The filter location on Intertherm furnaces is typically on the return-air side of the unit. Locating and replacing the filter is a quick, essential maintenance task that supports efficient heating, better indoor air quality, and lower energy costs. This article outlines where to find the filter, how to choose the right type, step-by-step replacement instructions, and practical maintenance tips for American homes.

Filter Location Basics For Intertherm Furnaces

Most Intertherm furnaces place the air filter in a slot on the return-air side of the furnace cabinet. In many models, the filter sits behind a removable front panel or service door near the bottom of the unit. The exact location can vary by model and installation, so attention to the following common setups helps prevent guesswork.

Common locations to check include the following:

The bottom front of the furnace, behind a removable panel or grille.
Inside the blower compartment, behind a service door that opens toward the living space.
In some manufactured-home installations, the filter may be part of a return-air grille in a hallway or utility closet.

Tip: If the filter is not obvious, consult the owner’s manual or the installation guide for the exact model. A quick photo of the furnace can also help a technician confirm the correct filter slot.

Choosing The Right Filter For An Intertherm Furnace

Choosing the correct filter type and size is crucial for airflow, energy efficiency, and indoor air quality. Intertherm furnaces commonly use 1-inch thick filters, but some models support thicker designs.

1-inch thick filters: The most common option for many Intertherm models. They’re available in fiberglass or pleated varieties and typically carry a MERV rating of 6–11, balancing cost and filtration.
4-inch thick filters: Some Intertherm configurations use 4-inch or deeper filters, especially in higher-efficiency setups or homes with higher particulate loads. These can offer higher MERV ratings (8–13) and longer replacement intervals.
Filter materials: Fiberglass filters are inexpensive but offer basic filtration. Pleated filters provide better filtration and higher surface area, improving efficiency and airflow. Gas and electric furnaces generally tolerate both types when the correct size is used.
Filter height and orientation: The filter must fit snugly in the slot with the airflow arrow pointing toward the furnace. An ill-fitting filter restricts airflow and reduces efficiency.

Important: Always verify the model’s documentation for the exact filter size and MERV range. Using an incorrect size or an overly restrictive filter can strain the blower and degrade system performance.

Step-By-Step Guide To Replacing The Filter

Replacing an Intertherm furnace filter is a straightforward maintenance task. Follow these steps to complete the job safely and effectively.

Turn off power to the furnace. If the system is gas-powered, switch off the thermostat and ensure no preheater or blower is running during replacement.
Open the access panel or door that covers the filter slot. Use caution to avoid damaging wiring or components inside the cabinet.
Carefully remove the old filter. Note its orientation, especially the direction of airflow indicated on the frame.
Inspect the blower area and the filter slot for dust, debris, or signs of mold. Wipe the slot clean with a dry cloth if needed, and remove any loose debris you see around the opening.
Insert the new filter with the airflow arrow pointing toward the furnace. Ensure a snug fit with no gaps around the edges that could bypass filtration.
Close the access panel securely. Restore power to the furnace and, if applicable, set the thermostat to a heating mode to confirm normal operation.
Run the system briefly to verify airflow sounds normal and there are no unusual vibrations or odors. If the airflow seems restricted, recheck the filter orientation and size.

For enhanced performance, check the filter once a month during peak heating months and replace as needed. If you have pets, allergies, or smoke exposure indoors, more frequent replacements may be necessary.

Measuring Filter Size And Handling Common Pitfalls

Correct sizing is essential for effective filtration and system efficiency. Use these guidelines to avoid common mistakes.

Measure the actual slot: Width x height x thickness. Don’t rely solely on model year or assumption; sizes can vary even within the same brand family.
Match the fit: Choose a filter that precisely fills the slot with a tight seal. Filters that are too small allow unfiltered air to bypass filtration; those that are too large won’t fit.
Record the size: Write the size on the filter frame or keep a log in the furnace area to speed future replacements.
Consider performance: If you notice dusty surfaces, lingering odors, or increased dust in the home, upgrading to a pleated filter with a higher MERV rating (within the furnace’s tolerance) can help.

Pro tip: When replacing, align the new filter so the air flows in the direction indicated by an arrow on the frame. Misoriented filters can reduce filtration efficiency and airflow.

Maintenance Schedule And Common Issues

Maintaining the Intertherm furnace filter location and replacement routine supports reliable heating. The frequency of replacement depends on several factors.

Household factors: Homes with pets, smokers, or occupants with allergies typically require more frequent changes.
Seasonal usage: In winter, when the system runs steadily, check filters monthly. In lighter use periods, you may extend to every 2–3 months.
Filter type: Pleated, higher-MERV filters typically last longer than simple fiberglass filters, but installation difficulty may vary with model.
Air quality changes: If you notice reduced airflow, cold spots in the home, or increased dust on furniture, check and replace the filter promptly.

Ignoring filter maintenance can cause several issues, including reduced heating efficiency, higher energy bills, and increased wear on the blower motor. Regular inspection also helps catch potential problems early, such as a dirty blower compartment or a degraded gasket seal.

Common Questions About Intertherm Filter Location

Q: Can I use a thicker filter than the slot allows?: A: No. A filter that is too thick may bind or prevent the door from closing properly. Only use the thickness specified by the model’s manual or the manufacturer’s guidance.
Q: What if I can’t find a filter slot?: A: Some newer or renovated installations may place the filter in a return grille away from the furnace cabinet. In such cases, check the return air duct near the furnace, or refer to the model’s installation guide.
Q: How long should a filter really last?: A: For homes with normal indoor air and low particulate matter, 1-inch pleated filters usually need replacement every 1–3 months. Higher-efficiency or deeper filters may last longer, depending on conditions.
Q: Does the Intertherm manual provide exact sizes?: A: Yes. The manual lists recommended filter dimensions and MERV ranges. Always confirm before purchasing replacements to ensure compatibility with your unit.

Table: Common Intertherm Filter Sizes And Notes

Filter Size (inches)	Typical Use	Notes
16 x 20 x 1	Standard 1-inch filters for many Intertherm models	Widely available; pleated options increase filtration without changing fit
20 x 25 x 1	Common in mid-sized installations	Check model guidance for compatibility; higher MERV options available
16 x 25 x 1	Alternate common size for some units	Ensure a tight seal to avoid bypass
14 x 20 x 4	Four-inch deep deeper filters	Typically higher MERV; longer replacement intervals; verify slot depth

Regularly inspecting the filter location and replacing the filter according to the system’s needs improves comfort, efficiency, and indoor air quality. Always refer to the specific Intertherm model’s documentation for exact dimensions, recommended MERV ranges, and replacement intervals.