filters in returns

[John Arnold]

There is a forced air furnace in the attic, with an electronic filter. So why do we have filters in the second floor ceiling returns?

[Jim Luttrall]

Looks like a rock catcher (cheap almost useless filter) that they may be using as a pre-filter or they just did not want to remove the filters. It may not cause much of a restriction if properly maintained but I would warn my clients of the potential problems.

[Bob Knauff]

Around my area that is the ONLY return for the furnace in the entire home. In a two story, the returns (usually one for each furnace for each floor and located in the attic) are located in the ceiling of the upper story in a main hall. Not a single return in the individual rooms except perhaps the Master Bedroom. The doors are undercut to allow room air out if the door is closed. All air must find its way back to the ceiling of the upper level.

[CHARLIE VAN FLEET]

Jim

why do you call it a useless filter???

cvf

[Jim Luttrall]

I said "almost" useless:D
The see through fiberglass filters let a considerable amount of stuff through. Once upon a time these were "treated" so that they performed better but in the day when price sells more than quality, price has pretty much done away with the treated filter.
Since we are a cooling climate with a/c running about 8 months out of the year and heat about 2-3 months (My a/c has been on for the last month or more) it is very important to keep as much crap off of the coil as possible. While the cheapo filters will catch the occasional dog, cat, or rock, they are "almost" useless at properly filtering dirt out of the air before it gets to the coil. If you are in a heating only climate they might work OK except for the stuff that builds up on the blower.
On the flip side, I don't like the super high priced "allergenic" filters either since they may do "too good" a job and restrict the air flow. I like the medium price / quality pleated filters or the 4" thick media filters.

[Darren Miller]

Actually John, I like that set-up as long as they keep changing the filter at the return.

With this particular arrangement, at least the return duct will be kept fairly 'clean'. Did you ever look into the return when the filter is at the unit and see NOBODY ever cleans grill or the duct. At least this way the duct is not a dust collector. Again, as long as the 1 inch filter is changed regularly.

[Jerry Peck]

Many installation instructions will state not to install more than one filter.

If there is an electronic filter, the filters at the return air grilles should not be there (yes, I like the filters at the returns as it does help keep the duct cleaner, but ... if the manufacturer says not to, then you are not allowed to).

The installation instructions work both ways: for you when you think something needs to be done better, and against you when you think it needs to be done your way but the manufacturer says not to do it that way - you should not be playing the game that you know better than the manufacturer, else you will find yourself being hung when someone knows your game and knows the installation instructions, that can totally disgrace everything else you put in your report.

[David Bell]

Electronic air filters have pre filters ahead of them, if those are removed the filter in the return grill is acceptable and will not affect airflow.

[Don Burbach]

While I understand Jerry's statements about the owner manual for the electronic filter.... putting a serviceable filter in the attic for many people is a sure recipe for failure. Many people have neither seen their attic or have had any service on their system unless it stopped heating or cooling.

While re-writting the manufacturer's care manual may be a no-no, augmenting it may make a lot of sense.

The manufacturer's true intent of the 'no other filter' remark may be something very similiar to the Viagra commercial that says that you should go to your doctor if you have an *rection that lasts more than 4 hours. The manufacturer may simply be pushing the 'you don't need another filter' point.

Like the bird in the bush axiom, it may be better to have one serviceable half-useful filter than a superior filter than may never get serviced.

Save the flame-war energy, this is a common sense approach to making a useful air filter system. The same instructions that say no other 'air filter' should be scanned for the usefulness and wisdom of putting the filter in an attic. Around here, 10 foot ceilings, hardwood floors, and access hatches in closet ceilings or hallways accessible only by ladder are frequent.

The avoidance of the risk of a fall servicing the electronic filter trumps discomfort any day!

[Darrell Udelhoven]

Normally, if you're using room filter grilles, you don't put a filter in the furnace/air handler, because it results in too much pressure drop. Don't filter at the blower.

Room Air Filter Rack Sizing
ACCA Manual D specifies on Return-Air Filter grilles a maximum of 300-FPM velocity.

Velocities higher than 500 fpm will decrease filter performance. Increase flow resistance, and possibly blow off collections of dirt. Measure Velocity 1” from RA grille face.

Average Free Air Area of most Return Air measured grille portion is about .95%; Clean Return Air Filter free-air-area is around .55%, this will vary; I use .50% for both.

You can put in real deep pleated filter grilles; always have a deep enough box to the RA duct so the filter area is not reduced to the entry diameter of the RA duct.

Therefore, measuring the louvered open area of the grille sq.ins. X's .45% initial measurement 900-sq.ins *.50 = 450-sq.ins., / 144=3.125-SF *X 300-FPM Vel = only 937.5-CFM of airflow; less than 400 or 375-cfm per/ton airflow for a 2.5-Ton system. I'd use two RA filter-grille areas

For a 4-Ton A/C use more Return-Air filter-grilles sized as large or larger than above.

Never sell units requiring more airflow than the duct system & filter area will support! Yes, this is shocking to everyone...!

You can't have too much RA Free-Air filter grille area...