Correct AC/Heating Filters

Old 03-07-15, 08:52 AM
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Correct AC/Heating Filters

My home has Lennox Heating/AC units. These units have a central MERV rated filter. In addition, the builder has installed low end pleated filters on the returns.

There is some debate as to whether the system needs both - can we remove the filters on the returns and work only with central MERV filter? What MERV rating is recommended?

Currently, the central filter is 16x25x5 MERv 10 Unit.

Old 03-07-15, 09:04 AM
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Hi, and welcome.

Since the forum you posted in deals with hot water and steam, and you are talking about scorched air, I've moved your post to a more appropriate area.

Last edited by NJT; 03-07-15 at 09:30 AM.
Old 03-07-15, 12:32 PM
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You do not need nor want both filters unless the ducts connected to the filter grilles attach to the furnace after the big filter. I normally use no more than a Merv 8.
Old 03-07-15, 12:57 PM
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Thanks for you both our help. How often do you recommend changing the MERV filter?
Old 03-07-15, 01:46 PM
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Ideally, filters should be changed when the pressure drop across them increases significantly over the pressure drop when new. You simply cannot state that a filter needs to be changed after so many days or hours of operation as it depends far too much on the amount of dirt in the air.

Probably the best differential gauge is a Dwyer Magnehelic with a range of 0 to 2 inches of water column. They are often available on Ebay for a price range of about $20 to $30 dollars.

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