How To Diagnose Heater Problems on a Carrier Model HVAC System

Be Sociable, Share! wintertime and a lot of people are going to have problems with their heaters. Unfortunately these things don’t normally happen when its just starting to get cold and in my case I just went through a problem in the middle of January at 1am just before two big storms are expected to hit my area.

    I am going to step you through what I did in hopes it might help you diagnose your problem on a Carrier or other Model Heater. And hopefully reviewing this might help this process stick in my mind a little better for the next time it happens.

    Needless to say it can be a little scary and if I was living on my own i might have more options but with family you need to make sure everything is working especially at the worst times.

    Here was my problem.. I woke up to a cold house and the heater cycling on and off without producing heat. The first thing I did was think the worst. You tend to do that in these situations so be prepared for that.

    The second thing I did was actually slightly smart. I had been thinking about what if the heater went out so I went through with my plan. I first checked on the heater and saw there was a problem. Before I started trying to repair it I first turned the gas line on to our gas fireplace and got that going to produce a bit of heat in the house. They don’t put out as much heat as a regular fireplace but I figured i had to get something going to replace the heat that wasn’t coming out of the heater right away. This way at least maybe it would maintain the homes temperature a bit longer until I could fix the problem or call someone in.

    In the middle of winter your heater will cycle on and off and normally there is about a 20 minute pause between cycles as it raises the heat one degree higher than your thermostat setting and then falls one degree below that number before the heater turns on again.

    When I first checked the LED thermostat there was no reading on it. It was blank with no numbers. That is one drawback of technology that these things either work or don’t.

    After pressing the buttons a few times I raised the thermostat setting to 77f and the LED numbers came back on. This is when I thought I was going to end up trying to run to the all night store to see if they had a Thermostat.

    I then went down to the heater and pulled the access cover off.

    Heaters are pretty basic but some of the parts are not easily tested. They also take knowledge about what parts to test.

    I could hear the main fan running on the heater and very little air was coming out of the vents. This smaller blower motor is providing air to the combustion chamber and although that does provide some air to the house its very little. The burner will not ignite if this blower fan is not operating.

    Second I looked through the Flame Port to see if there was any flame. There was no flame.

    Third I checked the main blower motor that produces the air that circulates through the house. It was not running.

    On my Carrier Model the computer control board is mounted in a compartment below the burner. You can see the circuit board through a access hole just like you can see the burner above.

    The Red LED light on the Circuit board was blinking… I then started to wonder if the circuit board was bad but this blinking is an Error Code that you can use to diagnose what is wrong with the Heater.

    Luckily I have the manual that came with the heater. If you don’t have one you can probably find one online in a PDF format and I suggest you print it out. You don’t want to be running to and from your computer while you’re figuring out what could be wrong. You want the Service Manual (not the worthless Owner’s Manual) for your exact Brand and Model Heater because they are all slightly different.

    At the back of my manual is a Flow Chart for Diagnosis. Follow the steps for diagnosing the problem and it should point you in the right direction.

    First the chart said is the red light on? Yes it is.. and if it wasn’t it would have pointed me to diagnosing if a circuit breaker had tripped or other problem.

    It then asked is the light blinking.. Yes it is.. and then it said the number is a 2 digit series that is seen by reading short blinks then long blinks .. My light was blinking 3 times short and three times long .. then repeating 3 short .. 3 long.

    So, my error code was 33.

    It then said look below and find your Error Code.

    I looked below and saw the box with 33 and it said there could be a few different things wrong that I should check. They all seemed to be related to a Flow Control Sensor.

    First was a Bad Blower motor. Well I just replaced that myself 2 years ago so I really hoped that wasn’t it..

    Second was the Capacitor on the Blower Motor. They are hard to check but there was no Physical damage to it. This could be a problem.

    Third was the cutoff switch that turns the heater off when you remove the lower access door to the large blower motor. I removed the door and taped that switch down so it would be closed. The only other way would be to wire across the switch and I had done that when I tested for the bad blower motor years ago.

    I then Reset the Heater by turning off the power switch on the side of the heater. I waited a minute or so and then turned it back on.

