I've read in many places that you should change your furnace filter
regularly as a dirty filter will lower the efficiency of your furnace.

Excluding extreme cases where a filter is exceptionally dirty, how
much of a loss in efficiency are we talking about for the average
"needs to be changed in the next couple of months" filter?

PS: I'm not trying to find a reason to stop changing my furnace
filters or anything. I'm simply curious as to how much impact a dirty
filter can have. Is it 0.1 - 2% or more like 10%?

I've read in many places that you should change your furnace filter
regularly as a dirty filter will lower the efficiency of your furnace.

Excluding extreme cases where a filter is exceptionally dirty, how
much of a loss in efficiency are we talking about for the average
"needs to be changed in the next couple of months" filter?

PS: I'm not trying to find a reason to stop changing my furnace
filters or anything. I'm simply curious as to how much impact a dirty
filter can have. Is it 0.1 - 2% or more like 10%?

I don't have a number for you, but you may want to cosider this.

If your filter is blocked, then less air is being fed into the furnace by the
cold air return ducts. If you don't have a perfectly sealed furnace chamber,
then the furnace might be drawing some of it's air from your basement, which
might be pulling air from the first floor dwn to the basement. This might
increase the mold growth in your basement (i.e. bringing relatively warm moist
air into the cold basement).

Just a thought.

Jonny R wrote:
I've read in many places that you should change your furnace filter
regularly as a dirty filter will lower the efficiency of your furnace.

Excluding extreme cases where a filter is exceptionally dirty, how
much of a loss in efficiency are we talking about for the average
"needs to be changed in the next couple of months" filter?

PS: I'm not trying to find a reason to stop changing my furnace
filters or anything. I'm simply curious as to how much impact a dirty
filter can have. Is it 0.1 - 2% or more like 10%?
Hi,
Why don't you experiment yourself? I saw a clogged up filter bringing
down room temp. to near freezing point in dead winter. No kidding.
When I took the filter out, it looked/felt like a black sheet of slate.
Tony

replying to Jonny R, Iggy wrote:
Just so any passersby have a little more answer. In my own testing I found no
difference on my electric bill ("efficiency" is only based on the blower motor
or air movement, in this case) when I went to annual and now even every
2-years of a change out...I used to smoke and no longer do.

I would've expected 10%, but I saw nothing when I had to wait 2-months after
moving-in for a custom filter size. The old filter's corrugations were
completely filled and the filter face was flat smooth with dust. I put the new
filter in and saw no difference on my bill and heard no relief from the blower
motor nor ductwork.

My findings agreed with the basic recommendation. If you can't see through the
filter when held up to the Sun or bright light, then and only then it should
be changed. Regardless of that, a new filter should go in after carpeting has
been changed out or after a dusty home improvement...so yuck doesn't sneak by
and into the HVAC system or your air.

I also found that high MERV ratings didn't effect my efficiency at all either.
So, I went with 3M's Ultra Allergen filters, even though they're almost twice
the MERV rating warned against by the HVAC manufacturer. They've kept my
system like new and dustless for a full 10-years now.

--
for full context, visit https://www.homeownershub.com/mainte...ct-565075-.htm

On Monday, September 4, 2017 at 5:14:10 PM UTC-4, Iggy wrote:
replying to Jonny R, Iggy wrote:
Just so any passersby have a little more answer. In my own testing I found no
difference on my electric bill ("efficiency" is only based on the blower motor
or air movement, in this case) when I went to annual and now even every
2-years of a change out...I used to smoke and no longer do.

I would've expected 10%, but I saw nothing when I had to wait 2-months after
moving-in for a custom filter size. The old filter's corrugations were
completely filled and the filter face was flat smooth with dust. I put the new
filter in and saw no difference on my bill and heard no relief from the blower
motor nor ductwork.

My findings agreed with the basic recommendation. If you can't see through the
filter when held up to the Sun or bright light, then and only then it should
be changed.

That test would seem to depend on what kind of filter you have, if at all.
Filters range from the cheap fiberglass ones that are less than an inch
thick to 5" thick pleated ones that have a high MERV rating. The latter,
you can't see through when they are brand new.

I agree that with any filter you'd have to block it up a substantial amount, raise
the pressure drop significantly for it to have any significant effect
on efficiency.

replying to trader_4, Iggy wrote:
Yeah, I assumed a 1" filter and not a fat-boy or electrical. I'd suspect a lot
of pet hair, drywall dust or sawdust would have a big effect and likely stop
air movement even entirely. In my case it seemed to be just a collection of
regular light and loose dust and pollen that may have actually helped to
filter new particles. Though, I was surprised the MERV rating didn't show up
as an efficiency drop.

--
for full context, visit https://www.homeownershub.com/mainte...ct-565075-.htm

The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

Who made that determination? Sure, the cheap 1" fiberglass crap filters
that are the minimum were likely put there back in the early days to
protect the eqpt. But today there are advanced filters available,
from 5" thick high MERV to electrostatic that go way beyond what is
needed to protect the eqpt. And they are sold and markteted to clean
the air for humans.




It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.

