best value furnace air filter?

[bright eyed]

hi,
After the fires, I decided we should change out the filter for the furnace - gross! It was long overdue.

I would like to buy the best value/quality/quantity, ideally online... wading thru the marketing and hype is not pulling up too many handy overviews and I figure there are some bright minds here who have waded through the mounds ahead of me?

I've read some of the archives - Fed said to get the basic, pleated, non-electrostatic...but didn't explain why the electrostatic were bad? The filtretes are the most heavily marketed and available and are electrostatic - that's what we got cuz that's what they had at the Tarjay...What materials or units of measure should I be looking for?

Willing to pay more if it makes a difference.

Happy to buy in bulk!
We need 20X24X1 size filters...

Your suggestions are appreciated!

 

[thefed]

The permanent electrostatic filters can be overly restrictive if forgotten about,taxing the blower motor. If you are religious about washing them, they are the most effective.

The disposable 'electrostatic' filters have been tested to be no more effective than the next pleated filter. More pleats means more filtering media which means longer life, but often more restriction as well.

Dont overanalyze this one...pleated filter w/ no gimmicks will be fine unless you will be religious about cleaning your permanent electrostatic (try aircare)

 

[ERD50]

They did a segment on this on 'This Old House' this past week.

My data point is this: only the cheap fiberglass ones seem to work well for us. Whenever I've tried anything more expensive or fancier, they seem to block too much air. I've noticed the flames in the furnace shut off and back on occasionally with the heavier filters on a long cycle (cold day) - apparently it gets too hot in the chamber with less air flow, and it shuts down until it returns to 'normal' temps.

They made a big deal about "more filtering is better", but I wonder if that is not just chasing a number? Oh, this filter is X microns, and that filter is Y microns - go for the 'better' number.... Is there really a practical difference? Think about it, you are talking an entire house, and a filter that is going to be in for at least a month. How much dirt can that filter hold? If that was spread all through your house, over the course of a month or two, it seems to be miniscule. I can get that much dust from under a bed or the refrigerator if I haven't cleaned back there in a while. And the cheap ones are still getting dust, we are just talking the difference in dust they collect.

During the heating season, we still seem to get a good two months before the filter seems to have much dirt. That's in IL, so the furnace is running more than a bit.

If yours was that clogged, you really need something to remind you to change it more often.

edit - didn't see thefeds post before I hit submit - yes, he is also talking about the air restriction, that is what I experienced.

-ERD50

 

[thefed]

ERD: If the flames are flickering from a regular pleated filter, the furnace is malfunctioning. THE BEST fiberglass filter will stop 20-25% of particles. not good. You might as well use nothing.

Most pleated filters stop in the 60-70% range, and are barely restrictive until fully loaded

Re: how much can they really hold? Most will offer an actual number for comparison... in a range of 200-400 grams. If you dig deeper, you will find the ASHRAE efficiency rating at certain dust loads if so desired. Again tho....too much analysis for a simple thing

But again, I urge you to have your furnace looked at if it continues to behave that way


Good luck

J

 

[OAG]

I have one of those Thick (about 3 inches wide) that gets changed about once a year (read someplace that Summer was the time to change these; something about A/C needs better flow).

If FL we had about a 1 inch thick filter (they cost about a buck each). I just would buy a box of 12 at Home Depot and having the box around reminded me to change them about every couple of months. Some say change them every month but even at 3 months they looked pretty clean when I changed them. Box of 12 would last about 3 years. I found it was also necessary to pull the grates at the air returns and wash them about every 3 months too. Amazing how much junk gets on them with the returning air.

 

[ls99]

At the risk of adding some confusion: One significant consideration is airflow. How's that?

Well, the ducting and registers are/were sized for some level of airflow with the original filters, which are likely to be the light mesh fiberglass type. This airflow theoretically would produce a specific amount of heat rise across the heat exchanger. Typically a good range would be 60 to 75 degrees F.

Therein lies the rub. Adding a high density filter such as HEPA type will reduce airflow, elevating the heat rise across heat exchanger. Heat exchangers are designed for an optimum temperature range. If they overheat due to low airflow, they develop holes (burn out) allowing combustion products into the conditioned space, your living space. This is most un-good.

What may be useful is measure return and supply air temperatures, see what the heat rise is. If there is 50 to 60 degrees heat rise then you can use more dense filtering, If heat rise is 90 degrees F or more, your system is already too restrictive, can't use high density filter.

Cheap option for better filtering is double up on the lowest cost filter, and change them frequently.

Actually there is a lot more to it, like duct size(s), often under-sized returns, register size, ducting material, duct lengths, leakage, burner size, combustion chamber size, blower speed, etc..... Covering all would take a novel.

 

[Goonie]

Before I ER'd, at the place I used to work, we had always bought the cheapest furnace filters we could find (20x16x1 IIRC). I had to dust my laboratory everyday, and never quite figured out why it always got so dusty so quickly. One day I went to buy a few cases of new filters, and they were out of the size I needed, except for the better quality (and slightly higher priced) "3M Filtrete Air Cleaning 1” Filters". We really needed new filters so I went ahead and bought them.

