First-time homebuyer finds filthy HVAC filter after inspector showed only the clean-looking end

A first-time homeowner checking the HVAC filter in a newly purchased house got an ugly surprise after discovering a "monstrosity."

The couple had been shown the filter during the inspection, but the cleaner-looking end was misleading. Once they removed it, the frameless filter was so severely clogged on the bottom that the owner turned to Reddit's r/HVACAdvice community for suggestions.

Here's what to know

According to the Reddit thread, after the homeowner and his wife bought their first house, they checked whether the furnace filter needed replacing. But instead, they found an old washable "hog hair" filter at the bottom of the furnace, caked with dirt, dust, and grime.

The original poster then asked the HVAC forum, "Wondering if you guys have any recommendations on what kind of filter and what kind of MERV I should get?" 

OP was referring to air filters' MERV rating (minimum efficiency reporting values), which determines their ability to capture different-sized particles. The slot was marked for a 25x16x1 filter, and the OP then asked whether a MERV 8 replacement would be a good choice. 

One commenter answered directly, saying, "Yes! MERV 8 would be an upgrade. Definitely get rid of the 'washable' one."

But much of the thread's advice pointed to using a simple disposable filter rather than keeping the old one in service. One suggested a "16x25 pleated 3 pack at home depot," while another said "MERV 8 or less is fine" for an older setup.

By the end of the thread, the OP said they had made the switch, writing that they bought an FPR 5 filter from Home Depot, which they understood to be close to MERV 8.

More background

The conversation also shifted to airflow concerns, not just the dirt on the filter. As one commenter warned when discussing filtration levels, "You don't want to go too high with the MERV because it will restrict the airflow making it harder for your motor to work and killing it eventually."

People in the thread also suggested checking the replacement filter again after two and four weeks and having the blower wheel and A/C coil inspected, since a filter this dirty can point to broader neglected maintenance.

Overall, the recommendations were to verify the system's required size, replace the old filter, and avoid assuming the highest filtration rating is automatically the best option.

Where can I learn more?

A dirty filter can be just the start of the problem. 

• One first-time buyer turned on their AC after a house sat empty for two years.

• In Michigan, a couple's filter turned black in 12 days, prompting duct-cleaning advice.

• Another homeowner's filter turned white after 10 days, with a humidifier suspected.

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