Furnace Filters: Flat vs. Pleated - which is better?

[Kalhoun]

Twice in recent months we've been told by furnace experts that a flat filter is better than a pleated one. How can that be? Pleated offers more surface area with which to filter the air through. We've never been told why flat is better. Any ideas? Thanks.

[kanicbird]

WAG:
The flat ones will allow more airflow then pleated when both are new. If you change the flat one more often (because it clogs faster) the airflow should always be better then pleated.

[Tess of the Derbyville]

The man who installed my furnace told me that the flat filters were the bare minimum needed to protect the furnace, but if you wanted any air cleaning at all you needed to pony up for the pleated ones.
Hopefully, Raindog will soon be along with a definitive answer.

[rbroome]

Twice in recent months we've been told by furnace experts that a flat filter is better than a pleated one. How can that be? Pleated offers more surface area with which to filter the air through. We've never been told why flat is better. Any ideas? Thanks.
I have never heard of anyone recommending flat surface filters. The pleated filters are always better, at least that is what I have always been told and believed.

[ombre3]

Ex Sears service tech here-------mostly AC/refrigeration.

I was always taught that pleated had more surface area and so better.

Also taught that the average filter was only good enough to catch very large things (like basketballs) ------otherwise mostly a waste-----but still and all a whole lot better than nothing.

[ombre3]

Ex Sears service tech here-------mostly AC/refrigeration.

I was always taught that pleated had more surface area and so better.

Also taught that the average filter was only good enough to catch very large things (like basketballs) ------otherwise mostly a waste-----but still and all a whole lot better than nothing.

Feel like I have to be a little careful on this forum that what seems to me to be an obvious joke just might be taken seriously.

Anyways-------that basketball thing was a most obvious exageration----but really was used by my Sears training instructor 30 years ago. All us techs got the joke back then. I hope you all do today.

[Carnac the Magnificent!]

WAG:
The flat ones will allow more airflow then pleated when both are new. If you change the flat one more often (because it clogs faster) the airflow should always be better then pleated.




There's some tradeoff between airflow and performance, but if you're interested in reducing pollen, dust, pet dander, etc from your interior, a high-performance pleated filter will outperform cheap, microspun filters. The higher the MERV rating, the better the filtration. Airflow certainly enters the equation, but shouldn't be an issue with most filters and most systems.

http://www.3m.com/us/home_leisure/filtrete/51_characteristics.jhtml

[raindog]

As a general rule, you want the best filter you can buy that doesn't create too much resistance. Your blower has to "overcome" a variety of resistances in your complete system. Those resistences include the ductwork, registers, the furnace itself, and the air conditioning coil (if you have A/C) among others.

But generally most blowers will overcome a good quality pleated filter. If the flat filter you were told about is the WalMart type fiberglass filter, I would say you got bad advice. (Unless your duct is undersized, or if your system already has high resistances making a pleated filter impractical)

All the posts in this thread (imo my professional opinion) are very practical. The 3M filter CtM cited is an excellent filter. In addition, this post (http://boards.straightdope.com/sdmb/showpost.php?p=6793296&postcount=2) , in this thread (http://boards.straightdope.com/sdmb/showthread.php?t=344660&highlight=furnace+filters) will give you the name of some other manufacturers of good quality filters. You won't go wrong with any of them.

Please note that in addition to the quality of your filter, the frequency of filter changes will have a lot of impact on indoor air quality. Post 10 (http://boards.straightdope.com/sdmb/showpost.php?p=6795153&postcount=10) of that thread has some basic recommendations as to how often you should change your filter. (given some variables)

Depending on your budget and needs, there are other options available, like an electronic air filter, 4" pleated box filters, electrically "charged" filters, charcoal filters etc. They can be very expensive but perform very, very well.

But a middle of the road approach is a good pleated filter, changed regularly. (keep in mind, some customers change them too frequently and throw money away) Good maintenance will extend the life of your equipment, reduce repairs, reduce utility/energy costs, and provide better indoor quality.

[ombre3]

Good point---

A good quality and not terribly expensive pleated filter will work just dandy--

as long as it is changed regularly. Once the filter plugs up---it will bleed through. And bleeding through means all that crap goes on the evaporator.

Dirty evaporator means poor air flow, means iced evaporator, means no cooling and a possibly liquid slugged compressor (==compressor dies). And who wants that to happen? Certainly not your technician.

Nothing trickier to access, messier, dirtier, cut your fingers and arms all to hell is there than to clean that evaporator once it gets dirty.

So use a cheap filter, preferably pleated, if you want to--------but for Gawd's sake CHANGE IT REGULARLY.

