just an opinion

the blue filters / disposable cheap ones primary function is to protect the equipment. If you are concerned about better air filtration, I'd recommend a Space Guard or Honeywell media filter. There are other brands also. EAC's are ok, but with the media filters there is no noise, no ozone, no cells to take out and wash, to die and have to replace.

 

also -

also- I'd suggest staying away from the 3M filtcrete style filters - 1" wide. They are VERY restrictive. Just an opinion.

 

Nothing wrong with the blue's as long as they're changed out regularly.

 

Ditto! also IMHO.

 

Cheap blue or otherwise,do not trap same particles ,these will build up on and in your indoor coil.

A wet coil is a very good filter,don't let it (the coil) be the best filter in your system.

The saving in using cheap filters will be spent on loss of efficieny,and coil cleaning,if you clean it before it damges the compressor or heat exchanger,IMHO.

That said ,every system is different.If you have ducted retuns ,a lot of stuff will gather there ,before getting to the coil.If your return are in the ceiling,a lot of stuff never gets to the return ducts.

It's often very simle to add a media filter and modify the return to allow the addition of the filter ,without a loss of airflow.

 

I believe ESP is the key

I am a homeowner who measured the system's ESP ("external static pressure") before and after some changes. In my case the ESP before changes was already way outside what was permitted for my air handler. The old Consolidated air handler clearly stated maximum ESP to be 0.50 (measured in inches water column) and I was about 0.70. Under those circumstances I should be running the least restrictive filters available. Those blue ones you can see through.

The change involved adding much return capacity and filter area. Now I have two 20-inch by 30-inch pleated filters (on a 3.0 ton system, 1200 cfm airflow) and the extra area lets me run the more restrictive filters without problem. ESP now measures about 0.60, but that is with a new variable speed air handler which is compatible with that. While I am actually using "Arm and Hammer" brand 8-MERV filters, I am convinced the 3M Filtretes would do no harm in this system. However many techs on this board will testify they hate to see homeowners buying those 3M Filtretes -- especially if they are not vigilant about changing them.

I am not a big admirer of those 1-inch filters. Am probably going to get a 4-inch media filter soon such as the Aprilaire 2200 model. That will probably add 0.10 inch w.c. to my ESP measurement, but will be able to do some real filtering.

Many houses are built with inferior return systems, and frequently also small filter areas. Those are the trouble signs for many homeowners. If you are able to get your ESP measured by a technician, he may tell you 1) you are in danger no matter what, or 2) you are OK with blue filters and in danger with those 3M Filtretes, or 3) you are running an acceptable ESP with the 3M Filtrete.

Hope this helps -- Pstu

 

I presently have a 4" media filter and am happy with it, but...

In my previous belt-drive old furnace I used nothing but the cheapest filters for more than 20 years. Always changed regularly, and very important, the flter was tightly fitted, and there were no downstream duct leaks allowing in unfiltered air. Blower sometimes ran continuous, also I had kids, a dog, and lots of carpet. No smoking and little greasy cooking in the house.

Over that time, the fan never collected an appreciable amount of dust, I may've pulled it once to wash it and replace bearings, and the evap stayed clean (to my surprise), never needed attention.

Lot's of better filters now on the market, but the cheapo's are not that bad either.

 

I was told at one time that 13% of what contacts the blue one inch filters goes right though it. Nothing wrong with the one inch pleated filters if you allow for enough surface area which is the major problem with them

 

How about the Honeywell (I think) 4" filters that fit into a 1" return grille? Are these as restrictive as the 3m 1" pleated?

 

No,but may still be tooo restrictive for your duct system.

Is there ample space behind the 4" filter in the wall????

 

For many years I did preventive maintanence service that included taking care of filters and checking the condition of coils. Here's what I found;

Standard blue fiberglass filters are less restrictive. This means that the air goes through them more slowly. As long as there is a good fit and seal around the perimeter of a standard fiberglass filter, they have provided good enough filtration to keep the HVAC system clean. I have never had to clean a coil due to a properly fitting fiberglass filter that is changed regularly.

