If you have been in this industry for any amount of time, you have come across a furnace giving a high limit code at some point. The high limit is a safety that shuts down the gas to a furnace in the case of excessive temperature rise. Excessive temperature rise can be due to a number of factors. Typically, we just change a filter and go about our way. 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 high limit opens due to temperature rise, the 24v signal which feeds the gas valve will be interrupted. When this happens, the board will turn the main blower on to cool the heat exchanger until the switch is closed again. This behavior will typically lead to a house cooling down and not keeping up, even though the furnace may be "running". The customer may find that the furnace then locks out for a period of time also.

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:

Solutions can vary here so we will start with the simple and work are way through till we find the issue.

1. Given a high limit error code is brought on by excessive temperature rise, we need to first verify the air side of the system.

• Check/Change the filter- Could be dirty, or its a high MERV filter with a lot of pressure drop. You may need to change out to a lower MERV filter.
• Go around the house and verify that all supplies and returns are open. A lot of people like to shut off registers to rooms they do not use. Explain that the furnace needs to breathe in order to properly work.
• Verify that the AC coil is clean and if the furnace is a 90%+ then make sure to pull blower and inspect the secondary heat exchanger
• Verify blower wheel is also clean. A dirty blower wheel can massively reduce air flow.
• Use our duct size tool in the bot to look at current duct work. Make sure its big enough to move the CFM needed.
• Then take a static pressure reading of the system. This will let us know which side of system is restricted, if not both. This will lead to a duct change, or at least the suggestion to change it.
• Static pressure ratings on furnaces are typically a max of 0.5"wc
• Verify proper temperature rise. You will want to aim for about the middle of the rise. This will give you a little room for filter loading later.
• After all of that verify that your gas pressures are correct as well.

2. IF all the air side checks out and you are still having issues, we will probably need to start looking at the electrical side of the system.

• First, pull all thermostat wires off the furnace control board. Use a jumper to simulate heat call. Verify operation. Also verify that the system is grounded properly. This means a dedicated circuit, all the way back to the panel. Be sure to look for junction boxes where the ground does not continue. Make sure all grounding points on the furnace are secure. If this checks out then move on to next bullet point.
• Verify the limit switch itself. Pull the wires, and test across the switch, if it has cooled you should see minimal ohms. IF your meter is reading "OL" then replace the switch. At this point use a jumper and see if it runs as normal.
• Verify also that the limit switch is not grounded to the cabinet. Measuring the from terminal to cabinet. Should be "OL"
• IF the switch tests good, then we need to make sure the 24v signal is getting back to the board. Use the units wiring diagram to see where the signal comes out and goes back in to the plug. We need to verify we have 24v at the input side of the plug. Do this by back probing the molex plug.
• IF you don't see 24V, work backwards toward your limit till you do find it. That is where you will find your issue. This could be a bad wire, or pinched wire, or a cut in the insulation.
• IF you see 24v at the molex plug input, then pull the molex plug and make sure the female pins are seated in all the way. Then plug back in and see if you get the same issue.
• If you do get the same thing, the board is not recognizing the input. This is where we may need to suggest a board replacement

Hopefully by the end of this you have found the issue that is causing the furnace to trip the high limit.