# Effectiveness of furnace filters? **URL:** \ **Category:** Factual Questions\ **Created:** [February 1, 2006, 3:33pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509 "2006-02-01T15:33:27Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [February 1, 2006, 3:33pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/1 "2006-02-01T15:33:27Z")
I had a visit from an HVAC person for an estimate for duct cleaning (that’s a whole nother question). Then she tried to sell me on elaborate filtering systems, costing up to $6,000. She claimed that “your furnace filter is only to protect the mechanism of the furnace, not clean your air, and is only 7 percent effective.” The 7 percent sounds like BS to me, since every square foot of air that the furnace blows out is passed through the filter. And, oh, how much healthier your air will be. She claimed that she had recently sold a filtering system to a woman whose allergy symptoms miraculously disappeared (she did not claim it would cure gout or leprosy). All those germs, dust mites, and off-gassing from the paint you used 6 years ago will all be filtered out. Now, first I am wondering if latex paint really continues to vent fumes 6 years after application, but mostly I am wondering how a filtering system can filter out gases (particulates I can believe). She discussed a high-end filtering system that she was not able to explain at all in any but the most shallow terms but what I was able to distill out of it was that it charges particles then uses some sort of electrical gizmo to attract the charged particles to take them out of circulation. OTOH you have companies selling high-end filters that claim to filter out particles down to a zillionth of a micron. I was getting the feeling she was trying to sell me a great solution to a non-existent problem. Imagine Quicky Lube trying to sell you a $75 oil filter because it filters out so much more stuff. So how effective is standard furnace filtering, particularly when you use a high-end filter? Is there any real benefit to an add-on air filtering system if I’m using good filters? ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [February 1, 2006, 4:00pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/2 "2006-02-01T16:00:50Z")
> [@CookingWithGas](#): > > The 7 percent sounds like BS to me, since every square foot of air that the furnace blows out is passed through the filter. Sounds to me like the furnace gal was contending that the filter only traps 7% of the total particulates that are in your air, allowing 93% to circulate through your house. ---
**Author:** ![Emerald\_Hawk](https://avatars.discourse-cdn.com/v4/letter/e/e495f1/32.png) [@Emerald\_Hawk](https://boards.straightdope.com/u/Emerald_Hawk)\ **Post date:** [February 1, 2006, 4:04pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/3 "2006-02-01T16:04:51Z")
Growing up in a house without forced air heating and cooling, I had terrible allergies, but in other houses with forced air and filters, my symptoms were better. That’s not science though, so take it for what it’s worth. I’m not completely useless though. I can at least give you a link to Consumer Reports latest test results of various air filters (expensive and cheap) so that at least if you decide to get one, you can get a decent one. [Here you go](http://www.consumerreports.org/cro/appliances/air-cleaners-1005/ratings/ratings-wholehouse-models.htm). > [@Consumer Reports](#): > > Quick Picks > > WHOLE-HOUSE AIR CLEANERS > > 1 Aprilaire $600 (pro-installed) > 2 Carrier $500 (pro-installed) > 9 American Air Filter $30 (do-it-yourself) > 11 3M Filtrete $15 (do-it-yourself) ---
**Author:** ![Emerald\_Hawk](https://avatars.discourse-cdn.com/v4/letter/e/e495f1/32.png) [@Emerald\_Hawk](https://boards.straightdope.com/u/Emerald_Hawk)\ **Post date:** [February 1, 2006, 4:08pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/4 "2006-02-01T16:08:15Z")
Oh, and also from the same article in Consumer Reports: > [@](#): > > Don’t assume that any air cleaner will improve your health. “There’s no definitive proof that air cleaners improve air quality in any way that benefits the health of those with or without allergies or asthma,” says Rebecca Bascom, M.D., professor of medicine at the Penn State medical center’s division of pulmonary, allergy, and critical care medicine. The American Lung Association and medical societies such as the American Academy of Allergy Asthma and Immunology also say there’s little evidence that air cleaners alone will reduce the effect of indoor pollutants for those with asthma or allergies. ---
**Author:** ![Man\_With\_a\_Cat](https://avatars.discourse-cdn.com/v4/letter/m/cab0a1/32.png) [@Man\_With\_a\_Cat](https://boards.straightdope.com/u/Man_With_a_Cat)\ **Post date:** [February 1, 2006, 4:46pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/5 "2006-02-01T16:46:05Z")
Hang on, because I can’t remember who it is, but any minute now, someone’s going to post here that works in this industry, and who has explained furnace filters in great detail before. _off to search for a thread to link_ ---
**Author:** ![Man\_With\_a\_Cat](https://avatars.discourse-cdn.com/v4/letter/m/cab0a1/32.png) [@Man\_With\_a\_Cat](https://boards.straightdope.com/u/Man_With_a_Cat)\ **Post date:** [February 1, 2006, 4:47pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/6 "2006-02-01T16:47:48Z")
[Ta-Da! The Furnace Answer Man](http://boards.straightdope.com/sdmb/showthread.php?t=344660&highlight=furnace+filters) ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [February 1, 2006, 6:00pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/7 "2006-02-01T18:00:02Z")
