How A Dirty Duct System Can Cause Limits to Fail

Having spent years in the HVAC parts business, I've gained a deep understanding of the trends and demands within this niche. One standout observation is the consistent high sales of furnace high limit switches on our site.

Understanding High Limit Switch Failures

Meet one of them: the high limit switch.

The high limit switch seems to be a rather popular item; in some sense, I think it's too popular. What do I mean by this? Well, as an HVAC technician myself, I understand the purpose of a high limit switch is basically a safety switch for your heat exchanger. Meaning, if the temperature becomes too hot for your heat exchanger, the furnace limit switch will shut the furnace off.

Beyond the Symptom: Diagnosing the Real Issue

In my experience with limits, I tend to look beyond the limit tripping to discover the cause. Why? Because from our experience of selling HVAC parts, I tend to think that, especially among homeowners, the ability to self-diagnose means that when a part is identified as the cause of why the furnace is not working, the homeowner, feeling joyful will just want to replace the part.

The problem, however, is that many homeowners either don't know or haven't actually thought about the fact that while the limit (rollout switch and pressure switch) might be the cause for the furnace not working, it is often just a symptom of a greater issue.

Case Study: Misdiagnosis of a Pressure Switch

I recollect a case in the winter of 2024 with a customer who wanted to order a pressure switch. Only after the correspondence of several emails with the customer did I start getting suspicious that the trouble was not the pressure switch.

He told me he was getting a "failed pressure switch" code. He had previously ordered a new one from a competitor of ours and had that switch installed. Only, three weeks later, the same problem came around with the new pressure switch. The customer felt that the competitor sold him a pressure switch that was clearly faulty. I doubted it as my competitors likely get the HVAC replacement parts from the same sources. I asked the customer why he felt that way?

He didn't have a really good answer, and it made me think the problem wasn't the switch but probably something else. Pressure switches are somewhat alike to limit switches. They are a safety feature installed in the furnace to be a device the furnace has to prove in order for the inducer motor to be allowed to come on. Many of the items on the furnace are safety features that expect the furnace to pass before the next stage of the operating system will commence. If any one of the safety devices trips, the furnace will fail to come on.

I began to ask my pressure switch customer how he had tested his pressure switch in order to determine that the pressure switch was the fault. The responses back were very vague. I then asked him the following:

1. Did you check to ensure that the collection box isn't full of water?
2. Did you check the pressure switch for continuity while the furnace was on?
3. Did you check the venting?

What really amazed me was that the customer became irked when I asked him that, as though he actually wanted to buy another pressure switch instead of figuring out the cause behind the pressure switch failure.

Eventually, I told him to call in an HVAC technician, as the likelihood that his pressure switch being bad was slim. Meaning if we had sold him a pressure switch the likelihood of us getting an email asking to send it back was too high.

Airflow: The Underlying Problem

This brings me back to the high limit switch, and likely the main reason why the switch keeps tripping is because the switch is doing its job and is not actually broken.

I often wonder how many of our customers are buying items that they actually don't need because I, being an HVAC technician, can say that I don't run into too many faulty limit switches, and the ones that I run into are simply doing their job. Meaning there is another reason behind the pressure switch failure code.

And what would those reasons be for why your limit switch be tripping?

One of the most common reasons your furnace's high limit switch will trip is not due to a manufacturer malfunction. Instead, it is related to a lack of airflow. Insufficient airflow causes the temperature inside the furnace to rise to a dangerous level, which then triggers the limit switch to shut off the furnace. One of the main causes of this issue is:

• Dirty air filters. For more information, check out our blog on filters.
• Dirty AC units and ducts.

Improving Airflow with Duct Cleaning

In a blog post by Cornwall ont, Quality-Heating.ca, the company goes over why furnace limit switch tripping is due in part to a lack of airflow. One of the simplest ways to fix this is by simply having the ducts cleaned.

It's a good blog post, and I am linking it in this blog. If you live in the Eastern Ontario area, you should check it out and contact Quality-Heating.ca for Furnace Duct Cleanings.

https://quality-heating.ca/duct-cleaning-cornwall

Conclusion: Understanding Cause and Symptom

In closing, making sure that you understand not just the cause but the symptom of the cause will greatly help you in knowing if a part that you order will correct the situation or if it will not.