Heil furnace 4 flashes: Limit switch string is open

Four flashes means a switch somewhere in the limit string has opened. The string includes the high limit and the flame rollout, and the code does not say which.

Applies to: International Comfort Products *9MPT and *9MPV gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker.

What it means

ICP's wording is the limit switch string — a series circuit that includes the high limit switch and the flame rollout switch, and on many furnaces an auxiliary limit as well. Any one of them opening breaks the string and produces this code. That matters because the two ends of the string have very different implications. A high limit opening means the furnace is overheating, usually from restricted airflow, and it resets itself when things cool. A flame rollout switch opening means flame escaped the burner compartment, which is a carbon monoxide risk, and it stays tripped until someone physically presses it.

Most common causes

• A clogged air filter, which is by a wide margin the most common cause on the limit side.
• Closed, blocked or covered supply registers, or a blocked return grille.
• Undersized or restrictive ductwork, or a duct that has come apart.
• A failing blower motor, or a blower wheel packed with dust.
• A blocked or restricted heat exchanger, on the rollout side.
• A blocked flue forcing combustion products back toward the burners.
• A failed or weak draft inducer not pulling flame into the heat exchanger.
• A genuinely failed switch or a loose connection in the string.

How to diagnose it

1. Establish whether a rollout switch is involved. If the furnace will not restart until a button is pressed, it is rollout and the situation is considerably more serious than a dirty filter.
2. Check the air filter first. Grey, matted or opaque to light means you have very likely found the limit-side cause.
3. Check that carbon monoxide alarms are present and working. If any has sounded, leave and call the fire service from outside.
4. Look for visible scorching, soot or melted wire insulation near the burner compartment — from outside, without dismantling anything.
5. Walk the house and count closed or blocked supply registers.

How to fix it

1. Replace the air filter with the correct size and type and open closed registers.
2. If a flame rollout switch has tripped, turn the furnace off and call a technician. Do not press its reset button to restore heat — it tripped because flame went somewhere it should not.
3. Blower motor faults, duct problems, heat exchanger and flue work are technician tasks.
4. Have working carbon monoxide alarms on every level of the home and outside each sleeping area, which is what NFPA and CPSC guidance call for.
5. Do not keep resetting a furnace that trips its limit repeatedly. Sustained overheating is what cracks heat exchangers.

Notes

A series circuit that reports as one code is a real limitation of this design. The manual reset on the rollout switch is what distinguishes the serious end of the string from the routine end, so whether the furnace recovers on its own is diagnostic in itself.

Related

• 7 flashes — Soft lockout, limit taking longer than two minutes to reset
• 6 + 4 flashes — Soft lockout, limit circuit opened during run

All codes for this manufacturer

Heil, Tempstar and Comfortmaker furnace codes — every code, with what differs between models.

Not sure this is your brand?

The same count means something else on other furnaces. What 4 flashes means on every brand puts them side by side.

Sources

• International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)