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Old 11-23-2008, 11:04 AM   #21
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I might have fallen victim to the hype, but when I designed our furnace system I went with the 4" thick pleated, disposable media filter. I only have to change it once per month and the large filter area helps to overcome the problem of pressure loss across the more restrictive filter media. It seems to be working well for us.

To Bright-eyed's question: This is the type of question Consumer Reports does a good job of researching. Their 2004 buying guide said that the Filtrete Ultra, Precisionaire 'NauturalAire Microparticle', and the other Filtrete filters did a good job (in that order) of capturing particles, but they were seemed focused on the issue of dust removal rather than addressing the issue of possibly decreased airflow through the furnace.
I can't find all my old CR Buyer's Guides, it's possible they do a better job in another report.
Meanwhile, ERD50's data gathering will likely produce some interesting results. If you want to do some cheap research on your own, you could drill a small hole before your furnace and one immediately after your furnace, insert a small mechanical stick thermometer, and check ont he temp difference when your furnace is running. Insert a "high efficiency" filter (i.e. one that is "tight" and catches a lot of dust) and if the difference isn't more that 70 deg F (thanks ls99) you'll know that at least you aren't harming your furnace.
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Old 11-23-2008, 12:09 PM   #22
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Quote:
Originally Posted by ERD50 View Post
BTW, this is a natural gas furnace, you mentioned oil - not sure if there is a difference on this spec (I would guess not).

The furnace is ~ 14 YO - 90% eff IIRC, it was high eff, but not the super-high. It has a draft inducer fan to suck combustion air in through the burners, they really look like jets of flame, but that's about it - standing pilot, nothing too fancy.

-ERD50
Old gas furnaces tend to collect rust and crud in/on the manifold. might try tapping on the COLD manifold, then vacuum out the crud from the COLD manifold. YMMV

Best I can tell fired heat exchanger for residential have same limitations (oil or gas).

From a HVAC site:
Airflow checks using a gas or oil furnace:
cfm = unit output (Btuh)/1.08 x temperature rise
1. Set the thermostat selector to call for heat and the fan switch to "on."
Note: Ensure that the fan is operating on the cooling speed.
2. Call for heat at the thermostat.
3. Measure the temperature in the supply air plenum in a location that is "out of line of sight" from the heat exchanger.
4. Allow the furnace to operate until this temperature stops rising.
5. Measure the return air temperature just entering the blower compartment.
6. Subtract the return air temperature from the supply air temperature. The result is the temperature rise.
7. Calculate the heating cfm using the above formula.
Example:
Gas Furnace
Input: 75,000 Btuh
Output: 60,000 Btuh
Supply air temperature: 130 degrees F
Return air temperature: 70 degrees F
Temperature rise: 60 degrees F
cfm = 60,000/1.08 x 60 degrees
cfm = 926
Note: Always clock the gas meter for the correct furnace input.
Reprinted with permission from the Split System Residential Air Conditioners service manual from Rheem Air Conditioning Division, Fort Smith, Ark. For more information, visit Rheem Manufacturing Company is one of the world's leading manufacturers of residential Air Conditioners, Heat Pumps, Gas Furnaces, Air Handlers and Commercial HVAC products..


In case you get bored, another method for checking airflow.
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Old 11-24-2008, 11:21 AM   #23
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Originally Posted by samclem View Post
I might have fallen victim to the hype, but when I designed our furnace system I went with the 4" thick pleated, disposable media filter. I only have to change it once per month and the large filter area helps to overcome the problem of pressure loss across the more restrictive filter media. It seems to be working well for us.

To Bright-eyed's question: This is the type of question Consumer Reports does a good job of researching. Their 2004 buying guide said that the Filtrete Ultra, Precisionaire 'NauturalAire Microparticle', and the other Filtrete filters did a good job (in that order) of capturing particles, but they were seemed focused on the issue of dust removal rather than addressing the issue of possibly decreased airflow through the furnace.
I can't find all my old CR Buyer's Guides, it's possible they do a better job in another report.
Meanwhile, ERD50's data gathering will likely produce some interesting results. If you want to do some cheap research on your own, you could drill a small hole before your furnace and one immediately after your furnace, insert a small mechanical stick thermometer, and check ont he temp difference when your furnace is running. Insert a "high efficiency" filter (i.e. one that is "tight" and catches a lot of dust) and if the difference isn't more that 70 deg F (thanks ls99) you'll know that at least you aren't harming your furnace.
Thanks for the info Sam - the filtrete we got supposedly is good for 3 months so I have some time to figure it out - and it does help offset the cost factor if it works well for that time.

