This is likely the most common “no heat” call culprit; The tripped high limit. For such a common symptom, it is one of the most misdiagnosed problems. There are a zillion ways technicians choose to correct it, and most of them are wrong. Completely, 100%, don’t pass go, wrong.
First, I hope you noticed I referred to a tripped limit as a “symptom”, not a “failure”.
Second, the typical knee jerk reaction is to replace the limit because it’s probably “weak”.
Third, I’m not sure how a component can be weak….it cant do 20 push-ups?
A tripped limit means the heat exchanger became too hot, causing the limit to open. Again, notice my choice of wording-“heat exchanger”- I did not say air temperature.
A common misconception is that high limits sense air temperature only. High limits sense air temperature (convection) and heat exchanger temperature (radiant).
Before we go any further, commit this to memory; Never alter a high limit safety, always use the limit that is recommended by the equipment manufacturer, and keep it located as per the equipment manufacturer’s direction.
The second thing to remember is a tripped high limit means there is a problem that needs to be corrected.
Assuming the limit is correct and is mounted correctly, there are three…make that four situations that can cause it to open; not enough air flow, uneven air flow, return air too warm, or too much fuel. The first thing to look at is the temperature rise through the equipment; it must fall within the range listed by the manufacturer of the equipment.
First, let’s start referring to temperature rise as Delta-T. Why? Because that’s what the big technicians call it… and it’s easier for me to type.
The range is typically found on the equipment data tag and would look something like this; “Temperature rise 35-75 degrees Fahrenheit”. The temperatures must be measured correctly, and here’s how; Start the burners and allow the system to reach steady state operation, about 10 minutes. If there’s an outside air supply to the return, be sure it’s closed. If there’s a bypass humidifier, close the bypass. If it’s a zone system, make sure all zones are open, and the bypass is closed.
Now, measure the supply temperature at a location that is not in the line of site of the heat exchanger so its not influenced by radiant heat.
Subtract the return air temperature from the supply temperature; there’s your Delta-T. If the Delta-T falls within this range and the limit is still tripping you may be dealing with uneven airflow. Try taking the supply temperature at different points in the supply plenum, the readings should be close to the same. If they’re not, look for restrictions to air flow like, a mismatched evaporator coil, a bullheaded supply plenum, or a partially plugged blower wheel. If the Delta-T is in fact high then you should first look for obvious air flow restrictions like dirty filters, dirty coil, clogged blower wheel vanes or closed dampers, then verify the equipment is not over fired.
If there are no restrictions to air flow, AND the equipment isn’t over fired, then it’s time to verify the air flow is correct. If you’re not sure how to verify firing rate or airflow, fear not! We have other articles on the site to show you how.