ICP (G)(F)97CMN Modulating Furnace Limit Tripping

Skylar Kaczanowski

Nov 13, 2025

Preventing High-Limit Trips on G?F97CMN Modulating Furnaces with Ion® Thermostats

A Technical Guide for Installers & Service Technicians

1. Overview

We continue to see a recurring field issue involving G?F97CMN modulating furnaces paired with the Ion® communicating thermostat, particularly in zoned applications. The most common symptom:

✔ Nuisance High-Limit Trips

—even though the Ion control reports airflow values that appear sufficient.

After joint review with ICP engineering and analysis of Ion zoning logic and heating airflow behavior, we identified several key adjustments and system conditions that significantly reduce overheating and improve overall furnace stability.

This guide consolidates the steps, recommended settings, and important system considerations.

2. Why High-Limit Trips Occur on Ion-Controlled Modulating Furnaces

High-limit events typically result from a combination of:

A. Insufficient Heating Airflow

Modulating furnaces reduce blower speeds at lower modulation rates. In restrictive duct systems or zoned systems with too few open zones, airflow falls below what the heat exchanger requires.

B. Ion Thermostat Behavior

Ion controls:

Do not directly control the blower during gas heating (the furnace board does).
Manage zoning dampers, airflow limits, and duct assessment values.
Override DIP switches and apply airflow strategies based on comfort/efficiency profiles.

But the Ion airflow estimate does not guarantee actual delivered airflow—especially under high static or zoned restriction.

C. Zoned Systems Creating High Static

Ion zoning uses:

No bypass
Automatic duct assessment
Airflow restrictions based on zone percentages

If a zone cannot handle the minimum airflow for heating, the system may:

Reduce airflow
Reduce stages
OR shut down—leading to high-limit events.

D. System Not Re-Commissioned After Installation Changes

Zoning requires correct readings for:

Zone sizes (CFM %)
Leakage
Static pressure
Accessory behavior
Airflow limits per zone

If duct assessment isn't re-run after airflow changes, the furnace may operate outside safe ranges.

3. Recommended Adjustments (Proven in the Field)

These steps have reduced or eliminated high limits on every affected installation we’ve supported.

Step 1 — Increase Heating Airflow (EFF2 Setting)

From the Ion service menu:

Press and hold the hat-icon (Service Menu) for 10 seconds.
Navigate to: Heating Airflow
Set to EFF2

✔ What EFF2 Does

Commands the highest allowable heating airflow for the furnace
Increases blower RPM across all heating stages
Provides the airflow needed for modulating heat without approaching limit temperatures

ICP engineering confirmed this as the preferred setting for installations with:

Zoning
Long duct runs
High static systems
Systems where comfort mode airflow is too low

Step 2 — Enable Inducer Boost

Still in the Service Menu:

Find Inducer Boost
Set ON

✔ Why this Helps

According to the Ion responsibilities slide:

The furnace—not the thermostat—controls combustion airflow.
Increasing inducer RPM increases combustion gas movement.

This reduces heat exchanger dwell time and decreases temperature rise.

Step 3 — Re-Run the Ion Duct Assessment (Critical for Zoning)

Ion zoning depends heavily on accurate duct assessment data:

The assessment measures:

Total system airflow
Zone percentage (CFM share)
Static with zones open/closed
Leakage
Minimum cooling/heating airflow capacity

If this data is inaccurate, the system may:

Restrict airflow too much
Mismanage zone staging
Run airflow levels too low in heating
Create conditions where high-limit trips occur

✔ When to Re-Run Duct Assessment

After enabling EFF2
After enabling Inducer Boost
After zone control wiring changes
After any ductwork changes
After contractor callbacks involving high static or airflow noise

4. Additional Insights From Ion® System Behavior

This section integrates important details that directly influence overheating issues.

A. Minimum Heating Airflow Must Be Met by Every Zone

Ion zoning requires every zone to support the minimum airflow for the equipment.

If a zone cannot support minimum airflow, the system will stage down or shut down.
That often triggers high-limit conditions during attempts to modulate back up.

B. Ion Airflow Strategies Reduce Airflow Based on Noise Limits

Airflow limits for each zone are set as “noise limits”—not target CFM.

If limits are left at default or set too low:

Furnace may be starved for airflow
Upper firing rates create high-limit conditions
Zones may only allow a fraction of needed airflow

C. Furnace Controls Heating Blower, NOT the Ion

“Ion Control does not control the blower during gas heating.”

This is critical:

Even if Ion reports a certain calculated airflow, the furnace blower table may be delivering less.
This mismatch is a common root cause of high limit trips.

D. Comfort vs Efficiency Mode

From the Furnace Setup Options slide:

Comfort Mode = Lower airflow
Efficiency Mode = Increased airflow

Comfort mode frequently leads to overheating when paired with zoning.
EFF2 setting bypasses this by forcing maximum heating airflow.

E. Avoiding Misapplied Dampers

Ion dampers operate:

Power open / power close
15-second timed movement

Using third-party dampers:

Creates incorrect timing
Causes static spikes
Breaks duct assessment calculations
Can lead to limit trips even with correct airflow settings
IO Controls does have a damper that works but uses a different actuator. As long as you don't mix them on a zone , it will be fine.

5. Recommended Best Practices for Installers

✔ Always use Ion dampers

Third-party dampers cause unpredictable airflow problems.

✔ Verify high heat CFM vs cooling CFM

Some zoning designs are sized for cooling airflow—not heating airflow.

✔ Confirm SW4-1 (Comm Relay Disable) is OFF

If ON, furnace communication changes and can cause incorrect airflow control.

✔ Review last 10 events in the Ion thermostat

High limit, zoning limit, airflow restriction, or static pressure alerts reveal root causes.

6. When to Contact Technical Support

If nuisance high-limit trips persist after performing:

EFF2 setting
Inducer Boost
Duct assessment re-run
Zone verification
Damper and static checks

Contact the JS Technical Support & Training Team for further analysis.

Johnstone TSA Team
📞 231-777-2727