As soon as you start getting into better filtering filters, you get into more restrictive filters. This means that the air is going to go through a more restrictive filter (of the same size) is going to be going faster. The faster air travels through a filter, the more likely it is that particulate in the air will get pushed through.

If you are going to use a "better" (more filtration) filter, you must increase the available surface area of the filter system or the better filter will not be of a benefit and may actually do more harm then good.

 

Less restrictive ,equals lower static pressue,which increases the cfms ,unless the speed tap is changed,so air would go thru a cheap filter faster ,not slower,assuming the same size filter.


Plus many better filter are pleated ,which increases the surface area ,in the same size filter frame,which reduces the velocity thru the filter media,that's one of the reasons they work better.

We do thousands of clean and checks a year,cheap filters,coil has some,maybe very little ,stuff on it.Most will need coil cleaning every few years.
Now we don't know if the customer ran it a few weeks without a filter,and there's likely some bypass air on most of them,which I agree is a factor.

We have installed a lot of better filters,in the last 7 -8 years,the techs have yet to find one that the coil needs cleaning.


New coils have the fins spaced closer and some type of enhanced fins,basicaly a stamping in the fins,which makes them a better filter then older coils.

 

Thanks for all of the input. From what I gather there is many differing opinions. It seems from what I hear that the cheap blue filters are ok, in protecting the system. It seems that everyone agrees that choking the system is a bad idea with a restrictive filter. Protecting the system is what I am concerned about - so, it seems that I am ok with the cheap-o blue filters. Again, thanks for all of the input.

 

From what I gather there is many differing opinions.

A few different opinions, some of them totally wrong.

It seems from what I hear that the cheap blue filters are ok, in protecting the system.

The cheep blue filters are only acceptable protection for a heating only system, and only if it is not a high efficiency furnace with a secondary heat exchanger.

It seems that everyone agrees that choking the system is a bad idea with a restrictive filter.

Yeap, many of those filters you see at the hardware stores will shorten the life of your system, or even kill it outright if your filter and duct system are not designed for them.
Even the lower rated ones can be very restrictive. The cheep AAF Dust Demon pleated filters are among the most restrictive disposable filters I have encountered, and they are only MERV 5 or 6.

Protecting the system is what I am concerned about - so, it seems that I am ok with the cheap-o blue filters.

If you have AC, the blue filters offer very poor protection. The average evaporator coil acts like it is a MERV 5 or 6 filter. Some secondary heat exchangers in high efficiency gas furnaces are better filters than the cheep blue filters too.
To protect the equipment you need a filter that is MERV 6 or higher. The cheep blue filters just don't cut it.

There are always exceptions. I have seen systems that stayed perfectly clean for many years with nothing more than the cheep blue filters. I suspect those same systems would have stayed just as clean with no filter at all though.

 

I know this site is not for DIY. Nevertheless, I'll ask a theoritical question:

The big deal with high filtering capacity filters is that it creates restrictive air flow and it will lead to damaged furnace soon or later.

But what about if you were to cut a hole in your return duct, install a 120cfm bathroom fan to suck the air out of your duct and into a highly restrictive filter and install another 120 cfm bathroom fan on the other side of the filter to send the air back into the return duct? In that case, the air flow would be barely impacted. I know that not 100% of the air would be filtered at once, nevertheless, you would achieve greater filtration with no impact on the furnace.

Do I sound to you like someone who took apart his brand new $2,000 clothes washer just to see how it works!!! (If you want to know, I can give you great tips on the Kenmore Elite HE3T- eh! eh!)

 

Sure that'll work,it's already being made,it's called a HEPA Bypass filter.

 

Firsthouse, what you have described would not even need to be connected to the return in order to work. I am not a pro but would anyone actually do that? Of course such a system would neither add nor subtract to the load on the air handler, ESP should be unchanged. Good real world advice would be to *measure* ESP and know whether you are in the danger range. Even the pros advice sounds incomplete to me when they talk around the problem with no knowledge of the ESP of the system.

