Pentair MasterTemp ERR SFS: Excessive Stack Flue Temperature

What ERR SFS Actually Means

The MasterTemp has a stack flue sensor mounted in the exhaust path above the heat exchanger. Under normal operation, combustion gases pass through the heat exchanger and transfer most of their heat to the water. By the time those gases reach the flue sensor, they have cooled significantly. If the flue temperature exceeds 480 degrees F, it means heat is not being transferred to the water and is instead going straight up the stack.

This is a critical failure mode. Hot exhaust gases can damage the vent pipe, surrounding structures, and the heater itself. The control board performs a hard lockout identical to ERR AGS: the front panel is completely disabled, and only a power cycle at the breaker will restore it.

ERR SFS is often related to the same root causes as ERR HLS and ERR AGS (insufficient water flow), but it can also indicate heat exchanger damage such as a cracked header or bypass leak that allows combustion gases to escape without proper water contact.

Step-by-Step Troubleshooting

Owner-Level Checks

Verify water flow immediately

1. Is the pump running? Are return jets flowing strongly?
2. Check for closed valves, a dirty filter, or a pump that has lost prime.
3. This is the same first check as ERR HLS and ERR AGS. Without water flow, the heat exchanger cannot cool the combustion gases.

Do not reset and retry without investigation

1. ERR SFS can indicate internal damage to the heat exchanger. Simply resetting and firing again could worsen the situation.
2. Look for any visible signs of damage: soot on the outside of the heater, discolored or warped vent pipe, unusual smells.

Tech-Level Checks

Test the stack flue sensor

1. With the heater cool and powered off, locate the stack flue sensor in the exhaust path.
2. Measure the sensor resistance and compare to the manufacturer's temperature-resistance chart. At room temperature (roughly 75 degrees F), the resistance should fall within the specified range.
3. If the sensor reads open or has wildly incorrect resistance, replace it. A bad sensor can give false SFS errors.

Inspect the heat exchanger for damage

1. Look for cracks in the copper headers or tubes. A cracked exchanger allows combustion gases to bypass the water, sending full heat up the stack.
2. Check for heavy internal scaling that insulates the copper from the water, reducing heat transfer.
3. Look for soot deposits inside the combustion chamber, which can indicate incomplete combustion or gas pressure issues.

Verify gas pressure and orifice sizing

1. Excessive gas pressure or wrong orifice size (NG vs LP mismatch) can cause the burner to over-fire, producing more heat than the exchanger can handle.
2. Check pressure at the gas valve test port. The MasterTemp requires minimum 4" WC inlet pressure for both Natural Gas and Propane, with a maximum of 14" WC. Manifold pressure should read -0.2 ± 0.1" WC (this is a negative-pressure gas valve).
3. Verify the heater's gas type matches the supply. A propane heater on natural gas lines (or vice versa) will not fire correctly.

Check for adequate water flow

1. Use the same flow verification process as ERR HLS troubleshooting. Minimum GPM by model: 20 (175/200), 25 (250), 30 (300), 40 (400).
2. Inspect the bypass valve assembly for sticking or incorrect adjustment.

Common Parts That Fix This Problem

• Stack flue sensor
• Heat exchanger assembly (if cracked or severely scaled)
• Bypass valve assembly
• Gas orifices (if wrong type installed)
• Gas valve or regulator (if pressure is incorrect)

Model-Specific Notes

• The 480 degree F threshold applies to all MasterTemp models. Higher BTU models (300K, 400K) produce more combustion heat and are more likely to trigger SFS if flow is compromised.
• The E05 and E06 error codes are related to the same stack flue sensor (open or shorted). If you see E05 or E06, the sensor itself has failed rather than detecting a genuine high-temperature event.
• On Propane models, verify the LP regulator is providing correct pressure. LP tank regulators can drift or fail, especially in cold weather.
• After any heat exchanger work, always test fire and monitor the stack flue temperature to verify proper heat transfer before leaving the job.

How to Prevent ERR SFS From Coming Back

• Maintain clean filters and adequate pump speed at all times during heater operation.
• Have the heat exchanger inspected and flushed annually if your water is high in calcium or minerals.
• Verify gas type and pressure at every annual service visit.
• Do not operate the heater below 68 degrees F water temperature. Condensation accelerates heat exchanger corrosion.
• Replace the stack flue sensor if resistance readings are erratic, before it fails completely and leaves you without this safety.