Filter size question

[davidlee]

How many square inches of filter area per ton of A/C is the normal recommendation?
My son just bought a house with a 3 1/2 ton unit and was told he may not have a big enough return air filter. He now has a 20x24.

 

[zmaxmotorsports]

Whats the size of the return drop itself?

 

[davidlee]

I don't Know. I will measure it when I go over there this weekend.

 

[simpler=better]

It's not too hard to retrofit a "media filter"

 

[davidlee]

simpler,
Tell me more. I have not heard of that filter. I assume I would have to replace the filterback grill to accept the deeper filter?

 

[manwithtools]

More filter media does not increase the size of the return opening. If the return area is too small that is needs to be addressed first.

Ideally you would like about 1 square inch of free grille area per 2 cfm of air flow. 3.5 ton would have 1400 cfm air flow approximately. This would result in a recommendation of 700 square inches of return grille area. He currently has 480 sq. in. of grille area. This might make the system louder than it has to be and the fan is not working within its design parameters.

 

[bonneyman]

Many homes have undersized returns. This is especially problematic with the new 14+ SEER units. They require alot of air, and are very sensitive to airflow issues. I've seen units that would work OK with a MERV 8 filter but freeze up (or whistle loudly) if the customer installed a high efficiency MERV 11 filter. (Same size, just slightly denser filtering media).

Many tract homes (and alot of custom homes) have duct systems that are made in bulk, and the installers don't always make allowances for heat load issues (high ceilings, large western exposure, more windows than the standard model, etc.). Returns are too small or not placed properly. The units therefore run behind the curve so to speak and something as minor as the wrong filter is enough to cause major problems.
Easiest way to see if your son has this problem is to install a dense pack 3M Filtrete filter and watch what happens. If it makes noises or ices up or loses cooling, he has a marginal airflow issue.

Hope this helps.

 

[davidlee]

Thanks for the info. I will do some more checking.

 

[simpler=better]

These guys know more than me, I'm just your average hack. But poking around online it seems like going from a 1" pleated filter to a 4-5" media filter takes a good amount of strain off the system.

The ducts are still probably all messed up but I'll leave that one to the pros.

http://yourhome.honeywell.com/en/products/air-cleaners/f100-whole-house-media-air-cleaner

 

[Falcon67]

We use 2x20x20 filters, 3 1/2 ton Ruud.

http://www.purolatorairfilters.com/key-pleat/5251123101?gclid=CLWs7LjU5swCFQEJaQodyeEPWA

 

[pseudorealityx]

500 fpm is typical max filter velocity.

 

[BD1]

Check your furnace manual. Some require more return air than others. If return duct is on the smaller side a option is to cut another return with filter on the opposite side of the furnace. Even if it is in the basement it just needs more air returning.

 

[Jackfre]

You can check with a manometer. Most systems are set at .5" external static. That means total static. With a manometer, take a 6" long or so 1/4" metal tube and put it in the supply and return plenum just outside the actual furnace, perpendicular to the air flow. Note the static in each. Don't worry about +/- numbers, just add the total of each. For example, if you have a .2 static on the supply side and .2 or .3 on the return the total of .5 says you have good balanced air flow. The comments about undersized return or "over filtered" units is spot on. On problem systems I would see a higher number usually on the return of say a .8 or so, showing major RA restriction. I'd go back on the return side and check on the return drop, say .7. Then I'd go up to the ceiling to the return main and see .2 or .3. The drop is undersized and possibly the return plenum.

The problem with putting in a new high merv filter is that it may operate fine when new and clean. As it get dirty, and it may not take much, then the system and owner starts to pay the price. How much do you want to babysit your hvac system? It gets overlooked...for a while.

The 5" pleats are offered by many companies today. I think Trion's Air Bear was the first of them. You can search around for them. Even if you have to change the filter rack, it may be your best bet.

Before the MERV ratings came out there was no better example of the old saw about, "figures don't lie, but liars figure". The industry threw around terms like efficiency and effectiveness and it was just a bunch of BS (I was the Purolator rep). Many residential furnaces didn't begin to filter until they were dirty. They had to be loaded up to work. MERV ratings are first day ratings so you know what you are getting.