Emerson 1171 Furnace Pressure Switch Review

We may earn a commission from qualifying purchases made through links on this page.

Emerson 1171 Furnace Pressure Switch: Overview & How It Works

If you are troubleshooting a gas furnace that won't ignite or is throwing draft pressure fault codes, replacing a failing vacuum switch is often the first step. The Emerson 1171 Furnace Pressure Switch is a professional-grade safety component and a reliable replacement for resolving common no-heat issues and ignition failures. For those managing broader furnace timing problems, pairing this with a Honeywell Home ST9400C universal furnace fan timer can ensure a complete system restoration.

By providing a critical fail-safe for draft inducer systems, it ensures your HVAC system operates safely—much like how a Filtrete MERV-12 air filter protects the blower motor from debris. By verifying that the draft inducer motor is operational and the flue vent is unobstructed before ignition, it serves as the primary solution for resolving furnace fault codes and no-heat issues. Because it prevents the dangerous leakage of carbon monoxide into the home, this switch is one of the most critical safety devices in any HVAC system.

Restore Safety: Ensure your furnace's safety interlocking system is operating to factory specifications with the Emerson 1171 Pressure Switch.

Maintaining a dry installation environment is key to component longevity. For those with damp utility rooms, we recommend using a GE ADEL50LZ dehumidifier to protect your equipment from moisture-related corrosion. For a broader look at managing your utility spaces, our best submersible utility pumps for draining water can help you keep these areas dry and safe, a critical step when managing a Basement Watchdog BW1033 sump pump system guide.

For the average homeowner, this part usually comes into play during a "no-heat" emergency. It works in tandem with other critical components, such as a universal furnace fan timer, to maintain overall system timing, to ensure the system cycles through its startup and cooldown phases correctly. When a furnace starts its cycle, the inducer motor kicks on first. If the pressure switch does not "close" (complete the electrical circuit) within a specific timeframe, the furnace control board will trigger a lockout or a fault code. While it is tempting to assume the switch is broken, the Emerson 1171 is designed to react to the environment—meaning it might be working perfectly but reporting a dangerous blockage in the vent.

This switch is best suited for those who have already performed a basic diagnostic check or for HVAC technicians looking for a dependable, OEM-spec replacement. Because pressure switches are calibrated to specific inch-of-water (IW) measurements, the Emerson 1171 is not a universal part. It must be matched to the specific requirements of the furnace model to ensure that the furnace neither fails to start nor, more dangerously, starts when the vent is actually blocked.

Key Features

Pros & Cons

Performance

In real-world HVAC scenarios, the Emerson 1171 Pressure Switch behaves as a binary safety gate.

Its performance is measured not by "speed" or "efficiency," but by accuracy and reliability. When the inducer motor creates a vacuum in the venting pipe, the diaphragm inside the 1171 moves, closing the electrical contacts. This tells the furnace control board, "The path is clear; it is safe to light the gas."

When we test the 1171 as a replacement for a fatigued or failed switch, the result is typically immediate restoration of the ignition sequence. A failing switch often exhibits "intermittent" behavior—the furnace might start three times and then lock out on the fourth. The Emerson 1171 eliminates this inconsistency, providing a crisp, reliable closure that ensures the heat comes on every time the thermostat calls for it.

It is important to remember that this component exists within a larger ecosystem of home utilities. Because furnaces are often installed in damp basements, the surrounding environment can affect component longevity. We suggest maintaining a controlled environment using a high-capacity dehumidifier to prevent premature corrosion of electrical contacts in your furnace and other critical tools. Similarly, if your furnace is located near a sump pump, ensure your Wayne ESP25 battery backup sump pump is operational—as discussed in our Wayne ESP25 battery backup sump pump review—to prevent flooding, which could either damage your furnace or trigger a safety shutdown via the pressure switch due to moisture in the inducer lines. For those with high-risk areas, a Ridgid SP-650C sump pump or a Basement Watchdog BW1033 is often a necessary companion to protect home HVAC equipment from flooding.

From a performance standpoint, the 1171 is a "restoration" part. It does not add smart features or increase the BTU output of your furnace. For homeowners looking to supplement their primary heating with more modern, efficient options, we recommend exploring our MrCool DIY-24-HP-WIF115 mini-split installation guide for those considering a transition to ductless heating alternatives for more modern, energy-efficient alternatives, such as the systems detailed in our Bosch BMS500-18HDN1 guide. However, for existing gas systems, the 1171 is essential. Instead, it restores the factory-standard safety parameters. While some users might be tempted to bypass a pressure switch to get heat during a freeze, this is extremely dangerous. The performance of the Emerson 1171 is designed specifically to prevent such disasters by forcing the system to fail-safe. Just as safety switches protect your heating, integrated home monitoring systems—such as those detailed in our Ring Alarm Pro guide or our SimpliSafe 12-piece kit guide—protect the rest of your home from external threats.

