Troubleshooting Common Furnace Problems With an HVAC Expert

Assuming the gas is on, I would be initially checking the pressure switch but there are a lot of possibilities based on the information without running some tests.

It sounds a lot like a pressure switch. See if this helps. https://dengarden.com/appliances/Everything-You-Ne...

There should be a light on the circuit board blinking to tell you roughly where to look for the problem. Since the blower is kicking on immediately, something has failed. Maybe the board or inducer motor since it’s happening before the unit even tries to light.

There are a lot of possibilities. I've written some other articles that might help you narrow it down. It's strange if it worked for the heating and everything else seems to be running, but not the fan for the AC. It could be a bad connection, or a problem with the circuit board or stat.

There should be a code chart on one of the furnace doors or in your install manual that tell you what these mean. Not all brands and units are the same so I'm sorry, I do not know off hand what it's trying to tell you.

You won’t get 24v if it’s not calling for heat. You’d have to tap into the low side of the transformer I suppose but I’d rather you test for resistance.

If the high limit switch has been tripped then yes, the blower would run until the temperature drops to the allowable range and then try to fire again.

There has to be something wrong with the wiring or the circuit board is shot. There is no other way a furnace would try to fire when in a/c mode. The fact that the blower is running full time regardless of mode tells me it's more likely the board.

Perhaps the board is the issue, but I can’t tell without running tests.

It's likely but hard to say without running electrical tests for sure.

Well I was going to say control board and though it says it’s ok if it’s not, it could say anything. Test for voltage on the ignition wire to be sure.

It seems as if the fan is running full time either because you have it set to “fan” or there is a short causing this. That said, I run mine full time and it doesn’t have much impact at all on my bill so perhaps your motor is going bad and drawing high amperage.

If it’s a spark missing then it would likely be the spark igniter or the spark module. Testing would need to be done to determine this unless you just roll the dice and replace one then the other or both. Keep in mind I cannot guarantee that but these are common failures that have this symptom.

The igniter is powered by the board so that is one possibility but if it's glowing and just not getting gas to light then that's a different issue and has a few possibilities.

Typically you'll see bad power or no power going from point A to point B. However, there are a lot of A's and B's, so you really need to know what you're testing. There are some "flags" that can give you the idea it's bad but electrical testing is the only true way.

Yes, it could very well be. Pressure switches can be “finicky” and don’t always just go out. They can act funny and then die sometimes.

While I can’t say what’s wrong, I can say I’d call the professional back and make him fix it properly.

You test to see that the spark igniter is getting proper voltage from the board. If not, it's the board and if yes, it's the sparker itself.

The igniter is what I'd start with.

A board can cause weird things since it's in control. When it goes the furnaces can work intermittently. I'm not diagnosing your problem, however. Just answering your question that yes, a bad board may still allow the furnace to run intermittently.

I would take a look at the pressure switch first. It can cause this type of behavior.

I do not know what the four flash code means as the codes vary per brand. The pressure switch just confirms that the inducer is venting properly. Just because the flash code (if it said pressure switch) says pressure switch it doesn't mean the switch is bad, just that it is why it won't let the furnace light. So, it could be bad (which seems it wasn't since you replaced it) or it could be a blockage in the chimney, the motor is bad or going bad, and a couple other less likely possibilities. Electrical testing the circuit would help if you know how to use a voltmeter and run these tests to help confirm what exactly the problem is.

No. It sounds like they never ran the wire to power the stat from the furnace. That would be my guess.

I hope you're using something besides your hand to do that, but it sounds like the capacitor if that's what's happening. Most often mounted to the front of the fan housing.

If the valve handle is “in line” with the piping, then it is open/on. If it is turned sideways in the direction of the pipe it is closed/off.

this could be the transformer, fuse, board, or all 3.

My initial thought is the blower motor or fan wheel. I would visually inspect that area to make sure nothing got sucked into the fan, and that the fan wheel isn't broken/wobbling/rubbing. Be sure you turn the power to the unit off before putting your hands in there.

You really shouldn't be able to smell gas in your furnace. If you mean the main line coming in then the problem is outside of the furnace.

I can’t honestly say because it’s not a repair I’ve ever attempted or been asked to do. One thing I can say is that if the unit was registered, then you should still be under warranty. Before going through all that I would get on Rheem's warranty site and run the serial number. A new module would be best to not only address the fuse but the underlying issue that caused it because I also know that a large majority of the motors never fail, but the modules do.

I don't know what a 2003 Lennox design is but if the testing shows a fan relay is bad then I would definitely try it before making a larger purchase since they aren't that expensive.

Yes. You can wire nut those together, and it won’t matter which wire is which in this case. Just don’t connect both to one or all together. (I probably shouldn’t have to say that, but you’d be surprised what I’ve seen people do.)

He would’ve had to change wiring or a setting to make that happen I believe. If your conditions are as hot as it is here then it’s also possible there is an outdoor temperature sensor and that it know it has to run in 2nd stage to keep up with the temperature delta. You may want to give it a day or two but if it seems something is wrong then make them come back. That said, if nothing is wrong they may charge you. Perhaps a call explaining your concern is something they’ll address over the phone with you.

