Furnace Filters and Why They Matter
Recommended filter traits:
- MERV rating of 3 or 5*
- Pleated (not flat spun fiberglass type)
- Inexpensive (about $3.00 to $5.00 each)
- Change filter every 4 to 6 weeks.
* If you are running a 4” wide furnace filter, a MERV 8 is the lowest MERV rating you are likely to find. This rating is acceptable on a 4” wide filter since you have more surface area than a 1” wide filter. Additionally, 4” filters generally will last longer than a 1” filter (somewhere between 3 to 6 months depending on conditions in the home).
Why?
Often times furnace filters are marketed to consumers in terms of their ability to provide clean, allergen-free air for the occupants of the home. While I do not dispute the benefits of clean indoor air, furnace filters are intended to protect HVAC equipment, not occupants. The above filter recommendations are aimed at ensuring optimal equipment function.
For purposes of ensuring clean indoor air in my own home, I run free-standing air purification filters (available at big box stores). These are designed to run high MERV rated (restrictive) air filters – think HEPA-grade filters. HEPA filters are usually equivalent to a MERV rating of 17 – 20. In this way, I’m able to clean my air without over burdening my heating and cooling equipment.
Have you ever been told ‘get the cheapest filter’? 99% of the time, customers are better served with an inexpensive, but not the cheapest filter. Here’s why:
The least expensive filters allow too much debris to pass through the equipment. Once debris accumulates on the evaporator coil (for air conditioning) it is nearly impossible to thoroughly clean. Once your evaporator coil is clogged with debris, it’s like having a permanently dirty furnace filter. Duct cleaning can substantially improve issues arising from clogged evaporator coils, but it’s an added inconvenience and expense. Better to have good filter practices, and keep equipment clean from the start.
Why not use more expensive, high MERV value, filters? (e.g. 11, 12, 13, those marketed as ‘allergen’)
Higher MERV rated filters have the same effect on heating and cooling equipment as a dirty filter; they restrict too much air flow. Heating and cooling equipment, including ductwork, is designed, installed and professionally balanced to ensure the unique characteristics of your home and equipment work together safely and efficiently. By using a filter designed for occupants, rather than equipment, it unbalances the system often leading to overheating, undercooling, premature aging and exceptionally poor efficiency. It’s like putting tractor tires on your Toyota Camry.
Heating and restrictive filters (and clogged/dirty filters):
In the heating season, the cold room air you are trying to warm up actually has a job to do. It needs to return to the furnace to keep it from over heating. If, a filter is too restrictive, an inadequate volume of cold room air reaches the blower compartment and heat exchanger, then equipment over heats.
Overheating equipment does 2 things:
- It prematurely ages equipment
- A safety device will trip causing the furnace to stop heating. In very cold temperatures, your furnace will not be able to reach the temperature programmed on your thermostat. Often the result is “short cycling”, a situation where your furnace turns on and off too frequently.
Cooling season and restrictive filters:
Just like the cold room air has a job to do during the heating season, so to does the warm room air in the cooling season. If the warm room air cannot return to the blower compartment and evaporator coil, the heat exchanger will get colder and colder until it freezes up. Once your AC heat exchanger (evaporator coil) freezes up, absolutely no air can flow through your forced air system, and your house will not cool adequately.