Changing Funace Filters in Time

Created: January 13, 2011
Last Revised: October 21, 2023

Old Furnace Filter vs New Furnace Filter

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Here's the question:

How dirty does a furnace filter get before it starts wasting energy up the chimney?

OR: Is there an easy way to know when it's best to change my furnace filter?

This article relates to any furnace that has a chimney and circulates heated air by some means.


Think about it. When you have a fresh, clean filter in your furnace, you get maximum air flow through the furnace's plenum chamber, which is where the heat from the fire is conducted to the air passing by it.
The more air that moves past the plenum chamber, the more energy is pulled from the fire and therefore to the building that the furnace is designed to heat.
That air doesn't get heated to the full temperature of the fire. The heat that does not transfer to the air simply goes up the chimney. The difference between the heat that can be used to heat the house and the heat that goes up the chimney is a measure of the efficiency of the furnace.
Many wood furnaces may have efficiencies as low as 50%.
Oil fired furnaces are a little better, at about 60% to 70%. (Don't bug me if I am a little conservative on this, because I am simply trying to make a point).
Conventional Gas Furnaces (those with regular chimneys)can get up about 85% or so.
Okay, an example: Let's say you have an 85% efficient furnace and your fuel bill is $1000 per year. At its BEST, you are sending $150 up the chimney each heating season.
What do you think is happening if you have a plugged furnace filter?
Pete's Answer:
You are worsening the partial vacuum between filter and the air distribution system and therfore reducing the energy transfered to the air passing the plenum chamber.
So--- if the furnace filter is plugged up, you are simply sending more of your heating dollars up the chimney and less of those heating dollars into the structure that you want to heat.

So, What Can Be Done?

I suggest installing a "Draft Gage" to sense the amount of heat-robbing vacuum in the furnace cabinet.
Here's the one that I use. Currently it costs about $34.00 US.
Draft Gage for Furnace Filter
Dwyer Mark II Model 25 Inclined Manometer, 3" WC

Contact any supplier of HVAC equipment to get one, or Google the name above to buy one online.
Our's is a manufactured home and it has the furnace on the main floor. The furnace is just inside the back door, in the laundry area, so I can see the gage anytime I come through that entrance.
DraftGage Installed in Furnace Cabinet
The gauge comes with several feet of tubing, so one has the opportunity to mount it someplace where it is easy to see.

There are a couple ways to determine when your filters need to be changed.
1. In my case, I use 3M Filtrete filters, so I called 1-800-3MHelps to find out the maximum vacuum that the filter should have.
Their suggestion seems to work out pretty well because, when I see the gage reading about .17 inches of water and change the filter(s), they are pretty well clogged up.
2. If you can't get a "number" from the manufacturer of the filters you use, simply read the draft gage when you install a new filter and then check the gage every month or so to watch the reading increase. When you see the reading increase from, let's say .10 inches of water to .12 inches of water, remove the filter and check it for build up. If the build up is minor, put it back and wait another month or so. Read the gage and look at the filter again. The filter will be getting more plugged up. Continue the process until the gage reads about .02 or .03 inches of water more than it did when you put the new one in. If the filter still doesn't look very dirty, I'd replace it anyway.
Filters vary a lot in their intial impediment to air flow. Usually the more stuff the filter traps, the higher the vacuum it causes. So, if you don't NEED a super-duper anti-alergen filter, you will probably save some money on your heating bill.

Here are a couple of examples of readings that I got just yesterday. I admit that I had not taken a reading when I put this filter in, but, when I checked it, it was pretty well clogged. It was the less expensive of the 3M filtrete filters, so I am pretty sure that it would have read .09 or .10 when I put it in. You can see that the gage reads .14 now.
Draft gage with old filter

The filter that I just installed is a "3M 1000". That's a very fine alergen stopping kind of filter, I think.
You can see that it reads .12.
Draft gage reading for new filter
If the draft gauge does nothing else it does make me more aware of the furnace filter and of the opportunity to save some energy and some money by paying it closer attention.
A last note: My furnace uses 2 of the 20" X 20" X 1" size. I watch for sales on furnace filters and I buy several sets at a time so I can always make a change when needed rather than having to put the filter on the gorcery list for "next time".