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How often to change furnace filter?

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  • Dec 7th, 2020 12:26 am
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Deal Fanatic
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Mar 8, 2003
9823 posts
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Richmond Hill

How often to change furnace filter?

I am using the 5" thick one, package says last up to 12 months, since we are all working from home and furnace and air condition are running more than normal, how to tell when to change? I am thinking at least every 6 months or even more often. Thanks in advance for your input and sharing.
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Deal Guru
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Nov 18, 2005
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Kingston
Ours is 2" thick and 24x24 (so the total area is roughly 24x24x2). Change it every 3 months. If you remove it and hold it up to a light to see through you can get an idea of how much "stuff" (technical term) there is on it.
Deal Addict
May 23, 2009
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Mississauga
I use run time hours (1000hrs) rather than month duration for my change schedule.

Mine is 1” and Merv 8 so relatively cheap when replacing often.

Since I run my fan often to circulate air I get the 1000hrs reminder roughly every two months and swap it out.
Jr. Member
Nov 13, 2019
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Toronto
bubuski wrote: I use run time hours (1000hrs) rather than month duration for my change schedule.

Mine is 1” and Merv 8 so relatively cheap when replacing often.

Since I run my fan often to circulate air I get the 1000hrs reminder roughly every two months and swap it out.
What thermostat do you use that shows runtime?
Deal Addict
May 23, 2009
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timofeewho wrote: What thermostat do you use that shows runtime?
I use a Nest but should be a standard feature on most thermostats these days. Most can now track and report historical usage so it just a matter of sending a reminder after x hours of use.
Jr. Member
Nov 13, 2019
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Toronto
bubuski wrote: I use a Nest but should be a standard feature on most thermostats these days. Most can now track and report historical usage so it just a matter of sending a reminder after x hours of use.
Oh, I have a Nest too but I had no idea about this. Where abouts can I find this reminder?
Deal Guru
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Oct 23, 2008
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timofeewho wrote: Oh, I have a Nest too but I had no idea about this. Where abouts can I find this reminder?
I have an Ecobee and it does this too.
I would just check into the preferences menu as I'm sure that's where you'll find it. If not, just admin settings.
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Deal Addict
May 23, 2009
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timofeewho wrote: Oh, I have a Nest too but I had no idea about this. Where abouts can I find this reminder?
In the app I click on the gear then click on equipment. You should see air filter reminder in there. It toggles only on/off and defaults to 1000hours.

If you need a reminder for more than 1000hrs then you will have to use a 3rd part app to track and email the reminder.

edit: I think Nest has changed the defaults, my last auto change notification was around 500 hours of runtime.
Last edited by bubuski on Dec 5th, 2020 9:52 pm, edited 1 time in total.
Member
Dec 4, 2009
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Monthly is my interval for the heating and cooling seasons. Use a (1”) Merv 6 or 8 and the costs are negligible.
Deal Addict
Jan 21, 2018
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Routine advice is 3 months for standard 1-2" filters, 6 months for 3-4" filters, 12 months for 5-6" filters.

This is a rough figure of course.

This is supposed to be based on average furnace usage for an average family of 4 in an average home in an average winter heating season. In reality it depends on the actual amount of usage. And 1 or 2 people in a home don't kick up a much dust as 4, so you wouldn't need to change the filter as often.

It comes from the industry that wants you to buy more filters, so it is probably exaggerated by double. I can't find any actual independent tests. Keep in mind that the only concern here is airflow - the furnace filter has negligible effect on reducing the particles in the air in your home - it's there to protect the furnace.

As it happens, I just changed out our 1" filter that's been in place for a total of 6 months of use between spring and fall. The used one is moderately grey. Being scientifically-minded, I did a simple test: I tried blowing through the filter onto my hand behind it. The force of my puff of air was greatly attenuated, but felt identical between the old "dirty" filter and the new one. I would therefore conclude that in my case the amount of dirt accumulated by the filter in 6 months has an insignificant effect on airflow.

filters.jpg
Deal Guru
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Nov 18, 2005
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bubuski wrote: I use a Nest but should be a standard feature on most thermostats these days. Most can now track and report historical usage so it just a matter of sending a reminder after x hours of use.
I have mine set to months and didn't realize that runtime hours was an option.

