTING GAS FURNACES |
| "Your comfort is important to us" |
| CALL 317-753-0279 |
| Troubleshooting Gas Furnaces To troubleshoot anything, it helps a lot to know how it works. In simplest terms the gas furnace heats the house by burning natural or propane gas inside a heat exchanger and a blower pushes air from the house over the heat exchanger to warm the air and then deliver it to the house. So the two general parts of the furnace are the burner section and the blower and blower section. Here the specifics and in the order in which events happens in most modern gas furnaces made within the last 20 years. This type of furnace uses a hot surface ignition system known as HSI. Sequence of Events 1. House thermostat calls for heat (house temperature drops causing thermostat switch to close) 2. That action turns on the electronic circuit board or ignition board inside the furnace and it then tells the combustion fan motor to come on. 3. When the combustion fan motor comes on, it sucks air in one end, and then out of the other end of the combustion chamber/heat exchanger. This causes an air proving switch (a pressure actuated diaphragm switch) to close sending a signal back to circuit board saying it’s okay to continue the ignition process. 4. The board then tells the HSI ignitor to come on. 5. About 7 seconds later the board tells the combination gas valve to come on and when the valve opens it send gas down to the burner where the glowing hot ignitor lights the gas. 6. A sensor that is positioned in the flame confirms there is flame present and tells the board it is okay to continue combustion. If the sensor did not feel the flame then it would tell the board to stop with combustion and then shut off the flow of gas. 7. The heat from the burned gas heats up the heat exchanger and then the board sends a signal to the blower motor to start. Blower motor comes and delivers warm hot thru the ducts to the house. 8. When the house warms the thermostat is satisfied and it opens shutting off the burners in the furnace. The blower continues to run until the heat exchanger is cooled and then the blower shuts off. Combustion Fan Not Working: With this sequence of events it is easy then to see where a failure occurs by observing how far along does the furnace go in the sequence before it quits. For instance if the combustion fan doesn’t come on in step 2 then we need to go back to the circuit board or the thermostat to find the culprit. If you place a jumper wire across R and W on either the thermostat or the terminal connection block inside the furnace and the furnace comes on then the thermostat is bad and needs to be replaced. If nothing happens then it could be two possibilities. One possibility is that either the control low voltage or high voltage isn’t present. Fuses could be blown. If the fan motor won’t come on when you place the thermostat fan switch to the ‘on’ position, then you need to check to see if there’s high voltage present as the furnace. Check the 15 amp fuse in the disconnect switch on the side of the furnace or the circuit breaker in the distribution panel that feeds the furnace. A burn mark in the fuse window or a tripped breaker would indicate a problem. Note the furnace may have a short circuit to ground if the fuse is blown or the circuit breaker is tripped. If you reset the breaker or replace the fuse and either trips again then that is the case and that should be figured out before going further. A non contact voltage sensor held next to the black wire inside the furnace or placed inside the opening left when you unscrew the disconnect fuse on the side of the furnace should tell you if there’s voltage present. If the voltage sensor glows bright then you should be okay. Next check the low voltage 3A or 5A fuse found on the circuit board. If you can’t see the ‘S or Z’ shaped element inside the clear plastic of the fuse or you see a burnt mark on the fuse then it is time to replace fuse. Note of caution here; if the fuse is blown there might still be present a ‘short’ in the low voltage circuit. If you place a fuse back in and it blows immediately then that is the case and the low voltage circuit need to be troubleshot to find the cause of the short and then fixed before placing anymore fuses in. The fuses are inexpensive so blowing one is not that big a deal. So if you confirm you have voltage then the only other problem it could be is malfunctioning circuit board because you have both low and high voltages and the with jumper on R and W you have eliminated the thermostat as the problem. Ignitor Doesn’t Glow: If the combustion fan is working and the HSI ignitor doesn’t glow then we have to go back a step or two to determine the cause. Let’s do an easy check before we go any further to see if the ignitor is good. Using a non contact voltage sensor check the wires feeding directly to the ignitor to see if there is voltage present. Be patient. The voltage will only be present for about 7-15 sec so don’t jump to conclusions. If you don’t have a voltage sensor but you have a voltmeter then disconnect the wires from the ignitor and check for the 120 volts. If your meter or sensor indicates 120 volts then the ignitor is bad. You can confirm that with an ohmmeter if you have one by checking continuity. If it reads ‘infinite’ then it is open and needs to be changed. There also might be burn marks or missing sections. In any case replace the ignitor. Be careful not to touch the surface of the ignitor when replacing as your skin oils will cause it fail early. If there is no 120 v present at the ignitor then it is either the circuit board or the air proving switch which is not working. Let’s check the air proving switch first. The air proving switch or air pressure switch has to close in order to send a signal to the board to continue ignition process. There are a couple of quick checks you can do at this point to confirm the circuit board is working. You could place a jumper across the switch terminals or suck on the small plastic or rubber tubing that is connected to the switch. The switch may not close for two reasons. If there is blockage in either the small rubber or plastic tube that connects to the switch then it cannot sense the pressure the fan is creating. Sometimes there is water present that has accumulated from either water vapor condensing inside the tube. Water could also be present because drain back up inside a condensing type furnace. If