The key to maintaining HVAC efficiency and clean indoor air is knowing where the filter sits in an Intertherm furnace. While many Intertherm models share a common design, the exact location can vary by model and installation. This guide explains how to locate the filter, identify the correct size, and replace it safely. It also offers tips on filter types, maintenance schedules, and common issues that can arise from improper filtration.
Where The Intertherm Furnace Filter Is Located
In most Intertherm furnaces, the filter is on the return air side of the system. A common arrangement is a panel at the bottom front of the furnace that hides a horizontal filter slot or framed opening. With the front panel removed, the filter slides in with the arrows pointing toward the blower and into the return duct. Some older or differently configured units place the filter inside the blower cabinet or in a side-mounted slot within the furnace body. If the return air is ducted externally, the filter may live in a wall-mounted or ceiling-return box feeding the furnace.
Tip: Always check the installation manual for model-specific directions, as some Intertherm units route return air from adjacent rooms or hallways, altering filter location.
Typical Locations For Intertherm Models
- Front-bottom panel: The most common placement on gas and electric furnaces; look for a removable panel with a slot behind it.
- Blower cabinet area: Some models have the filter inside or just in front of the blower housing, requiring panel removal.
- Side access: A few configurations mount the filter slot on the left or right side of the cabinet.
- Air return box integration: In homes with an external return, the filter may sit in a wall or ceiling-mounted box connected to the furnace.
In all cases, the filter should be accessible without disassembling major components, and the arrows on the filter frame must point toward the furnace to ensure proper airflow.
Identifying The Correct Filter Size
The correct filter size is determined by measuring the slot or the existing filter frame. Common residential sizes for Intertherm units include 16×20, 16×25, 14×20, and 20×25 inches with a 1-inch thickness, though some installations use 2-inch or thicker media. If the slot is hard to reach, remove the old filter and measure its height, width, and thickness, then purchase the closest standard size. Always verify the exact dimensions before buying.
table:
| Filter Size (Inches) | Common Slot Location | Notes |
|---|---|---|
| 16x20x1 | Front-bottom panel or blower access | Most common; ensure tight fit to avoid bypass |
| 16x25x1 | Front-bottom panel or side slot | Popular in larger return ducts |
| 14x20x1 | Front panel or internal slot | Fits many compact models |
| 20x25x1 | Front or large return box | Used in bigger returns |
Step-By-Step: Find And Replace The Filter
- Turn off furnace power at the thermostat or circuit breaker to prevent dust and debris from moving while you work.
- Carefully remove the front supply panel to access the return air area and locate the filter slot or housing.
- Check the filter orientation; the arrow on the filter frame should point toward the furnace blower.
- Slide out the old filter straight, avoiding bending the frame, and note its size for replacement.
- Insert the new filter with the arrows facing into the furnace, ensuring a snug fit without gaps.
- Replace the front panel securely and restore power to the furnace. Run a quick system test to confirm proper operation.
Choosing The Right Filter Type For Your Intertherm
Filters come in several types, and the right choice depends on air quality needs and system design. Disposable fiberglass filters are inexpensive and common in older Intertherm units but offer modest filtration. Pleated filters provide higher MERV ratings and better particle capture, which is beneficial for households with pets or allergies. A filter with MERV 8–11 generally offers a good balance of efficiency and airflow for most homes. Avoid overly dense filters that restrict airflow and strain the blower, especially in older furnaces.
When selecting a filter, consider: thickness (1-inch is standard, but some units use 2-inch or thicker), airflow requirements, and ease of replacement. If in doubt, start with a pleated 1-inch filter and adjust based on airflow and dust accumulation.
Maintenance Schedule And Signs It Needs Replacement
Filters should be checked monthly, especially during peak heating seasons, and replaced as needed. A typical replacement interval is every 1–3 months, depending on usage, household contaminants, and whether you have pets or smokers indoors. Signs that a filter needs replacement include reduced airflow, visibly dirty or clogged media, increased dust around vents, and higher energy bills due to restricted air movement.
Even with a filter in good condition, the system should be inspected periodically for seals and duct leaks that can bypass filtration. Maintaining a clean filter helps protect the furnace’s blower motor and improves indoor air quality.
Safety Precautions And Installation Tips
Always prioritize safety when servicing a furnace. Turn off power at the thermostat and breaker before opening any panels. For gas furnaces, avoid any ignition sources and ensure the gas valve remains shut if you suspect a leak. Use a flashlight to inspect the filter slot and never force a filter into a slot that is too tight. Ensure the filter sits flat and fills the slot tightly to prevent air from bypassing it.
Handle panels gently to avoid damaging screws or the blower housing. If the filter reveals signs of corrosion, water intrusion, or significant wear, consult a licensed HVAC technician for a diagnostic check.
Troubleshooting Common Filter Problems
- Air feels weak or cool: Check that the filter is not clogged and seated correctly; a clogged filter can reduce airflow and heat transfer.
- Noisy operation: A poorly seated filter or an undersized filter can cause the blower to work harder, producing noise. Re-seat or replace with the correct size.
: Ensure the filter is the exact size for the frame; replace with a properly sized unit if gaps persist. : Consider a higher-efficiency pleated filter if the air feels dusty, and ensure ducts are clean and sealed.
Frequently Asked Questions About Intertherm Filter Location
Q: Can I place the filter anywhere in the return air duct? A: No. The filter must be located where the return air enters the furnace and oriented with the arrow toward the blower to maintain proper airflow and protection for the heat exchanger and blower.
Q: What if I can’t find the filter slot? A: Check the owner’s manual, inspect behind the lower front panel, and look for a side slot or an internal cabinet area. If unsure, contact a professional to avoid damaging the furnace.
Q: Are washable filters acceptable? A: Some systems can accommodate washable or reusable filters, but many Intertherm units rely on disposable pleated or fiberglass filters. Always confirm with the furnace model’s specifications.
Q: How do I know if the filter is the right size? A: Measure the slot or the existing filter frame and verify the exact width, height, and thickness before purchasing. Use a filter that fits snugly without bending the frame.