Troubleshooting Heating Problems
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Below we have listed some Heating Problems and Solutions. I hope this will help you troubleshoot your furnace, solve your problem and save you money!
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ignitor will not glow. If a furnace has a bad ignitor, open limit switch,
open rollout switch, open pressure switch or bad control board. This is what
I see in the furnace sequence of operation:
1. Thermostat calls for heat.
You should be getting 24 to 28 volts AC between the W (white) and C (com)
terminals on your control board.
*Make sure furnace safety switch is pressed in when testing.
2. Draft inducer motor starts.
If draft inducer does not start then you either have a thermostat problem,
thermostat wiring problem, transformer problem, bad relay on control board
or a draft inducer problem.
3. Pressure switch attached by a small plastic or rubber tube
to the draft inducer senses the negative pressure produced by the draft inducer and closes.
Should have 24 volts to ground across both terminals on the pressure switch.
If pressure switch is not closed with draft inducer running check for
stopped up vent or stopped up drain line on a condensing furnace.
4. Limit Switch and rollout switch/switches should all be closed.
Press in on the reset button in the center of each rollout switch to make sure
each rollout has been reset. You should have 24 to 28 volts from each terminal
to ground. On most furnaces the ignitor will not glow unless all the safety
controls (limit, rollout switch/switches, pressure switch) are closed. If a
limit or rollout switch is open then you might have an over heating problem and
a possible dangerous condition. If the Limit or rollout continues to trip I
would strongly recommend calling an HVAC service company to check your furnace
5. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound.
If you hear a hissing gas sound then more than likely either the ignitor is
broken or the relay on the control board that sends power to the ignitor is
broken. If the ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace
will shut down, and try ignition again. Most of the time after three tries
the furnace will and go into a lock out condition until you turn your power switch back off and on again.
You might need a new ignitor, rollout switch, limit switch, pressure
switch or control board.
Problem: Your furnace will not ignite the gas to produce heat for your home. When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:
1. Thermostat calls for heat.
2. Draft inducer motor starts.
3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes.
4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas.
Solution: You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. Please see "limits, rollout switches & furnace control boards" further down on this page. The furnace's control board might not be supplying the voltage to the ignitor. Please click here for our Furnace Control Boards Page. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning. Please see the pictures below to help you identify a flame sensor. Here is a link to our Furnace Flame Sensors Page.
Below are pictures to help you identify what a flame sensor and ignitor look like. On the left is a picture of three different types of flame sensors. On the right is a picture of a hot surface ignitor. Make sure you do not touch the gray glass part of the ignitor. If you do it will shorten the ignitors life.
Problem: My furnace ignitor is going out too much!
Below we have listed some reasons why your furnace ignitor might be going out so often. We hope this will help you from having to replace your ignitor so often!
*Please remember to turn off all electrical power to your HVAC system when trouble shooting or working on HVAC equipment. We most certainly do not want to see anyone get hurt or damage their equipment!
Below are listed some reasons that might cause your ignitor to go out prematurely:
1. Handling the ignitor improperly by touching the gray glass part.
2. Installing an ignitor that is not made for the furnace.
3. High home line voltage. Check your voltage to see that you are in the 110 to 125 volt
range. Anything over 125 volts is going to cause premature light bulb failure and furnace
ignitor failure. Call your electric company if you are getting high voltage in your home.
Your electric company should be able to place a transformer on your line to lower your line voltage.
4. Debris from the heat exchanger or bugs/spiders can get on
the ignitor while the furnace is not calling for heat and when there is a call
for heat the debris/bug can short the ignitor out. It is a good idea to keep
your furnace and heat exchanger clean so this does not happen.
5. The ignitors have just so many on off cycles in them.
The reasons listed
below concern the furnace cycling off and on too much. These
reasons if they occur, will also cause you to have a high heating bill.
1. Over-sized furnace. A furnace that is too large for you home cycles too much. If your furnace is sized properly on your coldest day the furnace should run almost continuously. Shutting off very few times. I went to an HVAC training meeting on energy conservation. The speaker said, "The inefficiency of a furnace is in the On, Off cycling." He made this statement that on the coldest day your furnace should almost run continuously if sized properly.