I have no idea how many runtime hours are the right answer as it would seem to be primarily a function of your filter's surface area (LxWxD).
I was thinking house size too, but I think that a bigger house would cause more runtime so maybe it is just about runtime and surface area.
Actually, If your furnace is multi-speed (most are, some 2-speed, some variable speed), it would also be a function of the furnace speed x runtime since running at a higher speed means more airflow through the filter than at low speed. I don't think our thermostats have that level of functionality so runtime might not be much more "accurate" than months.

BUT number of months will vary a lot between homeowners. My furnace is geothermal so it runs longer than a gas furnace because it heats slower.

So I guess if you check monthly for a while using visual inspection you have to determine the right period for your home. Guidelines won't be very useful.
Deal Addict
Nov 24, 2004
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Toronto
We keep our furnace fan running 24-7 while we are home (to circulate air and even out the temperatures in our old, poorly insulated house).

On the advice of my HVAC guy, I use the cheap filters, but change them monthly.
Penalty Box
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Oct 13, 2008
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I use 16x25x1 Merv 8 filters.

LENNOX ML296UH070XV36B-55 installed October 2019 for $2450.

Nest 3rd Generation thermostat.

Detached 1800 sq ft 2 story.

I change the filter monthly from December to April. Summer time I change it bi-monthly.

Our AC Unit (York 2-ton AY024MA321A) that was installed by the builder in 2006. We don't use this much in the summer.
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Deal Addict
Jan 21, 2018
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jdmfishingonly wrote: I use 16x25x1 Merv 8 filters.

Detached 1800 sq ft 2 story.

I change the filter monthly from December to April. Summer time I change it bi-monthly.
That's probably way excessive.
Deal Fanatic
Feb 4, 2015
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Canada, Eh!!
Use mostly cheap fibreglass filters, sometimes use electrostatic up to merv 8.

Check every 2-3 weeks and change when overly dark compared to new.

Since using cheaper filters do tend to change more often even if can get a few more weeks... so anywhere from 3 weeks to 8 weeks.

Did not know about the hours reminder vs month. Thanks. Just selected to 1000 hours and have 310 hours left it says which means would be changing after 3 weeks usage.
Gonna keep at 1000 hours and then adjust after a few cycles if needed. Did try 700 hours and that would mean change in 1-2 days which would only be 2 weeks usage!!
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Deal Addict
Jul 13, 2009
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Scote64 wrote: That's probably way excessive.
I set mine to 400 hours and it comes to about 1-1.5 a month. I try to vacuum often in the house but the filters definitely go dark grey after 400 hours. I'm trying to stretch filter life but the nastiness is hard to ignore.
Running HRV and fan often during the day so it doesn't feel stuffy in winter time. Summer we'll open a window when we can but there's construction not far and it does get dusty.

As for filters, I buy them from Amazon from their warehouse deals, $2 to $3 each and the only condition is the box is damaged. The box comes damaged but the filters inside are perfect, clean and zero issues.
HRV filter I clean every season and it's nasty.
Deal Addict
Jan 21, 2018
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Vancouver
I know it looks dirty and nasty, but keep in mind that your furnace filter does not have a significant effect on the air quality in your house - its job is only to protect the furnace fan.

Next time you take out a dirty filter and you're about to put in the new one, take a minute and try the blow-through-it test. Do you detect any difference in how much the new filter and the old filter attenuate the force of your blowing? If so, probably good to change. If not, you are changing the filter for no good reason.
Sr. Member
Jun 4, 2020
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Clarington, ON
Flame away. Have my old school honey well vision pro 8000. It alerts after a month. I change monthly using the cheapest filters I can buy.

Not into the fancy ecobee/nest bullshit

Edit: furnace fan runs24/7/365

Edit 2: without a properly designed zones system the fancy thermostats with remote readings are pretty useless
Deal Expert
Aug 22, 2011
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I change mine once a month, as I'm running the HRV 24/7 on a 24min on and 40min off cycle, all year round.
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Dec 20, 2018
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KevinM56081 wrote: Flame away. Have my old school honey well vision pro 8000. It alerts after a month. I change monthly using the cheapest filters I can buy.

Not into the fancy ecobee/nest bullshit

Edit: furnace fan runs24/7/365

Edit 2: without a properly designed zones system the fancy thermostats with remote readings are pretty useless
How are the smart thermostat useless or bullshit?

They're pretty straight forward unless you're still using a USR 56k modem connection or something?

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