What thickness filter 1" or 2"

89c4l98 · May 18, 2010

I had a Coleman TG8S 3ton drive single stage 1200 cfm furnace plus a matching 3.5 ton coil and a consenser 3 ton installed the furnace called for a 16x25 filter. There is no internal filter so we installed a external filter cabinet in the return. The external filter cabinet can take a 1" or 2" thick filter obviously both 16x25.

What are the pros and cons of a 2" thick filter? what would happen if i put two 1 inch filters in the cabinet? would i get reduced air flow or better filtration? should i just stick with a 1" filter?

Edit: I'm using a standard merv 8 pleated filter

ljbrandt · May 18, 2010

Never put two 1" pleated filters side by side..you will severly restrict airflow and may damage your equipement. Best to go with a 2" filter vs the 1" ...more surface area = less pressure drop for the same amount of filtration as a 1".

skippedover · May 19, 2010

At a MERV 8 value, there's probably not much difference in the static loss across the filter. Once you get up to MERV 12+, that's a different story. What you will gain with a 2-inch filter is longer useable life. That means you only have to change the filter every 120 days instead of every 60 days. With higher MERV ratings, the static drop across the filter is very high for a 1-inch thickness. It's not uncommon to have higher MERV rated filters with 4 or 5 inch pleated media to give the high MERV rating and still maintain a nice, low static loss. High static results in less airflow, higher operating costs and in some extreme cases equipment damage.

pstu · May 19, 2010

General observations

At 2.78 sqft of filter area, the air velocity at the filter face calculates to about 430+ feet/min. Do the pros on this board have any worry about the wisdom of this? I hear complaints all the time about high MERV 3M Filtrete filters on account of pressure drop, but rarely are the complaints expressed in terms of air speed at the filter face. Nor are they usually associated with a measurement of external static pressure (ESP) in the actual installed system.

To illustrate how air speed (i.e. CFM) matters, you can take a look at the attached performance data on Aprilaire 2200/2400 media filters. A filter that has 0.25 inch (water column) pressure drop at 2000 CFM, will have only 0.09 inch at 1000 CFM. The relationship is non-linear with respect to airflow.

People on this board often scoff at "rules of thumb" but some are more rational than others. The rule of "1 sqft filter area per ton" translates to 400 feet/min. The rule of "200 sqin filter are per ton" translates to closer to 300 feet/min. The OP is already on the bad side of either of these rules of thumb, the question is whether it is professionally wise to criticize that.

I am of course not a pro. Instead I am a homeowner who wants to learn from experience, and let it be vicarious experience if at all possible. For my own house, I contracted with a pro to install additional returns so the airspeed at filter face is well under 200 feet/min. One benefit is that dirty filters (even those 3M Filtretes) are very unlikely to over-stress the air handler due to high ESP.

Hope this helps -- Pstu

dash · May 19, 2010

You really should have the statc tested,some companies can have techs can do this on a checkup call. Call around and see if you can find one, if yours can't.

Testing will tell how restrictive of a filter you can use, safely.

What thickness filter 1" or 2"

89c4l98

ljbrandt

skippedover

pstu

dash

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