Overview

If your Mysa for Central HVAC thermostat is rebooting during heating calls, it's possible that a high-limit switch in your furnace is being triggered. This article explains what the high-limit switch does, how it affects your Mysa thermostat, and how to diagnose and resolve the issue.

What Is a High-Limit Switch?

A high-limit switch is a built-in safety device found in most furnaces. Its main job is to prevent the furnace from overheating.

When airflow through the furnace becomes restricted—often due to a clogged air filter—the heat exchanger can overheat. To prevent damage, the high-limit switch will:

• Shut off the heating system

• Cut power to the thermostat (including the 24V signal that powers Mysa)

• Wait until the furnace cools down before restoring power

This power loss is what causes your thermostat to reboot during heating calls.

Common Symptoms of a High-Limit Issue

You may be dealing with a high-limit issue if you notice one or more of the following:

• Your Mysa thermostat reboots during or shortly after a heating call

• Heating runs, but the system doesn't reach the setpoint

• The thermostat shuts off intermittently while heating

• This behavior often starts in the fall or early winter, when heating is first used for the season

What Causes High-Limit Issues?

The most common cause is a dirty or clogged air filter. Over time, filters trap dust, pet hair, and other particles, which restrict airflow and cause the furnace to overheat.

🔁 Homeowners should replace their air filter every 3–6 months to maintain proper furnace performance.

While clogged filters are the most frequent cause, other mechanical issues (e.g., malfunctioning blower motor, blocked ducts) can also trigger the high-limit switch. In such cases, a certified HVAC contractor will be needed.

How to Diagnose High-Limit Issues

If you're unsure whether a high-limit issue is causing the rebooting, you can look for patterns in Mysa’s runtime data or monitor behavior during heating.

Diagnostic Checklist:

• Check runtime gaps: Look for periods when the thermostat went offline during or after a heating call.

• Look for heating triggers: If the temperature drops close to the heat setpoint (within 0.5°F), this suggests the furnace should have turned on.

• Review temperature recovery: After a reboot, if the current temperature is higher than before, it may indicate heating ran briefly before being cut off.

• Compare scenarios: If the thermostat stays online when heating isn’t running, but reboots during heating, it’s likely due to a power cut caused by overheating.

What to Do if You Suspect a High-Limit Issue

Here are the steps you (or your customer) can take:

• Replace the furnace air filter (if it hasn’t been replaced recently).

• Check the furnace diagnostic lights (if accessible) for any fault codes or warning signals.

• Set Mysa’s system mode to Off temporarily. If the thermostat stays powered without calling for heat, this confirms the power loss is related to heating activity.

• Contact an HVAC professional if the problem persists after changing the filter.

⚠️ Mysa thermostats do not reboot on their own unless prompted by a firmware update. Multiple reboots during heating calls typically point to a power issue, not a Mysa device issue.