Taking the door off and the unit functioning as a result typically means the fan isn’t getting enough return air meaning it’s tripping the high limit which has nothing to do with the pressure switch. Those should be unaffected by the door being off. So while I can’t say exactly what issue your furnace is having, this is typically what I’ve seen. Check your filter, return duct for blockage, maybe the fan in that it’s not running slow or noisily.

The pressure switch is typically a round disc-shaped part with a hose connecting it to the inducer fan. When the fan kicks on, it creates suction if working properly. That suction pulls in the switch making an electrical connection that allows the next step in the cycle to commence.

Great breakdown of info! So all I can do is narrow it down but this is how I'd proceed. First suspect, pressure switch. You can test with a voltmeter to make sure it's closing once igniter begins to glow. Or by jumping it out and see if it fires then. If not, check for flue blockage and listen to inducer to see if sounding slow or something in there. If not, could be gas valve. Test that voltage is coming to it. If not, I'd suspect the board. Keep in mind this is just a process, not a diagnosis. A voltmeter and knowledge of what you're doing with it is important. BE SAFE! Don't do anything you're not comfortable with.

There could be a lot of things but assuming the furnace worked fine before and the a/c worked properly and no other changes were made, I don’t know how to explain it. A blockage would’ve affected the A/C too. Perhaps they changed the fan speed setting when they put it in.

Improper venting is the most common cause of excess moisture. I can't say what may be improper, but that is usually the case.

You may have an issue with the pressure switch. See if this article can help to see. https://hubpages.com/home-improvement/Everything-Y...

If it’s all off, then you must have a short in the stat wire.

Something has failed and the blower is kicking on as a default, most likely.

The blower always continues to run until the temperature in the exchanger is depleted and satisfies the limit. This is normal.

I would need to run some tests to figure out why your unit is kicking off and on or witness what part of the cycle it's stopping. As for the heat exchanger, you'd probably need to have someone run a smoke or CO detection test and you're likely still to be under a parts warranty. Get a CO detector to be safe.

Only if they stuffed something in the chimney (flue) or intake (if high efficiency furnace). Otherwise no.

In my world "control" would be the circuit board (control board) or ignition module (ignition control).

I'm not a big "brand guy". I recommend you go with whoever will do the best job installing the unit. Regardless of brand, if it's not installed properly, it won't work properly. At least not for long. I assume all these units have the same efficiency and features so if it just brand name as the difference, focus on the installer.

You should have a chart explaining the red light codes. In my experience, a solid red means the unit has power and is awaiting command.

It depends on how low it's going. The thermostat will often have an "anticipator" that purposely lets the thermostat drop a degree or 2 below set point so that it isn't constantly kicking off/on.

I'd say you're correct. There shouldn't be any sparking on the board and boards just get replaced, not repaired.

That is efficiency. The flue is to be hot but it is the amount of wasted heat that determines how efficient you are. 80% efficient means 20% waste. Etc...

Next would be to run electrical tests to see if the actual motor is the problem or the pressure switch or perhaps the board. If it is, in fact, the inducer motor you'd have to replace that whole assembly. Rather easy to do but the cost is dependent on what you have.

I'd guess that your thermostat may be the issue.

A weak, slow closing gas valve could cause this. I assume this is an older unit and I suggest a service maintenance to be sure.

It could be or it could be the harness/connector itself. These pins/connectors will sometimes get loose and cause this type of issue.

The furnace should cycle properly to the point of blower coming on before failure so if the capacitor is bad, I don't think it's the only issue.

It sounds perhaps like the low voltage wiring isn't correct. Not something that would just start out of nowhere though unless there is a short in the circuit. This is my guess from afar.

Well since you've replaced the sensor I'd check the hi limit or the board. Perhaps one of those are the culprit. I would need to run electrical tests to confirm though.

No. It should have nothing to do with the lights in the house.

If the furnace is going through its processes and then after lit, the fan doesn't come on, and the unit shuts down again it would likely be the blower which if changed should change the capacitor on the blower too. That's really the best I can offer you from here.

A pilot light (little flame) always stays on if that's what you see. If it's the burners, then I've only seen that on pretty old units, but there isn't enough info for me to offer any more info other than it shouldn't do that so you should have it looked at.

This is programmable? It means it's getting back to an "occupied" set point. I.e., You were gone, so program for one temp and then another for your return...it's in "recovery".

If this is a standing pilot unit, then I would say perhaps the thermocouple though standing pilots are only on older furnaces now. Newer furnaces do not have those anymore.

Possibly your blower motor is bad. That's what I would be checking first.

Well I'm inclined to think the pressure switch is bad and would try replacing it since it should be cheap.

While it doesn't hurt to try cleaning the sensor, I'm not sure that's your issue. Usually a unit fails, for whatever reason, 3 times before it locks out so I would try and see what's happening prior to that. That might give you a better idea of where in the cycle the breakdown is occurring.

It likely means you don't have power to the unit or perhaps blew the board fuse.

I'm sorry but I can't answer this because Honeywell doesn't make furnaces and this would be in regard to what I believe is a zoning system which I cannot troubleshoot over the net. Sorry I can't help on this one.