you remove the tube and blow or suck thru it and water comes out then that might clear up that problem. Now you have to address the furnace condensate drain problem. The tube also can have a leak in it so that the correct pressure never gets to the switch. If that is the case then replace or repair the tubing. If the tubing is not the issue then there might be a clog or obstruction in the flue. Obstruction can be caused by animals enter the flue discharge opening and either dying or building nests there. The opening can also be clogged with ice or snow. If the furnace brings its’ combustion air from outside there may be a clog or obstruction in that intake opening as well. It is also possible there is insufficient combustion air available from inside especially if the furnace is installed in a closet and grilles openings have been blocked. Inspect the furnace intakes and exhaust and clear any obstructions. Now if the both the tubing and flue and combustion air paths are clear and the switch still doesn’t operate then place a jumper on the switch terminals and then if the ignitor glows then the switch needs replacing. Ignitor Glows Gas Doesn’t Ignite: This problem has at least four causes. The gas valve is inoperative, the board is malfunctioning, the ignitor is mispositioned, or the there is a blockage in the spud orifice on the manifold feeding the burner with the ignitor. To check the gas valve operation measure the terminals of the gas valve to see if there is 24volts present when the ignitor is glowing. If there is and you don’t hear a ‘click’ sound coming from the gas valve then the solenoid of the gas valve is open and the gas valve needs to be replaced. If there’s no 24 volts present then the circuit board needs replaced. If you smell gas when the ignitor is glowing or just after it glows then the gas valve and orifice are okay and you need to adjust the position of the ignitor by bending it’s mounting bracket. Try to get it closer to middle of the burner. If there’s 24 volts and it clicks and it still doesn’t fire then we need to check the spud orifice to see if there is a clog (possibly caused by a spider egg). This is easily fixed by unscrewing the spud from the manifold and then clearing the clog with a small instrument. Gas Lights And Then Goes Out: There are three possible causes for this problem; the thermostat heat anticipator, the flame sensor, or the circuit board. An easy fix of this problem is to clean the flame sensor. Remove it from the furnace and clean it with steel wool. Put it back and if it lights then great. If that doesn’t work then you might need to replace it if the ceramic insulator looks cracked. If it still doesn’t stay lit then it is probably the circuit board. If you have a mercury bulb thermostat then it might have a heat anticipator that is set incorrectly. It should be set to the amp draw of the ‘W’ or heat circuit on call for heat. You can measure this with a digital clamp on ammeter. Whatever amp reading you get then adjust the heat anticipator slide adjustment to point to that number on it’s scale. If it is too far one way or the other from that setting then the furnace will either cycle to quickly or the temperature of the house will overshoot. Note if you have a digital or electronic thermostat there is no heat anticipator. The heat anticipation setting is automatically felt and adjusted by the electronics. Main Blower or Fan Doesn’t Come On: This has several causes including circuit board, blower motor, or blower motor capacitor. Let's check the voltage coming from the circuit board to the blower motor. If there’s 120 volts present then the problem is with the motor. If there’s no voltage then the board is bad and needs replaced. If there is voltage then check the capacitor first. Easy way to do that is replace capacitor with a same size capacitor. If the motor starts with new capacitor then that’s that. If not then try spinning the motor. If the wheel is hard to spin check to see if there is an obstruction and if there is no removable obstruction then replace motor. Flame Quits and Blower Keeps Running: Burner has shut off on one of its’ safety switches; the limit switch, flame roll-out, upper limit switch, or flue limit switch. Limit Switch: If the filter is dirty, or the blower wheel cups are full of dirt, or the cooling coil or condensing coil is matted with dirt, or return grilles are physically blocked or there is some other obstruction in the ductwork then it will cause the discharge air of the furnace to be hotter than normal (more than 140 degrees). This will cause the main limit switch to open and kill the burners to keep the heat exchanger from getting too hot. A malfunctioning blower will also cause the limit switch to trip. The limit switch is usually in the middle of the furnace above the burners with two wires going to it. If you jumper the switch and the burners work then make sure there is no airflow issues like above or correct the airflow issues and then retry without the jumper. If that doesn’t work then replace the limit switch. Make sure you put in the correct temperature switch that you took out. Flame Roll-out: If the flame jumps out of the burners or there is a partial blockage of the flue then there are small disk shaped sensors mounted on the side of the burner assembly that will sense the heat and shut off the burners. Flame going where it shouldn’t is caused either by a hole or crack in the heat exchanger or a blockage in the flue pipe. Correct the problem and then press the small button in the center of the flame rollout to reset it. If that doesn’t work then replace the flame rollout. Upper Limit: If the furnace is a downflow type (blower on top) then you will find another limit in the upper section of the blower housing. Most of the time this switch is a manual reset type. The same problems that cause the limit switch trip cause the upper limit to trip Flue Limit Switch: This switch is similar to the flame rollout except it is located near and on the flue pipe before it leaves the furnace. You will find the switch mostly on 80% or standard low efficiency furnaces but some of the condensing furnaces have them mounted on the body of the combustion blower housing. If it trips then there is some problem with venting that is causing the hot flue gas to back up and then tripping the switch. Correct the problem then hit the reset button. If it still doesn’t work replace the switch. |