[thirdname]

Our furnace cme with a sort of blue plastic mesh filter, VERY coarse, but it does get some dust. It's supposed to be cleaned once a month. We didn't clean it for two years. Hope it didn't do any damage.

Anyway, if our furnace came with this really open filter, should it be able to handle a half-decent pleated filter?

[raindog]

Our furnace cme with a sort of blue plastic mesh filter, VERY coarse, but it does get some dust. It's supposed to be cleaned once a month. We didn't clean it for two years. Hope it didn't do any damage.

Anyway, if our furnace came with this really open filter, should it be able to handle a half-decent pleated filter?
In most houses, yes.

As to damage in the past, it's likely to have damaged the A/C system rather than the furnace, if any damage has been done at all.

Have a service tech service your system in the late spring---once the temp outside is 75-80ish outside. (not before) They should be able to tell you if there is air flow problems.

[Joe Mahma]

I've heard the flat fiberglass filters referred to by guys in the trade as "rockstoppers".

[raindog]

I've heard the flat fiberglass filters referred to by guys in the trade as "rockstoppers".
That's about the long and short of it.

[thirdname]

In most houses, yes.

As to damage in the past, it's likely to have damaged the A/C system rather than the furnace, if any damage has been done at all.

Have a service tech service your system in the late spring---once the temp outside is 75-80ish outside. (not before) They should be able to tell you if there is air flow problems.

It's a heat pump system, so the two are the same as I understand it. I looked at the evaporator and there seems to be a little dust but nothing really caked up.

[raindog]

It's a heat pump system, so the two are the same as I understand it. I looked at the evaporator and there seems to be a little dust but nothing really caked up.
Then don't sweat it.

However......Heat Pumps are "critical charge" units and it's a good idea to have it looked at every other year or so to check the charge and basic preventive maintenence.

It's best to be done when the temp outside is 80ish or better. If you haven't had it checked in the last few years, it's worth the hundred bucks.

[BoringDad]

as long as it is changed regularly. Once the filter plugs up---it will bleed through. And bleeding through means all that crap goes on the evaporator. I would challenge the assumption that it will bleed through once clogged. What it will do once clogged is reduce air flow (which is bad) and increase the differential pressure across the filter. This increased differential pressure may increase blowby around the filter, but is unlikely to be enough to force dust through holes that initially were able to catch the dust.

[raindog]

I would challenge the assumption that it will bleed through once clogged. What it will do once clogged is reduce air flow (which is bad) and increase the differential pressure across the filter. This increased differential pressure may increase blowby around the filter, but is unlikely to be enough to force dust through holes that initially were able to catch the dust.
Absolutely correct.

But whether the filter fails, or whethrt there is "blowby around the filter" the practical result is the same.

There will be an increased [actually measurable] pressure differential, and to the extent there are leaks around the filter there will be dust bypassing the filter. (and likely sticking to the evaporator)

But, as you noted, the greatest risk is the reduced airflow. In heating it will overheat the furnace and cause it to [possibly] 'cycle' on 'high limit' safteys. The additional heat will cause heat stress to the heat exchanger.

It's worse in the summer. Reduced air flow will cause not all of the liquid refrigerant in the evaporator to boil off into vapor. (to "evaporate") Since the compressor in your backyard is nothing more than a vapor pump, sending liquid refrigerant (as opposed to vapor refigerant) outside will ultimately destroy the compressor.

A neglected $4 filter, will destroy a $800 compressor (that's in your backyard), or, in time, a $2000 furnace.

Air flow is essential to the efficient, safe, and cost effective operation of both your heating and cooling systems. (not to mention the quality of the air you breath)

[raindog]

One other thing....

It might not "poke a hole" in the filter, but I've seen furnaces with high static drives actually buckle cheap filters once the pressure differential was great enough.

[Carnac the Magnificent!]

A neglected $4 filter, will destroy a $800 compressor (that's in your backyard), or, in time, a $2000 furnace.

Air flow is essential to the efficient, safe, and cost effective operation of both your heating and cooling systems. (not to mention the quality of the air you breath)



Raindog, I'm probably using the wrong terms, but I've heard that you're supposed to get both the outside compressor and the inside compressor (or is it the condensor?) cleaned periodically. Problem is that the inside ::whatever:: is usually located inside a duct and that accessing it requires cutting the duct to clean the unit. Because it's never cleaned, I've heard, it leads to premature failure of the compressor.

I'm not sure what I'm talking about, as is obvious, but can you address? I'm talking about a straight AC system, not a heat pump.

[ombre3]

Absolutely correct.