2. A dirty filter or blower squirrel cage causes too much cycling. Your furnace will go off on high limit continuously because it is not getting enough airflow through the heat exchanger. The furnace goes off on high limit to keep from overheating the furnace and possibly causing a fire. This is why it is very important to keep those filters changed.
3. A dirty restricted air conditioning evaporator coil causes restricted airflow that causes too much cycling. Make sure your coil is not stopped up on the underneath side. I usually have to take off the coil end plates and use a vacuum cleaner and coil cleaner to get all the lint and dirt off. Make sure you clean in the direction the coil fins are running. If you do not, it will bend the fins over and cause more restrictions. A good tool to use is our
coil fin cleaning tool.
4. Improper thermostat heat anticipator setting. Look at your gas valve and see the amp draw. Set you thermostat's anticipator setting to this amp draw plus one amp more. If your gas valve says, ".4 amps" then set your thermostat to ".5" amps. This will give you a little longer run time and a longer off time. If you have an electronic thermostat: Many of the electronic thermostats have temperature differential adjustments or like on the Honeywell thermostats have screws where you can adjust to make your furnace cycle a little longer.
Below we have a pictures on the left of a label off a White Rodger's gas valve and on the right a Honeywell Round thermostat anticipator. The label shows that this White Rodgers gas valve model #38C03 Type 300 draws .23A or .23 amps. The anticipator shown in the picture on the right is set on .4 amps. If I were using this White Rodgers Gas Valve and the Honeywell Round thermostat I would set the anticipator one amp more than the gas valve draws, on .3 or a little past the.3. The little copper looking pointer can be moved to make your furnace stay on longer by using your finger to move the pointer. Again, you should look at your gas valve's amp draw to determine your setting.
6. Too much gas pressure causing the furnace to overheat because too much heat is being produced. This causes the furnace to cycle too much on high limit. Does your furnace’s gas pressure sound like a jet when it is running? Your gas pressure on your gas valve might be set too high. This produces too many BTU's or too much heat. I would recommend a HVAC technician adjust this problem. Turning the adjustment screw clockwise gives more pressure. Counter clockwise gives less gas pressure. If you attempt to adjust this yourself, do not adjust more than 1/4 turn clockwise or counter clockwise. Delayed ignition, blow back, and a mini explosion, could occur. Again, I would recommend an HVAC technician do this adjustment. Getting this adjustment right will save you gas utility costs, and wear and tear on your furnace in the long run.
Pressure Switch, Limit, Roll out Switch or Furnace Control Board Problems?
Problem: My furnace's ignitor does not glow? This could be a pressure switch, limit, roll out switch, or furnace control board problem.
I see this problem many times during the heating season here in Louisville, KY. When your thermostat calls for heat the draft inducer (please see pictures below) should start which creates a draft in your vent pipe. If your vent piping is open and your pressure switch is working properly the pressure switch will close the connection between two wires and send a signal to the control board saying, "Yes it is OK to continue with the ignition process."
Above are pictures of a draft inducers showing where the pressure switch tube attaches. Pressure switch tubes sometimes attach on the top bottom or near the center of the draft inducer. These are just two of hundreds of different draft inducer designs. Some are metal and some are plastic. Most of the time draft inducers are plastic on 92% furnaces & metal on 80% furnaces. Make sure the opening where the pressure switch tube is attached to the draft inducer is open. Sometimes a small wire can be used to reestablish the hole. Make sure your water drain on a 92% furnace is open. If too much water backs up into the draft inducer it will not allow the pressure switch to close.
Also, all of your limit controls and other safety devices must be closed (a closed circuit between the two connections) to allow the furnace ignitor to glow and start the ignition process. If there is a break down in the pressure switch or other limit safety controls the furnace, for your safety, will not proceed with the ignition sequence. The computer chip inside the board says, Wooooh, something is wrong here! Most furnaces will try this sequence for a total of three times then lock out. On most furnaces the only way to get them out of lock out mode is to turn the switch (looks like a light switch) on the side of the furnace to off and then back on again.