Deep Dive: How the Pressure Switch Works

To fully appreciate the value of the Emerson 1171, one must understand the physics of a gas furnace startup sequence. A gas furnace cannot simply "turn on" the burners; it must first ensure that the combustion byproduct (carbon monoxide and other flue gases) has a guaranteed path out of the home. This is achieved through a process called "drafting."

The sequence begins when the thermostat sends a 24V signal to the furnace control board. The board first activates the draft inducer motor—a small, high-RPM centrifugal blower. This motor creates a negative pressure environment (a vacuum) inside the heat exchanger and flue pipes. The Emerson 1171 is connected to this airflow path via a flexible rubber hose.

Inside the 1171 is a calibrated diaphragm and a set of electrical contacts. As the inducer motor ramps up, the vacuum pulls on the diaphragm. Once the vacuum reaches a specific threshold (measured in inches of water column), the diaphragm snaps, closing the electrical circuit. This "click" is the signal to the control board that the venting system is operational. Only after this signal is received will the board proceed to the next step: opening the gas valve and igniting the burners.

If the switch fails to close, it is usually due to one of three reasons: mechanical failure of the diaphragm, electrical oxidation of the contacts, or a lack of actual vacuum. The Emerson 1171 is engineered with a high-tensile diaphragm to prevent "stretching" over time and gold-plated or high-conductivity contacts to ensure that the signal is sent cleanly every time. This precision prevents "nuisance trips" where a furnace shuts down despite having a clear vent.

Detailed Buyer's Guide: Choosing the Right Pressure Switch

Buying a furnace pressure switch is not like buying a lightbulb; you cannot simply choose the "brightest" one. The Emerson 1171 is calibrated to a specific pressure set point. If you install a switch with a pressure rating that is too high, the furnace may never ignite because the inducer cannot create enough vacuum to close the switch. Conversely, if the rating is too low, the furnace might ignite even when the vent is partially blocked, risking carbon monoxide leakage.

How to Verify Compatibility

• Check the Part Number: The most reliable method is to find the part number stamped on the side of your existing switch. If it says "1171" or matches the Emerson cross-reference list, you are in good shape.
• Verify the Pressure Rating: Look for a number followed by "WC" (Water Column) or "IW" (Inches of Water). This is the precise vacuum level required to trigger the switch. The replacement must match this number exactly.
• Inspect the Port Size: Ensure the vacuum hose barb on the Emerson 1171 matches the diameter of your existing rubber tubing. While most residential units are standardized, some older or high-efficiency units have varying sizes.

When to Replace vs. When to Clean

Before purchasing the Emerson 1171, perform the following checks to ensure you aren't replacing a working part:

The Condensate Trap: In high-efficiency furnaces, water can collect in the inducer housing or the pressure switch line. This "water slug" blocks the air pressure. Cleaning the trap often fixes the "fault code" without needing a new switch.

The External Vent: Check the PVC pipes exiting your home. Snow buildup, bird nests, or debris can block the exhaust, preventing the vacuum from forming. If you replace the switch but the vent is blocked, the new switch will (correctly) refuse to let the furnace start.

Installation and Setup Guide

Installing the Emerson 1171 is a straightforward process for those comfortable with basic electrical and mechanical work, but it requires strict adherence to safety protocols. Failure to follow these steps can result in electrical shock or, more seriously, gas leaks.

Step-by-Step Installation

1. Power Down: Switch off the furnace power at the breaker or the service switch. Verify the power is off using a non-contact voltage tester. Never work on a live furnace control board.
2. Gas Safety: While not strictly necessary for a pressure switch replacement, ensure you are working in a well-ventilated area and that no gas odors are present.
3. Document Wiring: Before disconnecting the old switch, take a clear photograph of the two wires connected to the terminals. The order usually doesn't matter for a simple switch, but documentation prevents errors and saves time if you need to reference the original setup.
4. Remove Old Hardware: Unscrew the mounting bracket and gently pull the vacuum hose off the old switch. Inspect the hose end for any tearing or "mushrooming" that might lead to an air leak.
5. Mount the Emerson 1171: Secure the new switch into the existing mounting holes. Ensure it is seated firmly to prevent vibration from the inducer motor from loosening the connections over time.
6. Connect Vacuum Line: Push the rubber hose firmly onto the barb. Ensure a tight seal; a loose connection here is the #1 cause of "installation failure," where the switch is new but the system still won't ignite.
7. Wire the Switch: Attach the furnace wires to the terminals. Ensure the connections are tight and no bare copper is touching the furnace chassis, which could cause a short circuit.
8. Test Cycle: Restore power and set the thermostat to call for heat. Observe the inducer motor starting, then listen for the "click" of the pressure switch closin