The blower runs at a default often when a failure occurs. Unfortunately, it doesn't tell me anything about why it failed. Sorry, I can't really offer any advice based on this information.

I'm not really familiar with this though I've heard of scenarios where it's happened. You may want to try a couple of searches to get info about this or contact a plumber. If you're getting shocked, you need someone to address it and likely should be a professional but probably a plumber, not a heating contractor.

Without being there, I'd say you have a dirty flame sensor or intermittent issue with the pressure switch. The blower coming on after three attempts is a default when going into lockout.

When it comes to circuit boards in furnaces, there isn't really any repairing them, and you may have a bad terminal which would require a new board. Make sure to check that the stat only needs 24v.

If your furnace is blinking a code, compare it to the chart either in the user's manual (you may be able to find that online) or, there is probably a chart on the furnace to tell you what the different codes mean.

It sounds like you have power to it, but I'd check that first. The hum is likely a transformer if it's a rather soft hum, (not a motor stick hum). I'd check the fuse on the board. It is probably a 3 amp car type fuse. If that is good, I'd check power to and from the transformer, (120v in 24v out). If all that is correct, then I'd be leaning towards the board being bad, but I can't really confirm that without being there.

It sounds like you need to light the pilot. Turn to the pilot. Get a match or lighter by the pilot, then press down on the button. Once lit, hold the button down for another 30-45 seconds. Next, let go and turn the switch to "on." I also wrote an article about this in regards to water if you need more insight. They are basically the same process.

The thermocouple confirms that the pilot is and stays lit. If the pilot lights but won't stay on, it may be because the thermocouple is bad and telling the unit it isn't lit, thus shutting down the valve.

If the igniter is getting constant power when there is a call for heat, and that's what's burning it up, then the circuit board is likely bad or the igniter is wired in wrong. But that is typically difficult to do.

I think this article will help you find and see what is entailed in the repair.

https://dengarden.com/appliances/How-to-Change-an-...

There are fan speed terminals that determine what speed the fan will run at during heating, cooling, and fan modes. If this wasn't an issue before then, it's not likely the issue. Check your filter and perhaps your evaporator coil (if you can) as they could be plugged and blocking air.

While I can’t say what the issue is for certain, you’re probably resetting the board by flipping the switch.

It sounds like you don’t have power to the unit or perhaps blew a fuse on the circuit board. Check your breaker and service switch as well as the little 3-amp fuse on the board.

Likely it’s a bad transformer or board but that can’t be for certain without testing.

Yes, you are fine running the A/C.

It may have a blown transformer, or perhaps the little three-amp fuse that is often on the board could have blown. You can easily see if that fuse is blown by just removing it and looking at it.

It may have been a fluke if it's not a regular occurrence. That does happen sometimes.

Are you sure that was the problem? It could be the batteries or the fact that you may have changed something else.

Perhaps the fan/high limit? Dirty filter or blocked airflow? These are a couple of common suspects.

I don't see you mention a pressure switch... it kind of sounds like it could be on its way out. These are one of the few parts that can slowly die whereas others usually either work or don't.

Your landlord should be calling in a technician to repair that. There's likely a short or something overheating causing it to trip unless the breaker is just bad. Either way, it needs to be tended to.

While water is difficult to track the source/problem via the internet, I can say that a 5-year-old Lennox may well still have a parts warranty so if you get a tech you might only have to pay labor. Your heat exchanger definitely has a warranty if it's the issue.

Not to seem curt, but it would be time to consider a new furnace. I can't say for sure without testing, but it sounds like you have a bad heat exchanger and that can be dangerous if it, in fact, is what is the problem.

I'm not sure what's happening there, but I can say that something is shorting or drawing too much amperage. Electrical testing would be required to determine the source.

If you are confident that your wiring is correct then the usual suspect is the circuit board.

There are a couple of parts in a furnace that can either be going bad, or just doing what they were supposed to do due to something happening at that moment. A dirty flame sensor can be intermittent, so cleaning it is essential, or it will become more frequent. Perhaps the pressure switch is going bad, or maybe lousy timing sent a draft down or through the flue, which can trick the switch into thinking it's not getting the right pressure. If that happens more than a couple times in a week (assuming no heavy winds), you may want to replace it before it goes all together. These are common occurrences that are easy to check or fix yourself.

A transformer can smell pretty good when it burns up. I would still recommend that a technician check that for you. It's probably not a bad idea to shut down the power to the unit if it's not running anyway.

There should be a service switch near or on the unit that you can turn off, or at the breaker. It sounds like a short, but I'm not sure where.

You could check the power at the transformer for 24v/120v just in case, but it may indicate the board is bad. Is everything else working? There is a slim chance that the bulb is bad, but it's happened before.

I believe it's the pressure switch. Check that the chimney isn't blocked or the inducer seeming weak or grinding but if those seem ok I believe that's the problem based on what you've told me.

The blower is supposed to run after a cycle until the fan limit tells the motor it's cool enough to turn off. If that's not happening then likely you have a bad fan limit or problem in the board. Keep in mind these are just educated guesses without running any tests to confirm.