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#4665733 - 02/14/18 11:41 AM When to Change my 4 Inch Furnace Filter?
Gebo Offline


Registered: 09/18/02
Posts: 2077
Loc: VA
I just installed one of those 5 inch furnace filters. I attached one of those vacuum differential devices as well. When furnace blower runs in propane mode is shows .20 inches of water. When furnace runs in HP mode is shows .30 inches of water. Reason being the fan runs slower in the Propane mode.

My question is how high does the resistance need to go before I replace the filter?


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#4665743 - 02/14/18 11:48 AM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
SVTCobra Offline


Registered: 12/11/09
Posts: 701
Loc: Battle Creek, MI
First thing I would do is make sure the Magnehelic is level and vertical, otherwise the readings won't be accurate.

Can't help answering your question though.
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#4665758 - 02/14/18 12:04 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
oldoak2000 Offline


Registered: 07/23/14
Posts: 119
Loc: North TEXAS
Originally Posted By: Gebo
I just installed one of those 5 inch furnace filters. I attached one of those vacuum differential devices as well. When furnace blower runs in propane mode is shows .20 inches of water. When furnace runs in HP mode is shows .30 inches of water. Reason being the fan runs slower in the Propane mode.
My question is how high does the resistance need to go before I replace the filter?
..


Interesting, I woulda thought the CFM should be higher for the propane furnace?
how many btu is the furnace, and how many btu is the heatpump ??

If you have the heatpump model, there should be a chart on in the specs that show performace at specific cfm's (your ultimate answer can be calculated from that).
However, IF you have ECM/variable blower motor, it can automatically compensate for filter loading to maintain ideal performance (assuming a perfect/near-perfect install).

5" media filters are the LEAST likely to get 'loaded up' prior to the 6-month swap time; too much older than that you'll get mite population munching the dead skin cells buildup (dust) right ON the filter (releasing THEIR allergens into the flow) frown

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#4665844 - 02/14/18 01:31 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
Nick1994 Offline


Registered: 02/19/13
Posts: 10097
Loc: Phoenix, AZ
Slip a new one in and see what the pressures are for a new filter. If it's the same or close and it looks ok put the old one back in.
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#4665873 - 02/14/18 01:55 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Nick1994]
Gebo Offline


Registered: 09/18/02
Posts: 2077
Loc: VA
Originally Posted By: Nick1994
Slip a new one in and see what the pressures are for a new filter. If it's the same or close and it looks ok put the old one back in.


The above readings are with the new filter. I was wondering if we had some super-duper HVAC genius's here on BITOG. I'm sure we do.

IF .2 and .3 are my starting points, at what pressure differential is my filter getting too restrictive?
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#4665920 - 02/14/18 02:44 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
road_rascal Offline


Registered: 12/09/02
Posts: 829
Loc: Stillwater, MN
The old house we owned before we moved had a 20x20x5 Air Boss filter. I changed it every 6 months. This was based on several HVAC people I know.
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#4665970 - 02/14/18 03:37 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
Yah-Tah-Hey Offline


Registered: 05/26/14
Posts: 3395
Loc: Columbus,Nebraska
Ask Lennox

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#4666104 - 02/14/18 06:11 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
Rand Offline


Registered: 08/20/03
Posts: 12638
Loc: NE,Ohio
usually 2x a year

unless it runs a ton a few extra months esp the spring or fall when it wont run as much wont hurt.

ie if you are feeling cheap you can stretch it to 8months easy enough but maybe not 1 year.

And the highest usage months are summer and winter usually in our climate.
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#4666120 - 02/14/18 06:35 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
MasterSolenoid Offline


Registered: 09/26/17
Posts: 161
Loc: One Step Beyond
Same gage I installed on my furnace.
I'm only a home owner, not a contractor.
This is just my 2 cents.

You want to find the maximum static pressure of your furnace.
Don't make the mistake of guessing.
The info should be on a label inside your furnace.
If you can not find it, call the manufacturer.

My furnace is max static pressure of .50" of water column.
My filters are one inch thick.
New filter reading .32"
After one year ... reading of .44
So, I could go longer, but change at one year.

Like other poster said, mount the gage as it was designed to be.



I also noticed, when the fan runs higher RPM, the gage will give a higher reading.
I only have natural gas, but my furnace fan is two speeds.
My fan only goes to higher RPM after 10 minutes of running. Which is hardly ever.