Above is a picture of three different types of limit controls. The two on the
ends should reset automatically when the furnace cools down. The rollout
limit switch in the center has to be manually reset by pressing the button in
on the top. All limit controls are there for your safety. Never by-pass these.
How do you test a pressure switch and other limit controls? This is for people who are experienced with electrical equipment and the use of a Volt Ohm meter. *Please never by pass a pressure switch or limit control. The pressure switch and limit switches are there for your safety. In the top picture I am testing a rollout limit switch to see if it is good. You would test by setting your volt meter to "Volts AC" and test the rollout switch by placing one meter probe on one terminal of the rollout switch and your other meter probe to a good ground. Below I have the red probe touching the top terminal of the rollout switch and the black probe touching a ground (body) of the furnace. You can see the I am getting 25.85 volts which means the rollout is good on the top terminal. I would next test the bottom terminal of the rollout by touching the red meter probe to the bottom terminal on the rollout switch and the black terminal to ground. If I get 25.85 volts on the bottom terminal the rollout is good. If I do not get any voltage on the bottom terminal then the rollout has tripped and can be reset (if equipped) by pressing in on the little button or replaced. If your rollout switch is tripped you probably have a stopped up heat exchanger or a leaking heat exchanger. I would recommend calling in a service technician to find out why the rollout switch tripped. If you have an open limit control either the furnace has over heated or the limit has gone bad. Problems that would make a limit open up would be dirty filters, dirty evaporator coil causing a restriction in the air flow or a slow blower motor (check the capacitor on the blower motor to make sure it is good). A weak blower motor capacitor will cause the blower to run slow and eventually fail. Pressure switches, and limit switches can be tested with a meter in the same manner. With the furnace calling for heat you can test each of the terminals on the pressure switch to ground to make sure the pressure switch is closed. You should be getting between 24 to 28 volts from each terminal to ground if the pressure switch is closed. If the pressure switch is open you either have a stopped up vent, drain line (if you have a condensing furnace) or bad pressure switch. Below we have three pictures of pressure switches. The picture on the left shows the full view of one pressure switch. Please keep in mind there are many different types. The picture on the right shows the two terminals that you can use to test to see if the pressure switch is operating properly. The front terminal is burnt and discolored. This is a clue that this pressure switch has a problem. The picture on the bottom is a black Goodman, Janitrol plastic pressure switch.
Above pic shows me testing a rollout switch.
Above are two pictures of pressure switches. The pressure switch on the left came off a 92%
furnace. It has two connections for pressure tubing. One tube would go to the water collection
box and the other tube would go to the draft inducer. The switch on the right has a burnt front terminal.
Above is a picture of a Janitrol Goodman pressure switch.
How do I test to see if a pressure switch is operating properly?
Set your meter to volts AC, turn your furnace on so it is calling for heat. The draft inducer should start. There should be either two or three wires going into your pressure switch. Probe one lead of the pressure with one lead of your meter while touching the other lead of your meter to ground. Ground would be any bare metal part of your furnace. This must be bare metal. I always try to touch the other lead of my meter to the furnace's switch box. If you have a two wire pressure switch you should be getting 24 or more volts between both leads to ground. By this I mean if you touch one terminal of the pressure switch with one lead of your meter, and touch the other lead of your meter to a ground, bare metal part of your furnace, you should get 24 or more volts (24 to 28 volts). If you do not get 24 or more volts with the furnace running then you have a pressure switch problem. Your vent could be stopped up, The tube that runs from your pressure switch to the draft inducer could be plugged up or the draft inducer hole could be plugged up. On high efficiency condensing furnaces the water drain line could be stopped up, causing a water back up and blockage in the pressure switch tube. I usually take the drain hose loose from the condensing furnace and use a wet vacuum to open the drain back up. I hope this will help you in troubleshooting your gas furnace pressure switch.
Problem: Draft inducer will not start, ignitor will not glow or gas valve will not open.
