The Honeywell Thermostat Reset Button Won't Fix a Dirty Evaporator Coil (and Neither Will a Table Fan or Dehumidifier)

Full disclosure: I'm an HVAC controls consultant who's been specifying and troubleshooting building climate systems for nine years. I've personally made—and documented—seven significant mistakes, totaling roughly $18,000 in wasted budget. The most embarrassing one started with a Honeywell thermostat reset button.

Here's the scene I keep walking into. It's July, it's 3 p.m., and the lobby is 79°F. The Honeywell thermostat says Cool On. The air handler is running. But nothing is getting colder. Someone suggests pressing the Honeywell thermostat reset button. You press it. The display blinks, the system restarts, and maybe you get fifteen minutes of lukewarm relief. Then it's warm again. So someone sets up a Honeywell table air circulator fan and drags a dehumidifier out of the storage closet. All three are treating the wrong problem.

The Surface Problem: The Thermostat Is Not Actually Lying

A thermostat is a sensor and a switch. It reads the room air temperature from where it sits. If the room is 79°F, it shows 79°F. It's not lying. It just isn't looking at the evaporator coil.

The Honeywell thermostat reset button clears fault codes, reboots the control board, and resets stored settings. According to Honeywell Home support, it does not unclog a filter or remove dust from a coil. I've pressed it more times than I'd like to admit, usually while the building owner watched.

The Deeper Reason: A Thermostat Can't See Blocked Airflow

The real culprit in a no-cooling call is usually airflow. Specifically, a dirty evaporator coil.

When the evaporator coil gets covered with dust and pet hair, air doesn't pass through the fins easily. Less airflow means the coil gets colder than it should. Then frost forms. The frost blocks more air. The room stays warm while the thermostat keeps calling for cooling.

This is why the reset button looks like a hero. If you turn the system off with a reset and wait a little while, the frost melts. Turn it on again, and you get cold air for a while. Then the frost builds again. It's exactly the kind of loop that convinces people to keep resetting instead of opening the panel.

A Nest Thermostat Has the Same Blind Spot

I'll say something slightly heretical for an HVAC controls guy: a Nest thermostat is not a diagnostic tool. Neither is a Honeywell one. Both will happily show that cooling is active while the actual airflow is half of what it should be. The brand of smart thermostat doesn't matter when the problem is below the thermostat, in the coil.

Why the Honeywell Table Air Circulator Fan and Dehumidifier Are Comfort Band-Aids

I like the Honeywell table air circulator fan. It moves air. Wind chill is real. A breeze can make a 79°F room feel closer to 74°F—as long as the air being moved isn't warm and humid. But if the evaporator coil is frozen, the fan is just pushing air that the AC isn't cooling. It's a comfort tool, not a cooling fix.

A dehumidifier is a little more strategic. In a humid building, high humidity makes 78°F feel like 84°F. Lowering humidity makes a space feel more comfortable at a higher thermostat setpoint. But a portable dehumidifier rejects heat into the same room it's sitting in. If the AC is already struggling, the dehumidifier can bump the room temperature up while pulling moisture down. That's not a great trade.

The Real Cost: A Mistake That Put a Checklist on My Wall

In August 2021, I got called to a small medical office that had already replaced parts without solving anything. The tenants were complaining about a 4-degree temperature swing. The HVAC contractor had swapped the thermostat, then the control board, then the condenser fan motor. None of that fixed it.

I opened the return air compartment and saw the evaporator coil. It looked like an old wool blanket. The filter hadn't been changed in months, and the coil was carrying years of built-up fiber. Frost covered about 60 percent of the coil. A $120 coil cleaning and a new $15 filter fixed it. The $900 in unneeded parts were not refundable.

That's when I created our checklist. In the past 18 months, we've caught 23 similar issues, and none of them required a thermostat reset. Every one had a coil that needed attention.

This isn't just a comfort problem. According to the U.S. Department of Energy (energy.gov), a dirty air filter can make your HVAC system work harder and reduce its efficiency. The same is true for a dirty evaporator coil. A system that runs longer and harder costs more to operate. One office we worked with dropped its summer utility bill by roughly 30 percent after coil cleaning and filter changes—but that was one building, not a guarantee.

And if the coil stays frozen, the risk is bigger than a high bill. Liquid refrigerant can flow back into the compressor, which can shorten its life. Don't hold me to this, but in January 2025 I quoted a commercial coil cleaning at $125 to $300 depending on access. Verify your own local rates. It's still a lot less than a $3,500 compressor replacement.

How to Clean Evaporator Coil (The Short Version)

If you've never done this, know your limits. The process isn't glamorous, but it's basic. And if you're not comfortable near electrical panels or refrigerant lines, stop at the inspection step and call an HVAC tech.

1. Turn off the system at the thermostat and at the breaker or disconnect. Any ice on the coil must melt completely before you touch it. Do not chip ice off the fins.
2. Take out the air filter and inspect the coil. On a furnace or air handler, the coil sits above the blower behind a metal access panel. On a mini-split, it's behind the front cover.
3. Remove loose dust with a soft brush and a vacuum attachment. Work carefully—fins bend easily and are sharp.
4. Spray a no-rinse foaming coil cleaner onto the fins. Let it sit for the time listed on the label, usually 10 to 15 minutes. Don't use a pressure washer.
5. Clear the condensate drain line while the panel is open. A blocked drain is the backup cause of humidity problems and water drips. I learned this after a one-week delay because I skipped it once.
6. Replace the filter, close the panel, and restore power.

When to Stop and Call a Tech (Honest Limitation)

I'm not going to tell you that cleaning the evaporator coil fixes every no-cooling call. It doesn't. If the coil is clean, the filter is fresh, and the system still blows warm or freezes, you might have low refrigerant, a bad expansion valve, a failing fan motor, or a duct problem. At that point, the most useful thing you can do is stop and call an HVAC tech.

That's the honest limitation. A thermostat reset is for clearing codes, not clearing coils. A fan and a dehumidifier can support comfort, but they can't replace airflow. The evaporator coil is where heat transfer actually happens. That's the first place to look.

Next time someone tells you to hit the Honeywell thermostat reset button, take a breath. Reset it if you need to clear an error code, but don't call it a fix. While you're waiting for the tech, the Honeywell table air circulator fan and a dehumidifier can keep people more comfortable. Just don't expect them to solve the problem.