If you have been in this industry for any amount of time, you have come across a furnace giving a rollout limit code at some point. The rollout limit is a safety that shuts down the gas to a furnace in the case of flame trying to rollout out of the heat exchanger. Rollout issues  can be due to a number of factors. Typically, we just rule that the heat exchanger is bad and replace it. But, that is not always the answer and really should be the last thing we look at. Since we are at the job, lets take a few extra minutes to examine the whole system. You may say "I don't have time to do that" or " I need to get to next job. The simple fact is, if you miss something you will have to make time to go back there again. This not only leads to wasted time and money for you, but now your customer is going to start second guessing having you work on there equipment and telling there friends and neighbors about their experience.

Symptoms:

When the rollout limit opens due to temperature rise, the 24v signal which feeds the gas valve will be interrupted.  The rollout switch is typically going to be a manual reset switch, meaning you have to push the button back in to get it back on. These are manual for a reason, the furnace created an unsafe condition and you need to figure out why. 

Tools:

You will need the following tools to diagnose this:

• Screw/Nut Driver
• Meter
• Temperature Probes
• Manometer- With static pressure tips
• Jumper Wires
• Spare Parts, Just In case

Solutions:

1. Airflow Issues- I know it doesn't sound right but high static pressure can cause low airflow and radiant heat buildup. You may feel the top of the furnace is hot to the touch. It should not be. This will save you a lot of work if airflow is the issue, plus gives you value to the customer for finding the real reason for the issue.

• Verify static pressure - typically anything above 0.8"wc could cause this issue. We see this a lot on variable speed furnaces, when running in a lower stage.

2. Verify the Rollout Circuit- 

• Verify wiring for the rollout switch loop- If any wires are open or the molex plug isn't tight, it can generate this error randomly due to vibration.
• Verify the switch isn't damaged
• Verify that the rollout switch is closed when reset. It should have a reading of ohms across it. If you read "OL" then the switch is bad
• Verify the rollout switch is not grounded. You should read "OL" to the cabinet. 
• If you your electrical looks good after this, you may have to replace board due to the fact its not reading the 24v input back from the rollout switch circuit.

3. Verify Heat Exchanger- You may have to reset the limit to observe furnace operation. 

• Look for signs of flame rolling out when main blower engages
• Perform a combustion analysis to determine if there is high CO in the exhaust.