We had decent air flow (unmeasured) w/ that stinky filthy totally clogged filter, so i'm sure it will only improve now!

I like CR, but sometimes their studies just don't do it for me.. since they seem to test for some aspects and not others and so many folks spend so much time discounting the research! I don't think they're as thorough as America's test kitchen ! haha
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Old 11-24-2008, 10:31 PM   #24
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The instructions on our new (in 2000) furnace/AC system said not to use the pleated filters. Said they caused to much strain on the system.
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Old 11-24-2008, 10:59 PM   #25
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There are lots of competing interests. The furnace company only cares that the big dust clods that might stick to the AC's A-Coil don't pass through and wants the furnace fan/heat exchanger to last through the warranty period, so they recommend a filter that lets most of the dust through but causes little restriction. On the flip side are those who are trying to turn their homes into a Class 4 clean room and install HEPA filters which are nearly as restrictive as a sheet of aluminum foil. They stop the dust, but also make for a very inefficient AC/furnace and possibly cause damage to heat exchangers and fan motors.

There's usually a satisfactory intermediate position.
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Old 11-24-2008, 11:49 PM   #26
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I use a "permanent" electrostatic and I AM religious about cleaning it
(hose it down about once a month).

But ever since I installed it, I've worried that maybe it restricts airflow
too much (that was 11 years ago, I guess I wasn't THAT worried).

I'd like to do the measurement procedure detailed above, but not sure
how to access the duct system so near the heat exchanger, not what sort
of temperature probe to use.
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Old 11-25-2008, 05:49 AM   #27
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Quote:
Originally Posted by RustyShackleford View Post
I use a "permanent" electrostatic and I AM religious about cleaning it
(hose it down about once a month).

But ever since I installed it, I've worried that maybe it restricts airflow
too much (that was 11 years ago, I guess I wasn't THAT worried).

I'd like to do the measurement procedure detailed above, but not sure
how to access the duct system so near the heat exchanger, not what sort
of temperature probe to use.
I have absolutely no knowledge of this subject, but I did get a good nights rest so here goes: Get two of those temperature things they stick in meat to check cooking temperature, drill little hole in duct near where the air come out (at top), drill another hole where the air returns (at bottom). After checking just put some tape over the hole (or it may whistle at you).

Someone will be along shortly to "fact check" me, but it seems it would work fine. Unless it is way off base, I plan to do that shortly; got me curious.
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Old 11-25-2008, 08:17 AM   #28
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A little update on my earlier measurements, and a few more tests to do yet. With the cheap, basic fiberglass mesh filter, I reported above that I was seeing temperatures that indicate a good flow (I have not tried those calculations yet, I will).

Then I put in the relatively cheap pleated paper filter, and the temps only increased a few degrees, so that doesn't seem bad.

It may have been the washable type filters that were causing my flame to shut down - I thought the pleated did it too, but I might be mistaken about that. I'm going to do this measurement with the washable filters in and see.

-ERD50
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Old 11-25-2008, 08:24 AM   #29
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- the filtrete we got supposedly is good for 3 months so I have some time to figure it out -
I wouldn't go by the calendar. It depends on the season, ours was barely run this summer we probably had a 3 month period with near zero usage. And of course, how much dust/dirt is in your house - I imagine pets could add dramatically to the load with shedding hair.

I check my filters after about 6-8 weeks of being into the heating season - usually fine, I let them go another few weeks.

My brother mentioned that his digital thermostat keeps track of the ON time, and lets you know when to change the filter based on hours of use. Better than time alone, but it still is dependent on many toher factors.