Amazing! They filtered out sooo much more dust, I could hardly believe it. I went from dusting the lab everyday, to maybe once a month!

There was some difference in the airflow, but not a lot. And the air cleaning capability of the filters certainly made up for the price difference.

Plus, after my initial purchase, I started waiting for sales and/or coupons on the Filtretes, which brought the cost down quite a bit too.

 

[ERD50]

What may be useful is measure return and supply air temperatures, see what the heat rise is. If there is 50 to 60 degrees heat rise then you can use more dense filtering, If heat rise is 90 degrees F or more, your system is already too restrictive, can't use high density filter.

ls99 - thanks for those numbers. I'll check them out. I have run numbers like that for A/C, and it seemed OK (16-18F delta IIRC?).

thefed - thanks, you may be right, maybe my over-temp sensor is too sensitive, or the system is too restrictive, I'll no more after making these measurements.

-ERD50

 

[bright eyed]

Hmmm - anyone have a range/amount that a filter should cost? I paid about $17 at tarjay and that seems really high!

I'm convinced most things can be purchased over the internet cheaper than in the box stores, but would love to have a target product - either by name or rating to look for...

thanks everyone! (almost) always illuminating...

 

[ERD50]

Home Depot had the cheap fiberglass ones for ~ $3. Fancier pleated ones for $7, but prices go up for more words on the package. I dunno if the words mean anything in real life, though.

ls99 - I hung the wired remote sensor of an indoor/outdoor thermometer in my furnace duct. Hung it in there in the main outflow, about 2 feet above the actual furnace. So far, reading ~ 127F max (that min/max memory came in handy) with ~ 65F in, so about 62F delta. I bought a pleated filter when I was out today, I will swap it after some more data and see.

I'm also going to weigh these filters before/aft and see how much dirt they collect. A guys gotta have some fun


-ERD50

 

[bright eyed]

ERD, you're unending intellectual curiosity is admirable! Let us know the results!

I should have taken a picture of the filter we threw out, I'm sure it had at least a pound of dust/crap on it! ick!

 

[ls99]

ls99 - I hung the wired remote sensor of an indoor/outdoor thermometer in my furnace duct. Hung it in there in the main outflow, about 2 feet above the actual furnace. So far, reading ~ 127F max (that min/max memory came in handy) with ~ 65F in, so about 62F delta. I bought a pleated filter when I was out today, I will swap it after some more data and see.


-ERD50

That is good. Some MFG recommend 70 to 72 max delta.


The only way to know performance is to measure it.

 

[ls99]

Home Depot had the cheap fiberglass ones for ~ $3. Fancier pleated ones for $7, but prices go up for more words on the package. I dunno if the words mean anything in real life, though.


-ERD50

FWIW. My Furnace has HEPA filter setup came with the house. They cost $25 to $27 at Lowe's, $48 at the local hardware store.

The heating system was total mess in terms of performance. 750 degrees F on the stack (chimney pipe) 130 F delta.

Added 3ea 6" return ducting to drop delta to 70 F. Dropped Nozzle on oil burner to .75 GPH. The system is still oversized in terms of BTU output.

 

[Yankees Rule]

I put the cheap filters in last month, looked at them this morning, nothing they are clean, So it looks as though we are not cleaning any dust out of the air, but then again we keep the place in the 60 to 64 range and the system does not come on that much. Cannot collect dust if its not on.

 

[ls99]

I put the cheap filters in last month, looked at them this morning, nothing they are clean, So it looks as though we are not cleaning any dust out of the air, but then again we keep the place in the 60 to 64 range and the system does not come on that much. Cannot collect dust if its not on.

You can turn the fan switch to "fan on" will run fan/blower continuously to filter air. Or try running with the fan on for several hours a day.

 

[Yankees Rule]

You can turn the fan switch to "fan on" will run fan/blower continuously to filter air. Or try running with the fan on for several hours a day.

Nah, too cheap.. Funny my sister came over and told me our furniture is cold to sit on. Turn that fan on and it would just move the cold air around.

 

[ls99]

Nah, too cheap.. Funny my sister came over and told me our furniture is cold to sit on. Turn that fan on and it would just move the cold air around.

Ah, the old Yankee frugality. Hey, it saves filters too.

 

[samclem]

ls99 - I hung the wired remote sensor of an indoor/outdoor thermometer in my furnace duct. Hung it in there in the main outflow, about 2 feet above the actual furnace. So far, reading ~ 127F max (that min/max memory came in handy) with ~ 65F in, so about 62F delta. I bought a pleated filter when I was out today, I will swap it after some more data and see.

This is a cry for help. Recognize it for what it is! We all need serious help.

 

[ERD50]

That is good. Some MFG recommend 70 to 72 max delta.


The only way to know performance is to measure it.

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

 

[ERD50]

This is a cry for help. Recognize it for what it is! We all need serious help.

I'm afraid it's too late

If I have time later, I'll try to find the Dilbert clip - something about having "the Knack".

-ERD50

 

[samclem]

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.

 

[ls99]

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.

 

[bright eyed]

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

 

[tibbar]

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.

 

[samclem]

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.