The thread **Mr Bus Guy** linked to has some information, and so as to not repost all of that I will simply say that it is a good idea to install a good quality pleated filter and change it regularly. Post #2 in that thread will give you some excellent manufacturers of filters. To add to that information I would point out that most people do not leave their blower on all the time—that is to say that the blower is only on when there is a demand for heating or cooling. However, that means the filter is only working when there is heating or cooling going on. If you live in an area where you would benefit from a humidifier, it would be an excellent choice to have a humidifier installed and then run your blower continuously. (24hrs/day) Then you are constantly filtering the air, and adding humidity as needed. (in the wintertime) You specifically asked about “add on” systems if you’re already using a high quality filter. IMO, the add on sytems are outstanding—but not needed for everyone. Currently, your filter is in **series** with the furnace blower—meaning that **all** the air is passing through your filter. The add on systems that are designed to be installed with your **central** heating/cooling system (as opposed to those that are designed to sit in the living room) are designed to piped in **parallel** —meaning that a ‘cross section’ of air is taken from the [return] air stream, filtered, and then put back into the [return] air stream, downstream of where it was taken. So, at any given time only a portion of the air is being filtered with the add on system. However, assuming your blower is left on continuously **all** of the air in your home is filtered in this fashion over a 24 hour period. They are fantastic systems. Why doesn’t everyone have them? Two reasons: Not everyone needs one—generally a high quality pleated filter will do the job adequately. The add on systems excel where there are allergies due to cat dander and things like that----and while I think that whole allergy/mold angle is oversold there is no doubt that the HEPA filter filters out very small particulate matter and help immensely. I recently sold one of these systems in a high end house and she simply didn’t like the amount of dust in the house. (she already had a high quality box filter) The second reason is cost. Here in the midwest they will cost between $1400-$2000 to install. What doesn’t get mentioned is the ongoing cost of filters; there is a pre-filter and a final filter and depending how often you change them you will spend between $100-200 per year to change them. Here is a [link](http://yourhome.honeywell.com/Consumer/Cultures/en-US/Products/Air+Cleaners/HEPA/Default.htm) to a Honeywell system that is excellent. I would typically charge around $1400-$1600 to install it in most homes. (prices vary by cntractor and region) I’m not sure what the sales person was suggesting for $6000—but for $6K I would be able to install Honeywell’s top of the line filtering system, (linked above) provide 3 years of filters, a top of the line programmable thermostat, install a new humidifier and 3 years of filters, a new high quality box filter system and 3 years of filters, re work the duct around the furnace, provide a 3 year maintenance program for both your furnace and A/C, and probably have money left to sub out the duct cleaning (a different topic and one I’m dubious about), wash your dog and detail your car. Do I think you need one? Maybe. If you have the cash, have allergies or simply like the idea of having **superior indoor air quality** , go for it. But I’d make sure I would get competing bids and make them detail what they’re providing. Make sure you get references and check the BBB. It’s a great system—make sure you have the right people installing it. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [February 1, 2006, 6:06pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/8 "2006-02-01T18:06:09Z")
One other thing… At the bottom of the Honeywell link I provided there is a “Find a Contractor” bar where you simply put in your zip code. If you’re looking for another perspective or competing bids you may find a contractor from Honeywell’s site. ---
**Author:** ![Man\_With\_a\_Cat](https://avatars.discourse-cdn.com/v4/letter/m/cab0a1/32.png) [@Man\_With\_a\_Cat](https://boards.straightdope.com/u/Man_With_a_Cat)\ **Post date:** [February 1, 2006, 6:28pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/9 "2006-02-01T18:28:52Z")
Ok **raindog** , since you’re here now, I have a related question: What about those devices that seem to be a high-powered UV light that gets stuck, I believe, in the plenum? Someone tried to see me on that - something about bacteria getting electrostatically wiped out, and eliminated from the air. Hokum, or worthwhile? ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [February 1, 2006, 6:47pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/10 "2006-02-01T18:47:12Z")
> [@Mr Bus Guy](#): > > Ok **raindog** , since you’re here now, I have a related question: > > What about those devices that seem to be a high-powered UV light that gets stuck, I believe, in the plenum? Someone tried to see me on that - something about bacteria getting electrostatically wiped out, and eliminated from the air. > > Hokum, or worthwhile? [qualifier] I am not an Industrial Hygienist, and this is not my exact field of expertise. I have had training in this area however, but most of that education was oriented towards educating/selling the product[s] you’re asking about. [qualifier] I am not a fan of them, and believe it is largely hokum. I won’t sell them or recommend them, even when a customer has wanted me to. I just don’t believe they perform. I had a conversation with a guy (among other conversations and research)who does a lot of this work in hospitals and other critical areas. He noted that the average blower moves air at about 500ft/min velocity. However mold spores and the like travel through the area of the UV lights at that speed/velocity and that is not enough time to effectively kill them. Essentially, the nasties would need to slow down Matrix-Like when in the immediate vicinity of the UV lights and stick around long enough to be killed. That just doesn’t happen. I think it’s more hype than performance. As I said, I am not an expert, but I don’t it. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [February 1, 2006, 8:01pm UTC](https://boards.straightdope.com/t/effectiveness-of-furnace-filters/342509/11 "2006-02-01T20:01:41Z")
**raindog** , thanks 1M for taking the time to provide all that advice. I’ll check the data I have and provide some more info over what they are offering for the $6K. It definitely does not include a dog wash. 🙂 We have a pet guinea pig, no other pets. My daughter had a cough for months that we thought might be due to allergies but it cleared up. When my 7-year-old son was an infant he had a respiratory condition that can turn into asthma, but that cleared up with treatment. So no current health issues. The whole episode started with dead-something smell and we wanted to see if it was possible that something was in the ductwork.