But whether the filter fails, or whethrt there is "blowby around the filter" the practical result is the same.

There will be an increased [actually measurable] pressure differential, and to the extent there are leaks around the filter there will be dust bypassing the filter. (and likely sticking to the evaporator)

But, as you noted, the greatest risk is the reduced airflow. In heating it will overheat the furnace and cause it to [possibly] 'cycle' on 'high limit' safteys. The additional heat will cause heat stress to the heat exchanger.

It's worse in the summer. Reduced air flow will cause not all of the liquid refrigerant in the evaporator to boil off into vapor. (to "evaporate") Since the compressor in your backyard is nothing more than a vapor pump, sending liquid refrigerant (as opposed to vapor refigerant) outside will ultimately destroy the compressor.

A neglected $4 filter, will destroy a $800 compressor (that's in your backyard), or, in time, a $2000 furnace.

Air flow is essential to the efficient, safe, and cost effective operation of both your heating and cooling systems. (not to mention the quality of the air you breath)

First few sentences is what I meant. Practically speaking----a plugged filter will cause lifting up and blow by of the filter onto the evaporator. (High pressure air gotta go somewhere you know---- even if restricted)

And that blow by can cause the worst sort of hardened crap you can imagine----on the evaporator. ----reducing air flow thru the evap to 1/2 or 1/4 or less.

Poor air flow equals iced suction line, liquid slugged compressor, and eventually (on units without low pressure safeties) Failed compressor and NOT covered by your 5 or 10 year warrantee.

Big bucks unnecessarily spent because some do not take care of their filter. .

[raindog]

Raindog, I'm probably using the wrong terms, but I've heard that you're supposed to get both the outside compressor and the inside compressor (or is it the condensor?) cleaned periodically. Problem is that the inside ::whatever:: is usually located inside a duct and that accessing it requires cutting the duct to clean the unit. Because it's never cleaned, I've heard, it leads to premature failure of the compressor.

I'm not sure what I'm talking about, as is obvious, but can you address? I'm talking about a straight AC system, not a heat pump.
First the terminology...
You're asking about the "coils"; the 'compressor' is essentially the 'engine' (more accurately, a vapor pump) that pumps the refrigerant through the system. You need not concern yourself with the compressor--that's the job of a service technician.

As to those coils.........

Outside, likely in the backyard, is the condensing coil. (the whole unit, which includes the condensing coil, fan, compressor and electrical stuff is called the condensor) As to maintenence you can do, simply hose it off periodically with a garden hose. If you keep the outside coil clean it will operate at peak efficiency and extend it's life. You can spray right into the coil---even when it's running. (It runs in the rain, right?) Just be careful not to spray into the electrical motor, or into the cabinet door. Spray right into the coil. One caution: high velocity hoses/sprayers may bend over the "fins" on the coil. Be patient and don't go mongo with super high velocity. Spray it off when it's dirty with grass, cottonwood etc and you'll be cool.

Inside, is the evaporator coil. (simply called the evaporator) You're right, you can't (easliy anyway) get to it to clean it. It may be in the duct, in which case it will be right on top, or right below the furnace. It's easy to identify: 2 copper lines run to it; called the lineset. That's an uncased coil. If those copper lines run into a painted cabinet that looks like it matches the furnace, that a cased coil. If you have a cased coil, you may be able to remove some screws from the front, remove the cover and 'slide" the coil out. That's not always easy---remember those 2 copper lines are still welded to the coil. In some homes however, the access is easy enough that the homeowner can do it. Most of the times however, it's best left to a service technician.

It may not need to be cleaned however. A skilled service technician will be able to tell---in either heating or cooling mode---whether there is reduced airflow; a sure sign of a dirty evaporator. (assuming the filters are clean and too many registers aren't closed) IMO, it's best checked in the cooling mode. If you have concerns, have a service technician go over your system early this summer when the temperature is 80ish or higher. (if the weather is cooler, reschedule. it really is best when the temp outside is at that point or higher)

Ask the service to check to see if he has any concerns about airflow---he'll be able to diagnose this from looking at system pressures, etc.

If you're concerned, and just want some peace of mind, it should take a service tech around 2 hours to uninstall, clean and reinstall. At $75/hr plus travel, and some nominal material costs we would charge around $200-$250, and may be higher if access is difficult. (prices vary wildly by region, contractor,etc, YMMV)

If you haven't had your system looked at recently, it's worth the investment to have it checked early this summer. We are in the midwest (SW Ohio) and most of our competitors charge between $75-$89 for a 'tune up.' (we charge $89) YMMV.

Hope that helps. Let me know if other questions.