Any filter, as it clogs, will start to impede air flow, raising the
pressure drop. I think we agree that it won't affect efficiency until
it's substantial enough to cause a significant pressure drop.

On Tuesday, September 5, 2017 at 10:19:28 AM UTC-4, wrote:
On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m

The air flow may be normal, but the blower could be using more energy
to run. Many newer furnaces have ECM motors, where the motor is
programmed to maintain a constant air flow. How exactly they do that,
IDK. How much of a factor that is in the overall energy usage, IDK,
but if the blower is working harder it will use additional energy.

On Tue, 5 Sep 2017 05:19:56 -0700 (PDT), TimR
wrote:

The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.
Which is just before it gets dirty enough for the extra dirt to
improve filtration.

On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

Agree. You have one on yours? I've never had one on any furnace
I've owned or seen. Of all the people who read here,
how many do you think have one? Nuff said.

On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

We have them at work, with alarms when they exceed their limit.

I've never seen one in residential. I just listen to mine, when I start to hear the filter noise rise I change them.

They used to make filters with some kind of a whistle that was supposed to alert you when they were dirty. I haven't seen one in a while.

On Tuesday, September 5, 2017 at 11:02:36 AM UTC-4, trader_4 wrote:
On Tuesday, September 5, 2017 at 10:19:28 AM UTC-4, wrote:
On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m

The air flow may be normal, but the blower could be using more energy
to run. Many newer furnaces have ECM motors, where the motor is
programmed to maintain a constant air flow. How exactly they do that,
IDK. How much of a factor that is in the overall energy usage, IDK,
but if the blower is working harder it will use additional energy.

yep that's a good point, if you have a system like that.

m

On 9/5/2017 9:24 PM, trader_4 wrote:
On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

Agree. You have one on yours? I've never had one on any furnace
I've owned or seen. Of all the people who read here,
how many do you think have one? Nuff said.

https://www.alpinehomeair.com/viewpr...SABEgKDWPD_BwE

On Sun, 10 Sep 2017 12:53:05 -0400, Congoleum Breckenridge
wrote:

On 9/5/2017 9:24 PM, trader_4 wrote:
On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

Agree. You have one on yours? I've never had one on any furnace
I've owned or seen. Of all the people who read here,
how many do you think have one? Nuff said.

https://www.alpinehomeair.com/viewpr...SABEgKDWPD_BwE
At least as many as those who have a thermometer on each side of the
filter. Filterminders (donaldson) are common equipment on engine air
filters

Also Google CleanAlert and Filterscan

or check
http://www.cleanalert.com/blog/how-t...filter-monitor

Unlike some of you guys I don't just pull this stuff out of my ass.

On Sunday, September 10, 2017 at 9:14:25 PM UTC-4, wrote:
On Sun, 10 Sep 2017 12:53:05 -0400, Congoleum Breckenridge
wrote:

On 9/5/2017 9:24 PM, trader_4 wrote:
On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

Agree. You have one on yours? I've never had one on any furnace
I've owned or seen. Of all the people who read here,
how many do you think have one? Nuff said.

https://www.alpinehomeair.com/viewpr...SABEgKDWPD_BwE
At least as many as those who have a thermometer on each side of the
filter.


thermometers on each side are not needed, there is no temperature change across the filter.

One thermometer is all you need.

If the airflow is restricted by a dirty filter, the temperature will deviate from the normal value.

mark

On Sunday, September 10, 2017 at 9:27:24 PM UTC-4, wrote:
On Sunday, September 10, 2017 at 9:14:25 PM UTC-4, wrote:
On Sun, 10 Sep 2017 12:53:05 -0400, Congoleum Breckenridge
wrote:

On 9/5/2017 9:24 PM, trader_4 wrote:
On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

Agree. You have one on yours? I've never had one on any furnace
I've owned or seen. Of all the people who read here,
how many do you think have one? Nuff said.

https://www.alpinehomeair.com/viewpr...SABEgKDWPD_BwE
At least as many as those who have a thermometer on each side of the
filter.


thermometers on each side are not needed, there is no temperature change across the filter.

One thermometer is all you need.

If the airflow is restricted by a dirty filter, the temperature will deviate from the normal value.

mark

I think he means two thermometers, one on the input to the air handler,
one on the output. That way you measure the temp drop. The normal
output value when it's 78F in the house and 70F will be different.
Seems to me that's how you'd do it. But then I just check the filter
once a year and see what it looks like. Like I said, I've yet to see
a system using air pressure or temp measurements to gauge filters
in a residential application.

On Sunday, September 10, 2017 at 9:44:45 PM UTC-4, trader_4 wrote:
Like I said, I've yet to see
a system using air pressure or temp measurements to gauge filters
in a residential application.

https://www.amazon.com/Simply-Conser.../dp/B009T7TJWA

On Tuesday, September 19, 2017 at 8:37:46 AM UTC-4, TimR wrote:
On Sunday, September 10, 2017 at 9:44:45 PM UTC-4, trader_4 wrote:
Like I said, I've yet to see
a system using air pressure or temp measurements to gauge filters
in a residential application.

https://www.amazon.com/Simply-Conser.../dp/B009T7TJWA

Like I said, I have yet to see one of those in use on a system.
Have you?