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This switch verifies safe draft conditions. It lets the system light only when the vent is clear and the inducer is running properly.
Specifications
- Part number: 18A291
- Brand: Ultrafryer
- Part type: Air pressure switch
- Depth: 3 5/16" | 84 mm
- Diameter: 3 9/16" | 90 mm
- Height: 3 In
- Length: 3.5 In
- Material: Metal
- Width: 3 In
- Setpoint: same inches WC, like -0.90 in
- Contacts: same type, usually normally open
- Ports: same number of hose ports
From the maker's and parts sellers' listings for this number. Compare them with the label on your old part.
What the air pressure switch does
An air pressure switch protects your equipment from venting hazards. It monitors the negative pressure created when the draft fan turns on.
The pressure switch lets the furnace light only when the vent is clear and the inducer is running. If draft pressure drops, the switch opens its circuit to stop the burners safely.
Specifications for part 18A291
If you have this part in your hand, check its physical measurements against the listed ratings to confirm a physical match.
- Height: 3 In
- Width: 3 In
- Length: 3.5 In
- Depth: 3 5/16" | 84 mm
- Diameter: 3 9/16" | 90 mm
- Material: Metal
Confirming a replacement matches
Matching a replacement pressure switch requires checking key specifications against your old unit. Never install a switch that does not meet the exact pressure and port needs.
- Setpoint: must have the same inches WC rating, like -0.90 in.
- Contacts: must use the same contact type, usually normally open.
- Ports: must have the same number of hose ports.
- Material and size: verify the metal housing and body dimensions fit your mounting bracket.
Signs of trouble and safe replacement steps
A cracked hose, a blocked vent, or a clogged drain can make a good switch look bad. Always check the hose and vent paths first before changing parts.
Follow these steps when you need to replace a pressure switch:
- Turn off electrical power to the equipment at the breaker and verify power is completely off.
- Inspect the tubing connected to the switch ports for cracks, moisture, or clogs.
- Take a clear photo of the switch wiring and hose connections before removing anything.
- Disconnect the wires and vacuum lines carefully.
- Mount the matching switch and reconnect the hoses.
- Move wires one at a time to the same spots as shown in your photo, then restore power.
Read your old part
- Find the stamped label or sticker on your existing metal pressure switch body.
- Read the part number printed on the label to verify it shows 18A291.
- Count the hose ports to ensure the port count matches your replacement.
- Check the setpoint rating on the label to confirm the exact inches WC matches.
When to call a pro
Hire a licensed HVAC technician for any step you are not sure you can do safely. If the switch continues to trip after installation, or if you suspect venting problems, call a licensed pro right away.
Questions
Can I jump the 18A291 pressure switch to test the heat?
No, never bypass a pressure switch. Jumping a pressure switch can allow carbon monoxide into the house. It is a critical safety device designed to shut down the burners if venting fails.
Why does my equipment fail to light even if the switch is good?
A cracked hose or a blocked vent or drain can make a good switch look bad. Check those items first. If the inducer fan cannot pull enough draft through the vent, the switch will stay open.
What should I do before unplugging wires from the old switch?
Take a photo of every wire before removing a part. Move wires one at a time to the same spots on the new part to avoid wiring errors.
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