It is common knowledge that when your furnace filters get plugged it makes the blower work harder---thus consuming more energy and shortening the life of the blower.
But, just like “common sense,” “common knowledge” is also not as common as one might think. In actuality, the opposite is true. The more restricted the ductwork---the dirtier the filters---the less energy is consumed and the easier the blower will run. (Obviously, in the long run more energy will be consumed because of the added time it will take to heat your house.)
The fact that this is true, seems almost counter intuitive; and hence the ease with which the myth became established. I have even heard HVAC technicians spew this wisdom and it is very common to hear home inspectors state it as if it were obvious.
In this first picture you can see the amount of amperage been drawn during normal operation of my fully operational furnace with just cleaned filters---6.8amps.
So now let’s restrict the air flow by installing a piece of plywood in the extra slots of the air cleaner. This will completely cover the air return to the furnace---making the duct more restricted than most dirty filters would ever make it.
If I am wrong the motor is going to work very hard, the amount of amperage drawn will be significantly higher and I will have a lot of egg on my face. But then,in all likelihood, this post would not be here.
As you can see by the second reading on the amp probe the amount of amperage is significantly less---3.7amps---meaning the blower is working much less hard. It is consuming approximately 50% less energy.
Essentially the blower is just going around and around and around. Because the blower isn’t doing any “work”----pulling air----it just sits there and spins.
So what is the real issue with dirty filters?
If they become too clogged there will not be enough air moving through the heat exchanger and it can overheat. This can shorten the life of the heat exchanger/furnace.
In some respects I wish the myth was true because a ruined blower motor is cheaper to fix than a ruined heat exchanger.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board