Goodman furnace HI: High heat at 100 per cent demand

HI means the furnace is running at full output. It is normal in cold weather and worth noticing if it happens in mild weather.

Applies to: Goodman GMVM97 and GCVM97 modulating gas furnaces with a two-character alphanumeric display. On two-stage models such as GMVC96 and AMVC96 the equivalent indication is high stage heat.

What it means

HI indicates the furnace is at 100 per cent demand — maximum firing rate. On a genuinely cold day that is expected and correct. What makes it worth documenting is the inverse case: a modulating furnace that sits at HI in mild conditions is not modulating, and the reasons for that are worth finding. Either the thermostat is not communicating a proportional demand, the furnace is configured as a single-stage appliance, or the house is losing heat faster than it should. None of those is a fault code, and none would be visible any other way.

Most common causes

Cold weather requiring the furnace's full output. This is normal.
A recovery period after a setback, where the furnace runs hard to catch up.
A thermostat not communicating proportional demand, so the furnace defaults to full output.
The furnace configured for single-stage operation.
A house losing heat faster than expected, from an open door, a failed window seal, or insulation problems.
A furnace undersized for the property.

How to diagnose it

Note the outside temperature. HI in genuinely cold conditions is expected.
Note whether the furnace ever shows lower demand percentages, which confirms modulation is working.
Note whether it sits at HI after the house has reached temperature, which would be unusual.
Check whether the thermostat is configured for a modulating furnace, since a mismatched thermostat produces exactly this.

How to fix it

Nothing to fix if the weather explains it.
If the furnace never modulates below full output, have the thermostat configuration checked. A modulating furnace paired with a thermostat that only sends on and off signals cannot modulate.
Check the air filter and registers if the house is not reaching temperature at full output.
If the furnace runs at HI constantly through mild weather, that is worth raising at the next service.

Notes

A modulating furnace that never modulates has been bought and not used. The efficiency and comfort advantages come from the long low-output runs, and a thermostat that cannot request them removes the reason for the furnace.

All codes for this manufacturer

Goodman and Amana furnace fault codes — every code, with what differs between models.

Sources

Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.