You can buy a pretty mature design 4-inch media filter such as the Aprilaire 2200, and it will add to ESP about 0.10-.15 inch w.c. depending on the airflow pushed through it. The installing tech will still need to ensure either 1) ESP is measured and within spec, or 2) professionally give the opinion the prior filters had more restriction than this one. Such a solution may not be cheaper than your bath fan idea, but would probably deliver better results.

Techs have observed equipment failures using restrictive filters, and testify one problem is the 3M Filtretes load up quickly and become even more restrictive. However to go so far as to recommend they never be used without measuring a system's conditions, is a "one size fits all" type of advice.

Hope this helps -- Pstu

 

Firsthouse, what you have described would not even need to be connected to the return in order to work. I am not a pro but would anyone actually do that?

It's done all the time and it is the right way to do it. It is called a self contained filtration system and is usually of HEPA filtration quality.

 

pstu, I like to tinker with stuffs. I realized there is a mistake in my original idea. You do not need two bath fans, just one is enough. Get a high priced and quiet 200-300 cfm bath fan directly screwed to your return air duct to suck up the air, direct this air through a highly restrictive 3M 1700 filter (their latest one - few months ago the highest was 1500) and direct the air from the other side of the filter back into the return air duct.

It has nothing to do with money. I simply like to tinker with things.

P.S. If anyone out there has a Kenmore Elite front loader clothes washer, remove the front bottom panel (3 screws) and you'll see on the side a transparent plastic bag containing the "secret" instructions for the tech guy. It will allow you to interpret the error codes the washer can display and you can even put the washer in test mode by simply pressing the right buttons at the front of the washer. When it spins at the highest setting, given the diameter of the drum, your clothes are spinning at 72 mph!

 

What about my improved design of using just one fan?

Thanks.

 

Yes,just one fan!lol

 

Robo states:
"Standard blue fiberglass filters are less restrictive. This means that the air goes through them more slowly. As long as there is a good fit and seal around the perimeter of a standard fiberglass filter, they have provided good enough filtration to keep the HVAC system clean. I have never had to clean a coil due to a properly fitting fiberglass filter that is changed regularly.

As soon as you start getting into better filtering filters, you get into more restrictive filters. This means that the air is going to go through a more restrictive filter (of the same size) is going to be going faster. "


Dash states:
"Less restrictive ,equals lower static pressue,which increases the cfms ,unless the speed tap is changed,so air would go thru a cheap filter faster ,not slower,assuming the same size filter."

Robo states:

Not so. While the lower static pressure does allow for more air, the air is moving slower due to having more area to go through. FPM reduces when static pressure is reduced.

Not so;
I don't see how there can be more area in a cheap filter,there is more filter area in a pleated ,either 1" or 4".

That's why 4" pleated of the same material,is less restrictive then a 2" or 1" thick filter.

With constant cfm ,more area equals lower PD thru the filter material,as cfm increases PD increases.

Velocity is thru the "opening" isn't it?So a 20X20 filter is the same velocity,for either type of filter.

Pleated being more restrictive,may reduce cfms,which would further reduce velocity,if cfms are reduced.

 

4" pleated filters are not a problem (never checked 2" ones). The 4" pleated filters have so much more surface area that they are not as restrictive even with tighter material. The 1" pleated are more restrictive because even with the additional surface area, the filter material is much tighter constructed with much less free area for air movement.

Also; a 1" standard fiberglass filter is 1" thick. It has the ability to catch particulate throughout the entire 1" thickness of the filter material. This type of filter, like the hogs hair filter, allows for "stacking" of particulate which still allows airflow around the trapped particulate. 1" pleated filters have only about 1/16" thick filter media which relies on "surface" filtering. Immediately, the surface of the pleated filter becomes blocked with particulate, causing it to lose free flow surface area and increasing velocity of air that does go through.

