How To Check Gas Pressure On A Furnace

Many homeowners encounter furnace problems, especially during the colder months. One frequent culprit behind a malfunctioning furnace is improper gas pressure. This can manifest in various ways, such as the furnace not igniting, producing a weak flame, or cycling on and off frequently. Understanding how to check your furnace's gas pressure can help you diagnose the problem and potentially save you a costly service call. However, it’s crucial to prioritize safety and know when to call in a qualified professional.

Understanding Gas Pressure in Your Furnace

Your furnace needs the right amount of gas to function correctly. Too little pressure, and the furnace won't heat properly; too much, and it can be dangerous, leading to potential damage or even a gas leak. Gas pressure is typically measured in inches of water column (in. w.c.). Your furnace's rating plate (usually located inside the furnace cabinet or on the door) will specify the required gas pressure. Knowing this number is essential for proper diagnosis.

Safety First!

Before you begin any troubleshooting, it's paramount to prioritize safety. Gas is a potentially dangerous substance, so follow these precautions meticulously:

Turn off the gas supply: Locate the gas shut-off valve on the gas line leading to your furnace. Turn the valve to the "off" position (usually perpendicular to the gas line).
Turn off the power: Switch off the circuit breaker that supplies power to the furnace.
Ventilate the area: Open windows and doors to ensure adequate ventilation.
No smoking or open flames: Keep the area free from any sources of ignition.
If you smell gas: Immediately evacuate the building and call your gas company or the fire department from a safe location. Do not attempt to troubleshoot the furnace yourself.

If you are uncomfortable with any of these steps, stop and call a qualified HVAC technician.

Tools You'll Need

To check your furnace's gas pressure, you'll need the following tools:

Manometer (or U-tube manometer): This is the primary tool for measuring gas pressure. A digital manometer is generally easier to use and more accurate, but a U-tube manometer can also be used.
Screwdrivers: You'll need a variety of screwdrivers (usually flathead and Phillips head) to remove panels and access the test ports.
Wrench: A wrench may be required to loosen or tighten gas line fittings.
Gas leak detector (or soapy water): Used to check for gas leaks after making any connections.
Safety glasses: To protect your eyes.
Gloves: To protect your hands.
Flashlight: To improve visibility.
The furnace's rating plate information

Step-by-Step Guide to Checking Gas Pressure

1. Accessing the Test Ports

The first step is to locate the test ports on your furnace's gas valve. There are typically two test ports: one for inlet pressure (the pressure of the gas coming into the valve) and one for outlet pressure (the pressure of the gas leaving the valve going to the burners). These ports are usually small, threaded openings with screws or plugs covering them.

Carefully remove the access panels to expose the gas valve and the test ports. Use your screwdriver to loosen and remove the screws or plugs covering the test ports. Be gentle to avoid damaging the threads.

2. Connecting the Manometer

Now, you'll connect the manometer to the test ports. Most manometers come with a flexible tubing that attaches to the test port. Be sure to use the correct size tubing for a secure and leak-free connection.

For Inlet Pressure: Connect the manometer tubing to the inlet pressure test port. This port is usually located on the gas line leading into the gas valve.

For Outlet Pressure: Connect the manometer tubing to the outlet pressure test port. This port is located on the gas line leading from the gas valve to the burners.

Make sure the connections are snug and secure. If using a U-tube manometer, ensure it's properly filled with water to the zero mark and that it's level.

3. Restoring Power and Gas Supply

After connecting the manometer, carefully restore the power to the furnace by switching the circuit breaker back on. Then, slowly turn the gas shut-off valve back to the "on" position. Be attentive for any hissing sounds indicating a leak. If you suspect a leak, immediately turn the gas back off and address the issue.

4. Starting the Furnace and Taking Readings

Now, start the furnace by turning up your thermostat. The furnace should go through its normal start-up sequence. Once the burners are lit and the furnace is running steadily, observe the manometer reading.

Inlet Pressure Reading: The inlet pressure should be within the range specified on your furnace's rating plate. Typically, this is around 7-10 inches of water column for natural gas and 11-13 inches of water column for propane. A slight variation is normal, but significant deviations indicate a problem with the gas supply or regulator.

