Dirty Air Filters: The Top Cause of HVAC Breakdowns in NH

A clogged air filter is the single most common cause of HVAC service calls in New Hampshire. A neglected filter restricts airflow across the indoor coil, which causes a furnace to overheat and shut down on a safety limit, or causes a heat pump or AC coil to ice over. The fix is a $5 to $40 filter and a minute of work. The cost of ignoring it is a $200 to $500 service call, and in some cases damaged equipment.

At A.J. LeBlanc Heating, we respond to dozens of these calls every year across southern NH. Here is what a clogged filter actually does and how to prevent it.

How a clogged filter takes down a system

On a furnace or boiler:

Restricted airflow across the indoor heat exchanger causes the exchanger to overheat. Modern furnaces detect this with a high-limit safety switch and shut down to protect themselves. The cycle then repeats every time the thermostat calls for heat: short run, overheat, lockout. Over time, the repeated thermal stress can crack the heat exchanger, which is a major (often $2,000+) repair or a forced replacement.

On a heat pump or central AC:

Reduced airflow across the indoor (evaporator) coil drops the coil temperature below freezing. The condensate that should drain away instead freezes onto the coil. Over hours or days, the entire coil ices over. The system then either shuts down on a safety, or you wake up to a flooded indoor unit when the ice melts. In the worst cases, liquid refrigerant returning to the compressor damages it.

On a ductless mini split:

The same airflow restriction principles apply, but the washable indoor pre-filters get clogged with finer dust and pet dander much faster than central system filters. Restricted airflow reduces cooling and heating capacity, can ice the indoor coil, and can lead to mold and biofilm buildup behind the filter that is hard to clean later.

How dirty is too dirty?

A simple visual test: hold the filter up to a bright light. If you cannot see light through the filter, it is past replacement.

What lives on a heavily clogged NH filter typically includes:

• Pet hair (the single biggest contributor for households with dogs and cats)
• Human hair
• Skin cells and dust mites
• Pollen (especially in spring and early summer)
• Construction dust from any recent renovation
• Outdoor particulates from wildfire smoke events

Filter replacement schedule

The right interval depends on filter type and household conditions:

• 1-inch disposable filters: check every month, replace at least every three months.
• 4 to 5-inch media filters: typically last six to twelve months.
• Electronic air cleaner cells: rinse every one to three months.
• Ductless mini split pre-filters: rinse every one to two months.

If you have pets, run the fan continuously, have done recent renovation work, or live with someone with allergies, replace more often.

MERV ratings explained

MERV (Minimum Efficiency Reporting Value) is the standard scale for residential filter effectiveness:

• MERV 1 to 4: basic fiberglass filters. Protect equipment but capture very little fine particulate. Avoid.
• MERV 8: the minimum we recommend. Captures pollen, dust mites, mold spores, and pet dander.
• MERV 11: better filtration for households with allergies. Captures most particulates, including some smoke.
• MERV 13: high-efficiency. Captures most bacteria and smoke. Some older systems cannot tolerate the higher static pressure without modification.
• MERV 16+ / HEPA: medical-grade. Usually requires a dedicated bypass filter housing.

Higher MERV is not automatically better. A system designed for MERV 8 and pushed to MERV 13 may suffer from low airflow and the same issues a clogged filter creates. If you want higher filtration, the right move is often a thicker (4 to 5-inch) media filter at a moderate MERV rating, which has more surface area and lower pressure drop than a 1-inch high-MERV filter.

How to change a filter the right way

• Turn off the system at the thermostat first so the blower does not pull dust into the duct system.
• Pull the old filter out slowly to avoid releasing trapped dust.
• Note the arrow on the new filter. It should point toward the blower (downstream in the airflow direction).
• Check the filter cabinet for any obvious dust accumulation while it is open. Wipe with a damp cloth if needed.
• Slide the new filter in, close the cabinet, and turn the system back on.
• Make a note (calendar reminder, sticker on the cabinet) for the next change.

When changing the filter is not enough

If you change the filter and the system still struggles, you may also need to address:

• Closed or blocked supply or return vents
• A dirty blower wheel that has not been cleaned in years
• A dirty evaporator coil from years of marginal filtration
• Undersized return ductwork that restricts airflow at the filter

Schedule a tune-up

If your system is struggling, your filter is older than you can remember, or you want a professional to confirm everything is running properly before the next season, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.

Frequently Asked Questions

How often should I change my furnace filter in New Hampshire?

Check 1-inch filters monthly and replace every three months. 4 to 5-inch media filters last six to twelve months. Pets, renovation work, and continuous fan operation all shorten the interval.

Can I use a higher MERV filter than the system is rated for?

Sometimes, but not always. Higher-MERV filters create more static pressure across the filter, which can starve the system of airflow. A licensed contractor can verify your system can handle a higher-MERV filter, or recommend a thicker media filter that gives you better filtration without the pressure drop.

My filter looks clean but my AC is freezing up. Why?

A frozen coil can also be caused by low refrigerant, a dirty evaporator coil from years of marginal filtration, blocked return vents, or a failing blower motor. Have a technician diagnose it.

Should I use a permanent washable filter?

Most washable filters are MERV 4 or lower, which is not adequate for indoor air quality. We generally recommend a quality disposable or media filter at MERV 8 or higher.

Does running the fan continuously help with filtration?

Yes, if the filter is rated for it. Continuous fan operation circulates more air through the filter, which improves indoor air quality at the cost of slightly higher electric use. Filters need to be changed more often in this configuration.