The mistake being made about cfm vs velocity is that it is the increase of velocity that causes the cfm decrease, not the other way round. Restrictive filters decrease cfm "BECAUSE" they have increased velocity. A less restrictive filter allows for more air flow "BECAUSE" the velocity is lower.

I always prefer the best filtration that a specific filter containment system can handle. I have just seen way too many systems being adversely affected by having a filter that is just not large enough for how restrictive it is due to the limited size of the filter rack.

The company I work for sells electrostatic filters...but I won't. I will only sell a better filter if the application for that filter is condusive to the proper operation of that filter without causing problems for the system. This usually means that the proper filter system for most applications is going to be a lot more costly then just sticking a more restrictive filter into the same filter rack. Again; I'd rather not have a sale at all then to sell what is going to adversely affect the system.

At least dash and I agree that the HEPA by-pass system is the best for those who want to really clean up the air for health reasons.

 

Here is another question – the dreaded rule of thumb question. Is there a general return size one needs? For example, for a 2 ton A/C – Should the return be X by X? For an 80% 45,000 btu furnace – Should the return be X by X?

I understand that there is no exact answer to this type of question, but I am just looking for some ballpark guidance. It seems that the real answer about filter is how restrictive of a return one has.

It seems that HVAC industry does little about educating the consumers about routine maintenance. To be, changing the filter is routine maintenance. Talking to friends and family, the HVAC techs/installers (from several companies) usually instruct the HO of where the filter is located and how often to change said filter, but what type/brand of filter is usually not discussed. This seems to be real mystery to most HO. Their seems to be a great void of filter knowledge when it comes to the HO.

 

If you mean return grille size,1 sq ft or more,per ton(400cfms) is a safe way to go.

Return duct size is too dependent on the rest of the duct system,have a Pro do the Manual D,to design.

 

Please check my math.

So at 400 cfm (1 sq foot) per ton:

1 ton = 12in x 12in return
2 ton = 24in x 24in return
3 ton = 36in x 36in return
4 ton = 48in x 48in return
5 ton = 60in x 60in return

Is this correct? A five ton unit needs a return that is 5ft by 5ft. That seems very large. Then again, I am only a HO and I don't do HVAC for a living. I guess this shows how much/many returns are undersized. In my neck of the woods most people (friends and family) have 2 to 3 ton units and most have a single 20in by 20in return.

HVAC is complicated. Thanks for all of the input. I sure do appreciate the information.

 

60"X60",is 5'X5',so that's 25 sq ft.

Area = length X width



It may not be all that safe,I'd want a 24"X24" (4 sq ft)on a three ton,probably should have said minimum.

 

Thanks Dash!

I am glad that I asked someone to check my math. In a way I guess I was correct - 25 sq feet is pretty big. Good thing that I was wrong on needing that much return.

I guess I have learned my lesson on doing recreational math.

 

I once watched a guy that sold after-market filter systems (MLM guy) do a demonstration about HVAC filters. Granted it was a 'sales pitch', but what he did got me to thinking:

He took one of those 'spun fiberglass' filters (blue) and held it horizontally. He then took a salt shaker and poured salt over the filter. He then shook the filter and an appreciable amount of salt went through it and onto the dark carpet of the restaurant (if he had done that in my home, he would have been vacuuming my house).

Anyhow, this got me to thinking about air filters. If a blue filter will pass salt, what else will it pass.

The reason pleated filters are pleated is because they have more surface area to filter with.

IMO, the better arrangement is a 4-5" think 'media' filter. This will allow more airflow due to surface area. It goes a bit farther than this; because of the larger surface area, the filter can be built to catch smaller particles with the same restriction. Thus you get less resistance yet more filtering capacity.

IMO the best of the media filters is the AprilAire 2200/2400. They have lots more area than the fixed cartridge style 4-5" filter elements.

I have installed the AprilAire for numerous folks that have Asthma (sp). A few of them were so serious that a trip to the hospital because they could not breath was a monthly (or more common) occurance. After the AprilAire filter, no more trips to the hospital.

