Furnace Filters


One of the most important home maintenance procedures a homeowner can do is to keep a clean filter installed in your furnace.

Why is it so important?
The reason we want to keep a clean filter in place is to preserve the life of the furnace. The primary job of the filter is to keep the heat exchanger clean. The heat exchanger is the most important part of your furnace. This is where the heat generated by your furnace is transferred to the air that comes out of your heat registers to heat your home. If the heat exchanger is dirty its ability to transfer heat is reduced causing it to overheat and possibly crack and fail. This is an expensive repair and sometimes results in an entire replacement of the furnace. Secondly a dirty filter impedes air flow resulting in decreased efficiency. In the summer cooling season a dirty filter can cause the A/C components to freeze up. This hurts efficiency and can damage the furnace.

What type of filter should I buy?
There are three basic types of filters. Disposable, washable and electronic. For this conversation we will talk about disposable filters only as they are by far the most common. Buy a filter with the same dimensions as the existing filter. It should fit in its slot just snug enough to allow installation. If it is too loose it will allow unfiltered air to pass. In that case an HVAC technician should be consulted. In addition to size, a number you need to be concerned with is the MERV rating. (Minimum Efficiency Reporting Value). The MERV rating will be between 1 and 20. The higher the number the more particulates the filter will remove from the air. But higher is not always better. Manufacturers like a lower MERV rating of 2 – 4. This protects the equipment while allowing for maximum airflow for efficiency. Higher ratings filter out more particles which may be beneficial for people with allergies or other health issues but they can restrict airflow, harming efficiency and even damaging the furnace. They will also get dirty quicker. If you feel you need this extra filtering (MERV rating of 12 or higher) you should consult a qualified HVAC technician as they may recommend increasing the airflow through the furnace. I recommend filters with a MERV rating between 4 and 8. These will protect the furnace well and clean the air somewhat for a reasonable cost of around 5 – 6 dollars each.

How often should I change the filter?
A one inch thick filter should last around one month. A four inch thick filter about 4 months. This can vary from home to home but inspect your filter each month and replace it if it looks dirty or clogged. If in doubt replace.

How do I Install my furnace filter?
First turn off your furnace. You do not want the furnace blower to come on while changing the filter. There will be a switch near the furnace or you can turn it off at the thermostat. The filter is most often located where the cold air return duct enters the furnace. Your home inspector will show you where yours is located. Install the new filter with the arrows pointing toward the furnace. When performing home inspections I often see filters installed incorrectly. Make sure you replace the cover to the filter slot. If your cover is missing it is important to replace it. This can affect the performance of your HVAC system.