-ERD50
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Old 11-25-2008, 08:38 AM   #30
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I've always kind of wondered what would happen if you never changed the filter. I got my answer from a co worker who said his furnace wouldn't heat the house any more, so he called a furnace repairman in. Turns out he had not changed the filter in 18 years. Guy was Asian born, just didn't know furnaces had filters. Bet that filter was about 4" thick with dust and lint.
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Old 11-25-2008, 10:05 AM   #31
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I've always kind of wondered what would happen if you never changed the filter. I got my answer from a co worker who said his furnace wouldn't heat the house any more, so he called a furnace repairman in. Turns out he had not changed the filter in 18 years. Guy was Asian born, just didn't know furnaces had filters. Bet that filter was about 4" thick with dust and lint.
CFB (peace be upon his bunniness) posted a story several months ago about a guy who had a visit from a furnace repair guy because his furnace wasn't working right. The repairman asked how often he changed the filter, and the customer said something like he just put a new one in whenever the old one was gone. He never saw the old ones, never took them out. As it turned out, the filters would get so full of junk that the negative pressure would become sufficient to collapse them and they'd get sucked into the ductwork. At that point he'd see the filter was gone and install a new one.

You do have to wonder how some people function in a modern world.
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Old 11-25-2008, 01:20 PM   #32
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I have absolutely no knowledge of this subject, but I did get a good nights rest so here goes: Get two of those temperature things they stick in meat to check cooking temperature, drill little hole in duct near where the air come out (at top), drill another hole where the air returns (at bottom). After checking just put some tape over the hole (or it may whistle at you).

Someone will be along shortly to "fact check" me, but it seems it would work fine. Unless it is way off base, I plan to do that shortly; got me curious.
The drilling of a diny hole sounds good - maybe there's even a plug
there already (sealing a tony hole).

Not sure about the meat probe. There's probably something meant for
this. I have a nice Fluke multi-meter, maybe something that would
plug into it ?!?
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Old 11-25-2008, 02:48 PM   #33
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Not sure about the meat probe. There's probably something meant for
this. I have a nice Fluke multi-meter, maybe something that would
plug into it ?!?
Meat probe will work fine. No need for gnat's a$$ accuracy.
Thermocouples, range converters, adapters, are pretty expensive. $$$.
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Old 11-25-2008, 09:48 PM   #34
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I only have to change it once per month ......
Another cry for help!
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Old 11-25-2008, 09:57 PM   #35
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Quote:
Originally Posted by samclem
I only have to change it once per month ......

Another cry for help!
I guess I need to get off the cough syrup! I should have written that I only have to change the pleated media filter once per year. I'd sooner go without a filter than spend $30 on one every month.

Thanks for the catch!
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Quoting myself for an update
Old 12-04-2008, 09:54 PM   #36
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Quoting myself for an update

Quote:
Originally Posted by ERD50 View Post
A little update on my earlier measurements, and a few more tests to do yet. With the cheap, basic fiberglass mesh filter, I reported above that I was seeing temperatures that indicate a good flow (I have not tried those calculations yet, I will).

Then I put in the relatively cheap pleated paper filter, and the temps only increased a few degrees, so that doesn't seem bad.

It may have been the washable type filters that were causing my flame to shut down - I thought the pleated did it too, but I might be mistaken about that. I'm going to do this measurement with the washable filters in and see.

-ERD50
Yes, it was the washable/re-useable type filter that was causing my flame to shut down. I took that one, washed it out well and the flame would shut down after 4 minutes, and the temp had only reached 115F in the duct. It would shut down for a minute, and keep cycling like that.

But the pleated seemed fine for me.

I still question the importance of any filtering beyond basics, since ~5 months of the year we pretty much have the windows open and/or no HVAC running, and those window screens are a lot courser than even the cheapest furnace filter.

-ERD50
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Old 12-04-2008, 11:01 PM   #37
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The repairman asked how often he changed the filter, and the customer said something like he just put a new one in whenever the old one was gone. He never saw the old ones, never took them out. As it turned out, the filters would get so full of junk that the negative pressure would become sufficient to collapse them and they'd get sucked into the ductwork. At that point he'd see the filter was gone and install a new one.

You do have to wonder how some people function in a modern world.
Umm... He probably read somewhere that furnace filters are "consumables".

So, how many were stacked in the duct, I wonder.
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