Edited by MasterSolenoid (02/14/18 06:42 PM)

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#4666173 - 02/14/18 07:45 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
WiskyBadger Offline


Registered: 01/14/13
Posts: 1635
Loc: Wisconsin
I change mine at a year, based on a couple hvac fellas.
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#4666195 - 02/14/18 08:02 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
Srt20 Offline


Registered: 07/14/14
Posts: 494
Loc: Wisconsin
I forgot about mine this fall. It had been in about a year, maybe a little less. It was so plugged, it burned up my fan motor. $150, and it was an easy replacement, but filters are alot cheaper and alot easier to replace.

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#4666240 - 02/14/18 08:42 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Srt20]
MasterSolenoid Offline


Registered: 09/26/17
Posts: 161
Loc: One Step Beyond
Originally Posted By: Srt20
I forgot about mine this fall. It had been in about a year, maybe a little less. It was so plugged, it burned up my fan motor. $150, and it was an easy replacement, but filters are alot cheaper and alot easier to replace.

I wonder what the static pressure was ?
That would be the difference in pressure before and after the filter.
Every Home and Furnace is different, that's why the filter Co. says to change every 3 Months.

But I like using the differential pressure gages.
I change it a little sooner than need be, but also know that I'm not changing them too soon or too late.

Another good thing with using the pressure gage is: there are cheap filters (spun glass) and there are
the best filter (HEPA) and several (pleated) in between.

Depending on the furnace and duct work, some people may use a (new) HEPA filter and be close to the maximum static pressure.
Then you have to use the next least restrictive filter.

Just my 2 cents.
I'm not a Contractor.

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#4666291 - 02/14/18 09:46 PM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
Gebo Offline


Registered: 09/18/02
Posts: 2077
Loc: VA
Mastersolenoid, thank you! I found the info on a tag in my furnace. .50 inches water pressure maximum. Thank you for your help! Most excellent.
_________________________
'98 LEX LS400 280K
'00 LEX GS300 195K
'02 4Runner 225K
'05 Lex LS430 85K
'06 Toyota Highlander 145K
'09 Lex IS350 85K


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#4666303 - 02/14/18 09:54 PM Re: When to Change my 4 Inch Furnace Filter? [Re: MasterSolenoid]
Srt20 Offline


Registered: 07/14/14
Posts: 494
Loc: Wisconsin
Originally Posted By: MasterSolenoid
Originally Posted By: Srt20
I forgot about mine this fall. It had been in about a year, maybe a little less. It was so plugged, it burned up my fan motor. $150, and it was an easy replacement, but filters are alot cheaper and alot easier to replace.

I wonder what the static pressure was ?
That would be the difference in pressure before and after the filter.
Every Home and Furnace is different, that's why the filter Co. says to change every 3 Months.

But I like using the differential pressure gages.
I change it a little sooner than need be, but also know that I'm not changing them too soon or too late.

Another good thing with using the pressure gage is: there are cheap filters (spun glass) and there are
the best filter (HEPA) and several (pleated) in between.

Depending on the furnace and duct work, some people may use a (new) HEPA filter and be close to the maximum static pressure.
Then you have to use the next least restrictive filter.

Just my 2 cents.
I'm not a Contractor.






































IDK I dont have gauges, but it was enough to fold the filter over. The blower motor had to be working hard.

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#4666459 - 02/15/18 06:55 AM Re: When to Change my 4 Inch Furnace Filter? [Re: Gebo]
dnewton3 Offline



Registered: 05/14/07
Posts: 7389
Loc: Indianapolis, IN
Typical filter dP should be no more than a few tenths of water column from baseline.
Example, if a "new" filter offers WC of .3", then by the time it gets to .6", it's time to change.


Anyone with minimal skills can build their own manometer at home. Just get some clear flexible plastic tubing, create a "U" shape, fill part-way with water, hang it vertically on a piece of cardboard and measure off inches above/below a baseline at atmospheric pressure. Put a couple of small port holes in the duct upstream and downstream of the filter; connect tubes. Viloa!



Edited by dnewton3 (02/15/18 06:58 AM)
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The act of preventative maintenance, in and of itself, is FAR MORE important than brand/grade/base choices among lubes and filters.
- under maintaining something is akin to abuse/neglect; that can kill equipment by shortening the lifespan
- over maintaining something has never been proven to be anything but a waste of time and money

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