Today I would like to discuss furnace filters with you. As a home inspector that services Cedar Rapids IA and Waterloo/Cedar Falls IA, I see a lot of furnaces on a weekly basis.I think a basic description of the method in which a furnace works is a good place to start.
A furnace heats our home by moving air over a heat exchanger via fan and through a series of supply and return ducts. So, what we have is a circuit that starts at the air handler which pushes air over the heat exchanger, into the supply/distribution ducts. Air then returns to the furnace via the cold air return ducts and ends up at the filter located just before the air handler. It should be noted here that a severly clogged filter will cause a rise in temperature inside the heat exchanger. This shortens the life of the unit and may lead to a crack in the heat exchanger. Bad News!
There are several different types of furnace filters. Some of the types are; electro-static, 1″ fiberglass disposable, 1″ cotton pleated disposable, 4″-6″ pleated, and last but not least, electronic.
Generally speaking the things that are constant in the heating or cooling process are fan speed and temperature of the heat exchanger or evaporator coils. (Some of the newer furnaces have variable speed ECM’s [electronically controlled motors] which vary fan speed based on perceived demand.)
So when we are talking about efficiency what does that leave? You Guessed it, the filter which can have quite an impact on the amount of air moving through the system. In order to heat or cool the air we must move it over the heat exchanger or evaporator coils respectively. The more air we can move the less time the unit needs to run in order to satisfy the thermostat. This helps our efficiency.
Now that we understand a little about the workings of our forced air furnace, let’s talk about how we can get the most out of them with the part we can control, the filter.
Most of the furnaces I see have a 1″ filter. If this describes your furnace you have a typical installation which is just fine. In this case I recommend a pleated cotton filter, if you must, in the winter because of the increased time spent indoors, and a less inexpensive fiberglass filter for the summer. Most folks find this combination agreeable to their lifestyles.
Without getting into MERV ratings and other details, what you should know is that the 1″ cotton filter will trade some of the ability to move air for the ability to trap smaller particles. With this in mind, you should check this type monthly and if they look dirty, change them. The manufacturers usually give a three month rating on this type of filter, but, that is entirely dependent on the conditions of the home! So, check them monthly until you feel comfortable going longer.
In the summer months I recommend the less expensive fiberglass filters. These also should be checked monthly and replaced if needed. They allow the maximum amount of air to pass through the system while trapping large particles and protecting the unit. Remember the more air we move, the better or closer to maximum efficiency we get.
The 4″-6″ pleated filters give the best of both worlds because they trap small particles while at the same time allowing lots of air to pass through them. This is due to the amount of surface area of the filter. If you were to take one of these and open up the pleats you would see a very large area of filter media. Conversly, the 1″ is not much larger then the area you see when you look at it.
Electronic filters are the best at “cleaning the air”. They trap the most amount of particles. Their down side is that they become less effective as they get dirty. They should be checked monthly as well. These are usually cleaned in the dishwasher.
As far as “cleaning the air” goes, we need to be realistic in our expectations when it comes to the little air handler in our furnace. Generally speaking, unless the fan switch is set to the on position instead of auto, the air handler is only running for a small portion of the day. Even if the fan is left on continuously the amount of air changes per hour accomplished is not sufficient to label our furnace as an air filtration system. It might be wise to keep this in mind when you are looking at those $15.00 1″ pleated filters at the local box store.
I hope this information is helpful to you. This is just some of the many things I pass along during home inspections to those that utilize my services.