Now I think we are down to the question of my health vs technical details. And I will leave the answer of that question to the purchsers of the filters.

I will say this: Every one of the folks I have upsold a better air filter system to has noticed a big improvement in the air quality in their home. That is without exception...

 

Question; "is salt an airbourne substance?"

No; that was not a test, it was a parlour trick.

When air passes through certain medium such as fiberglass, wool, nylon or most plastics, it creates a static charge on the surface of all of the tiny fibers of that material. It is the static charge that attracts the airbourne particulate.

Next time you see the salt test guy, ask him why an electronic air cleaner works when you can drop bb's through them.

 

i don't think salt is a valid test,but it makes a point.

Have the guy try fine particles of dust and see how many "fall" thru.

 

Is restriction caused by premium filters as much of a concern on variable speed furnaces?

Is there any danger with running a high restrictive filter if you furnace only runs in low fire mode?

 

Restrictive filters are even more of a concern with variable speed blowers. What a variable speed blower does is to monitor the amount of air that is being delivered by sensing the amp draw from the motor. When the amp draw decreases, the motor controls know that the amount of air has decreased and the motor ramps up to a higher rpm. This can only go so far though. If a restrictive filter gets so restrictive that the motor hits its limit, the motor will then shut down to a lower rpm to protect itself from overheating. This causes the blower to do what is called "huffing". The motor will ramp up speed to compensate for the restricted filter, then decrease speed when it hits the danger zone and continue this cycle until those very expensive controls burn out.

 

I used to push pleated when they first became popular. My main point was it was the layer of dirt that builds up on the surface of lets say a 20x20 is 400sq in. In a pleated the surface could be double that sruface. And then the mfg claiming the pleated could go 3 months yea right! Once I got a Dwyre SP gauge I changed my mind about it. I now recomend just the blue now for existing units.
But the filter I like the best and it is ment to be cleaned easly and is 1/6" thick and is practicaly just a window screen is the OEM type Rudd Rheem all plastic filters.
Now my dream idea of a great filter would be that made from the Rudd material only pleated for added surface and dirt the filter could hold giving my filter the over 30 day clean cycle the pleated ones say they do. Put all this in a kit small 2x2x24" box to include a compressed pleated filter and a frame kit to build any filter up to 24x24.

 

Robo states;
"The mistake being made about cfm vs velocity is that it is the increase of velocity that causes the cfm decrease, not the other way round. Restrictive filters decrease cfm "BECAUSE" they have increased velocity. A less restrictive filter allows for more air flow "BECAUSE" the velocity is lower."


How can the velocity increase thru a filter,other then the filter being slightly smaller then the ductor frame it's located in?If there is any increase in velocity,it causes an increase in static,static causes the decrease in cfm.

How do restrictive filters cause an increase in velocity as compared to the same size,cheap filter???

1.)Velocity = cfm divided by area(sq ft)
For velocity to increase ,area must become smaller,or cfms must increase.


Restriction of the filter causes the decrease in system cfms,not velocity.

The fan data is all based on static pressure change,increase the static and the cfms will decrease.If you decrease the area(duct size),velocity will increase,which will increase the static,decreasing the cfm.

 

Yes dash, a filter certainly does have less free area then ducting of the same size....about 40% less with a standard fiberglass filter and more with most others.

Yes dash, the same amount of air traveling through less space must travel faster.

No dash, a higher velocity of air through a filter restriction is not going to reduce the overall cfm of air in a system the same way a smaller run of ducting is. This is due to not having the static pressure pushing against the walls of the ducting but rather the decrease only in the filter that the air is pushing through.

Less free area = more velocity.....more restrictive filter = less free surface area......restrictive filter = more velocity of air moving through it.

The affect on lowering the cfm is just an added bonus to damaging a system with too restrictive of an air filter. That is why variable speed blowers huff when subjected to a restrictive filter.