Solution: Any combination of the above problem or problems could be caused by a bad furnace control board. I always troubleshoot and test the least expensive parts first, such as the pressure switch, limit switch and rollout switches to make sure they are all closed and operating correctly before I go to the control board. First, and most important things to remember is turn your electrical power off to the furnace. Most control boards are located in the blower compartment. Many control boards have a fuse located on them to protect the board from getting burned up if you have a short to ground. The fuse is usually a 3 to 5 amp fuse similar to the fuses that you might find in a car's fuse box. Please see picture below of the furnace control board:
If you fuse is blown you will want to check and see if any of the low voltage thermostat wires are touching the metal frame of the furnace or grounding out. Most of the time I see fuses that are blown from animals chewing through the thermostat wires and from the wires grounding out to the body of the furnace where the wires enter or exit the furnace. The vibration of the furnace over time sometimes wears through the wires insulation and causes a short to ground. I would try a new fuse first, then if the fuse blows again you are going to need to find out where the wires are shorting out. This can be quiet time consuming.
If your fuse is OK, tape your door safety switch shut so you can view the status LED light on the control board with the furnace power on. Please remember to turn the power back off before touching anything inside the furnace. I do not want to hear of anyone getting hurt or shocked! Turn your thermostat up so it is calling for heat. Let the furnace cycle and see if the Status LED flashes a code. The directions for reading the code are usually located on the furnace door. The code will usually tell you what is wrong with the furnace. Technology is amazing! Best of luck on repairing your furnace. Please remember to turn that power back off before touching anything.
Troubleshooting Electric Heat problems:
Problem: Electric heat will not come on or fan will not come on.
1. First and most important, Please make sure your electrical power is turned off before trying to repair or inspect any type of electrical appliance. I would recommend that you purchase an Volt Ohm meter for some of the troubleshooting procedures listed in this section. We have a reasonably price Volt, Ohm, Temperature testing meter on our Favorite HVAC Supplies & Tools Page.
2. Inspect the inside of the air handler or electric furnace for burnt wires. I find this to be a major problem. Electric furnaces use lots of electricity. Any connections that are the least bit loose inside the furnace will cause a heat build up, arcing and eventually a completely burned off connection. Before long the entire wire will be burnt off. Please see picture (compliments from one of our nice customers) below of a burnt heat sequencer. You can see where the heat build up and arcing has caused the terminals to completely burn off the left side! This looks like a three stack sequencer, because of the divisions in the layers of the sequencer:
I intend to include a wire repair kit with connectors before long on our site.
3. Check your breakers or fuses inside the air handler. Again, make sure your power is off. A good electrical tester multimeter would be nice to test your fuses. Many air handlers have fuses in them like the ones pictured below. You can remove these fuses and test them with a multimeter or if you do not have a multimeter, install new fuses to make sure they have not gone bad.
4. If you still are not getting electric heat then you should test your sequencers to make sure they are working. I made myself a tester so I could get portable 24 volts without having the furnace power turned on by using an old fan center connected to a junction box with alligator clips attached to the fan center. We sell new fan centers on our Fan Blower Controls Page.
Tester made from Fan Center.
Son's 13 Tennis shoe propping up tester. Ha!
When 24 volts is applied across the two bottom connections on this sequencer you should get continuity (a closed circuit) between the two terminals the one on the left M1 and the one on the right M2. You should also have continuity between M3 & M4. This might take 30 to 90 seconds after the 24 volts is applied before you get the continuity. If you do not get continuity then you might want to purchase one of the sequencers we have below.
Below are links to some of my favorite sites for Heating, Air Conditioning repair and advice:
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type of Heating or Air Conditioning Repair. We do not want to see anyone get hurt or shocked!
*Please always turn off all electrical power, and discharge the capacitor/capacitors (if working around capacitors) before attempting to inspect or repair any heating & air conditioning equipment. Check to make sure the electrical power is off with a reliable meter.
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Sincerely, Steve & Barbara Arnold
11506 Seatonville Road
Louisville, KY. 40291