How to Choose the Right Furnace Filter

The right furnace filter can make the difference between winter comfort and winter sneezing and congestion. Every house has dust particles, pet dander, and other particulate matter that is blown around easily whenever the furnace kicks on. Choosing the right furnace filter helps protect your and your family from breathing in this material, and also keeps your home heating system functioning properly and efficiently.

Most furnace filters are rated with a MERV (Minimum Efficiency Reporting Value) of 1-11. The higher the MERV rating, the more efficient the furnace filters is at removing particles from the air. A MERV rating of 11 means that the furnace filter has a 85-90% efficiency rating for removing airborne particles 3.0 to 10.0 microns in size. A MERV rating of 6 means the efficiency rating for the filter is only 35-50%. The higher the MERV rating is, the more efficient the filter will be at removing particles from the air.

Furnace filters come in five basic types:

1. Disposable fiberglass filters are the cheapest to buy but also the least efficient.
   
2. Washable ‘electrostatic’ filters are charged to attract dust, and can be washed and reused, but are still only about 20% efficient.
   
3. Pleated allergy filters use a denser material to trap dust particles and are usually about 50% efficient; better that disposable or electrostatics but still fairly inefficient.
   
4. Electronic air cleaners create an ionized field that ‘magnetizes’ particles and then traps them inside the unit. These units are expensive but are 95% efficient and some can even trap up to 80% of airborne viruses: A good choice if you have children or elderly people at home.
   
5. HEPA filters which block most particles but also restrict air flow, and so are not recommended for most residential applications. HEPA stands for High Efficiency Particulate Air Filters.

If you have installed a pellet stove or multi-fuel stove it is recommended that you purchase a special vacuum with a filter fine enough to trap the fine ash that must be cleaned out of the stove every seven to ten days. If you use a regular household vacuum or a shop vac, the fine ash will be blown into the air and will settle onto your furniture as white dust. You will also want to use a high end furnace filter such as a pleated allergy or HEPA filter if you use a furnace blower to circulate the heat.

In general, pellet stoves are extremely efficient and create very little in the way of ash and wood by-product, especially when compared to traditional wood stoves. But if you have never used one of these high-efficiency stoves before, you may notice a change in the level of dust and particulate matter in the air as compared to a gas or oil heating system.

Although electronic air filters run $800 or more per unit, if you use a wood pellet or multi-fuel stove as your primary heat source, you may want to invest in one of these units to keep the very small amount of fine ash generated by these stoves from becoming airborne. If you supplement a gas or oil furnace with a wood pellet stove, another choice is to run your furnace blower to circulate the heat from the stove, and change the furnace filters as frequently as needed.

Finally, you will want to periodically inspect any cold air exchange and the vent for the stove and make sure these are clean and free from the very, very minor ash build up that can occur over time.