    The heater started and operated without an Error Light. At this point I am thinking crank up the heat to get the house warm while its working so I went upstairs and set the thermostat to 74 and let it run without continuing on my diagnosis.

    This took a while because remember it was 64f in the house.

    When the heater turned off I returned and began where I left off.

    The final item on the list of maybes was the filter.

    Now I had just changed the filter at Thanksgiving .. I was actually proud of remembering to do it because I don’t always remember these things.

    With the lower access door off I was actually bypassing the filter and sucking air in from the basement and also down the stairs. So the Heater worked.

    I then checked the filter and it looked clean. I checked it up to the light and I tapped it on the floor to see if there was any loose dust. .. nothing. It really looked clean but…

    Then since the heater operated after the reset I thought to replace the lower door and see if it would continue working.

    When the heater started again the small blower motor for the burner turned on and it ignited and then the large blower motor came on and the heater ran but after about a minute the flame went out and the lower motor turned off.

    I am thinking to myself this is a sensor or switch… There is no place to get that at 2am and even if I called someone it would be hours until they woke up and drove over to help and then if they did they might not have the part on their truck.

    So then I thought about the situation with the lower door off and me bypassing the filter and I decided to pull the filter and try again.

    The heater started normally and ran.


    I inspected the filter really close and against a new filter of the same type. It is a basic paper filter. It seemed almost new compared to the one I pulled in November but It seems that this was the only problem.. well so far the heater has been running like normal for the past few hours so .. yeah it was the problem.

    Then I started to wonder why this seemingly clean filter was causing me problems and what was different? It wasn’t a defective filter or maybe it is.

    I came to the conclusion that my new Humidifier is the problem.

    In December it was very cold. It was in the teens overnight and the heater was running constantly and it took all the moisture out of the air causing me headaches from dry sinuses and also dry skin.

    I picked up a one gallon humidifier at a local store and I have been running it 24/7 at maximum ever since having to refill it twice a day so two gallons of water are being vented into the air.

    It is actually rated for a single medium sized room but I figure 2 gallons a day is really pretty good considering the heater was sucking out all the moisture before. My sinuses weren’t giving me problems and I wasn’t considering living in the bathtub to prevent my skin from getting dry.

    The only thing I can guess is that extra humidity must have mixed with the dust that was on the filter and blocked it off like cement.

    Like I said when i pulled the filter in November there was visual dust on it but the system was still working.

    When I pulled it this time there was a very slight discoloration so you could tell it wasn’t new but no visual dust.

    Well I hope that this was the only problem and I will be upgrading to the more expensive brand filters that have different layers of material to catch dust rather than the cheap paper filters which are mid-range filters above the cheapo green fiberglass ones.

    Maybe this will help or maybe I will just have to change filters every 2 months or less when I have the Humidifier running.

    Either way its not fun waking up to a heater that is not working. Hopefully this was the only problem and I won’t be replacing capacitors or switches… it seems that way so far.

    Final Note

    My problem turned out to be pretty easy but the diagnosis wasn’t that easy. I took for granted that my filter was clean because I just replaced it and it looked visually clean.

    I followed my disaster plan by first restoring some limited heat to the home and I could have resorted to two small space heaters and ran out and got some Kerosine for my larger heater if necessary. I wouldn’t have done that if I had not thought about what could happen and what would I do.

    I then stepped through the diagnostics flow chart that was in my heater’s manual. This resulted in some slight embarrassment at me not realizing that a filter that looked clean and was recently replaced was causing all my problems. Hey I can eat the embarrassment as long as the heater is working.

    While I was waiting for the heater to cycle I also looked up my options for a service call and I also recorded some part numbers of the items that could have been bad according to the diagnostic chart so I could tell the repair man my model, my error light code, what I had checked and what parts he might need.

    I tried to keep my cool which was actually not that easy in a cold house that was getting colder.. go figure. But I went through the plan the best I could.

    I guess at the very least I could have just called for emergency service and paid $200 an hour or something to have them come out and swap out my entire heater when it was only a filter.

    I was lucky this time and planning and having backup heat solutions was what gave me the time to figure things out. I hope my situation gives you a few tips on how to handle a similar problem if you wake up to a cold house in the middle of the night.


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