Outlet Pressure Reading: The outlet pressure is equally important, as it determines the gas flow to the burners. This pressure should also be within the range specified on the rating plate. An incorrect outlet pressure can lead to improper combustion and inefficient heating.

Record your readings for both the inlet and outlet pressure. Note any fluctuations or inconsistencies.

5. Shutting Down and Disconnecting

After taking your readings, turn down the thermostat to shut off the furnace. Turn off the gas supply at the shutoff valve, and then switch off the circuit breaker.

Carefully disconnect the manometer tubing from the test ports. Replace the screws or plugs on the test ports, tightening them snugly but not excessively. Ensure they are properly sealed to prevent gas leaks.

Replace the furnace access panels, securing them with the screws you removed earlier.

6. Checking for Gas Leaks

This is a critical step. After reassembling everything, check for gas leaks around the test ports, gas valve connections, and anywhere else you may have disturbed the gas lines.

You can use a commercial gas leak detector, or you can make your own solution by mixing water and dish soap in a spray bottle. Spray the solution on the connections and look for bubbles. If you see bubbles, it indicates a gas leak. Do not proceed if you find a leak. Immediately turn off the gas supply and call a qualified HVAC technician.

Interpreting Your Readings and Simple DIY Fixes

Now that you have your gas pressure readings, you need to interpret them and determine if there's a problem. Here's a breakdown of common issues and potential DIY fixes (remember, proceed with caution and only if you feel comfortable):

Low Inlet Pressure: If the inlet pressure is consistently below the specified range, it could indicate a problem with the gas supply from your gas company or a malfunctioning gas regulator.
- DIY Fix: Check your gas meter to ensure it's operating correctly and that the gas supply is turned on. Make sure other gas appliances are not running simultaneously, as this can sometimes cause a pressure drop. If the problem persists, contact your gas company.
High Inlet Pressure: If the inlet pressure is consistently above the specified range, it's a more serious issue that requires immediate attention.
- DIY Fix: DO NOT ATTEMPT TO FIX THIS YOURSELF. This is a dangerous situation. High gas pressure can damage your furnace and create a fire hazard. Call a qualified HVAC technician or your gas company immediately.
Low Outlet Pressure: If the outlet pressure is below the specified range, it could indicate a problem with the gas valve itself.
- DIY Fix: This typically requires professional attention. Attempting to adjust the gas valve without proper knowledge and equipment can be dangerous. However, as a simple check, ensure the gas valve is clean and free of any obstructions. Consult the furnace manual for specific instructions. If in doubt, call a professional.
High Outlet Pressure: If the outlet pressure is above the specified range, this can also lead to dangerous conditions.
- DIY Fix: DO NOT ATTEMPT TO FIX THIS YOURSELF. High outlet pressure can lead to incomplete combustion, carbon monoxide production, and potential damage to the furnace. Call a qualified HVAC technician immediately.
Fluctuating Pressure: If the pressure readings fluctuate wildly, it could indicate a problem with the gas regulator or a blockage in the gas line.
- DIY Fix: Check for any kinks or obstructions in the gas line. Ensure the gas regulator vent is clear of debris. If the problem persists, call a qualified HVAC technician.

When to Call a Professional

While some simple troubleshooting steps can be performed by homeowners, it's crucial to recognize when a professional HVAC technician is needed. Call a professional if:

You are uncomfortable performing any of the steps outlined above.
You suspect a gas leak.
The gas pressure readings are significantly outside the specified range.
You are unable to identify the cause of the problem.
You need to adjust the gas valve.
You suspect a problem with the gas regulator.
You experience any unusual noises or smells from the furnace.
The furnace continues to malfunction after attempting simple DIY fixes.

Remember, your safety and the proper functioning of your furnace are paramount. Don't hesitate to call a qualified professional if you have any doubts or concerns.

Preventative Maintenance

Regular preventative maintenance can help prevent gas pressure problems and extend the life of your furnace. Consider the following:

Schedule annual furnace inspections and maintenance with a qualified HVAC technician.
Regularly replace the furnace filter.
Keep the area around the furnace clean and free of obstructions.
Inspect the gas line for any signs of damage or corrosion.

By understanding how to check your furnace's gas pressure and practicing preventative maintenance, you can keep your furnace running efficiently and safely for years to come.