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Furnace Troubleshooting
As we hunker in for the cold wind and falling snow of the heating season, it is important to refocus our minds on heating systems and how to work on them. There are countless brands and types of heating equipment in customer’s homes and businesses, and one of the most important things about being a technician is to be able to troubleshoot and repair these systems. In a single day, a technician could service a gravity furnace in the morning and a high efficiency modulating furnace after lunch. This simply shows how important it is to listen to your customer describe the issue they are experiencing. Many times, in those details, you will find the problem, or encounter other issues. Here are a few ways you can work through troubleshooting a furnace.
  1. First, look over the equipment, ducting, drains, vent piping, and gas line. Many times there can be dangerous issues in plain sight. 
  2. Always turn the fan on at the thermostat. That will tell you the blower is working and the thermostat is functional. Many of the new thermostats communicate with the indoor and outdoor equipment.  They also provide you with fault issues and diagnostics.
  3. Review current and stored faults to determine system problems.
  4. Check and verify control board indicating current error or lockout.
  5. Once you have verified system-identified errors, reset the system. Then, check to see if the error is still present.
  6. If the error is still present, you will begin the next steps in the sequence of operation of the furnace.
All heating and cooling systems have a sequence of operations.  Brand to brand, these sequences will be similar. To correctly troubleshoot, one process has to be complete in order to move on to the next process. You will find the issue when you move from one completed sequence to another, and find that the sequence of operation is broken. 

Follow the next steps below to finish out the sequence of operations to find the issue:
  1. For a typical gas furnace, the thermostat is the beginning of the sequence. Make sure everything is working correctly on the front end of the system.
  2. The control board will then verify safety controls are intact and that the pressure switch is open.
  3. The induced draft blower will operate for a specified purge period.
  4. The control board will verify the pressure switch is closed.
  5. At this point a trial for ignition will occur. After ignition and flame is sensed, the blower motor will engage after the control board’s fan on delay is complete.
Heat Exchanger Testing
One of the most important parts of a furnace is the heat exchanger. Per the AHRI (Air-Conditioning, Heating, and Refrigeration Institute), there is a five-step sequence to test these parts to make sure they are running smoothly and efficiently. Checking to make sure the heat exchanger is working properly is one of the most important parts of troubleshooting a furnace. Follow the steps below to test heat exchangers.
  1. Look for flame disturbances.
    1. Start up the furnace. View the flame pattern, and observe if there are any distortions. Flame disturbance after the blower comes on is a huge indication that there may be a problem with the heat exchanger. There may also be other air leaks near the burners, which should also be addressed.
  2. Measure CO levels in the airstream.
    1. Leave the furnace running for about five minutes. Next, measure the CO level in the return air stream near the furnace, and record that number. Next, measure the CO level in the supply air stream. Generally, you’d like to measure a location one or more bends through the ductwork.
      1. If there is no measurable difference in your measurements, it’s likely the furnace is not leaking CO into the air stream. If the value is below 9ppm, you can move on.
      2. If the CO in the air supply is less than the CO in the return air, it may be that there was a measurement error. Repeat 2.a.
      3. If the CO in the supply air is greater than the CO in the return air, it is possible the furnace is generating the CO that is leaking into the airstream.
  3. Measure CO levels in the Flue Pipe
    1. Leave the furnace running for five minutes. Next, measure the CO in the flue pipe, making sure to use a properly calibrated combustion analyzer. If the CO reading is less than 200ppm, there is no action needed. However, If the reading is more than 200ppm, next steps must be taken.
  4. Verify Proper installation
    1. One of the last steps is to verify the furnace was installed in compliance with the manufacturer’s requirements and codes.
    2. Verify the following items: gas orifice size, pressure switch settings, gas input rate, manifold pressure, proper conversion for fuel type and altitude, vent lengths, duct static pressure, and the provision for adequate combustion air to the furnace.
    3. Check to make sure there are no damaged or disconnected wires or hoses.
    4. Check for misaligned burners
    5. Inspect the entire vent system and combustion air pipe
  5. Finally, visually inspect the heat exchanger
    1. Disassemble the furnace until you can inspect all the interior surfaces. Any crack or hole that could affect the unit will be easily visible.
    2. Do not use water, a camera, or smoking agents to check for leaks. Heat exchangers are not hermetically sealed, so a small amount of leaking is normal.
If there are any abnormal splits, cracks, holes, or damage, the heat exchanger must be replaced.

The most important part of troubleshooting a furnace is to follow the sequence of operations. Don’t jump ahead to an issue you believe is present, because you may miss a crucial step in the sequence, and spend precious time searching for faults that are not there. You could also neglect other issues by simply skipping over them to get to the key issue. Make sure to make the first repair, and then look to other potential issues. Test and cycle the system, allowing it to operate and stabilize.

Another key part of troubleshooting is to be professional and listen to the customer first. Determine the problem and what it will take to resolve it, and relay that information to the customer so they feel informed, both about the process and how much it will cost. If anywhere during the sequence you find safety concerns, be sure to document and address them, and have a conversation with the customer. Additional issues mean the customer could be looking at a more costly repair, or they may want to purchase an entirely different system.
Some tips to keep in mind:
-Some older furnace control boards do not have LED indicators.
-Having the proper tools and a good quality meter is important.
-Knowing how to properly use and test components and voltage is imperative
-If you are working with a system you are unfamiliar with, look over the installation/service manual to find the solution. If manuals are not present, search online for the brand and model you are servicing. 
-Some service manuals will provide a flow chart to follow.  It will walk you through the sequence of operations, test locations, and procedures.
- Always perform temperature rise/drops and static pressure tests. Temperature rise/drop ensures the system is operating correctly and within manufacture specifications. 
- Testing static pressure is crucial with ECM variable speed and X13 constant torque motors. Excess static over 1.0 will lead to noise complaints, increased power consumption, decreased efficiency, and failed motors.