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Question DetailsAsked on 5/14/2013

Is it ok to run my AC with only the 4" filter at the plenum, or do I need 1" filters at the return air intake too?

I have a split AC with a 20x25x4 media filter between the return air plenum and the blower. I also have a regular 1" filter downstairs at the return air intake. I was told by a HVAC contractor today that I only need the 4" filter and that having both filters will put extra strain on the blower and is bad. Is that right? Thanks!!

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8 Answers

Voted Best Answer
1
Vote

Chances are he is correct, static pressure could be checked to confirm that.


However being in the business for years, we have only designed one system for a customer that wanted to keep dust out of the return ducts by using the setup you describe.


This required larger ducts then usual, because of the additional resistance of two filters in the same return duct system.


Filters in te return intake, are usually the original install, having a much better Media Filter is usually the result of replacing the original system.

Source: www.bayareacool.com

Answered 7 years ago by BayAreaAC

0
Votes

This is not a typical residential design but is found at times, and is used in commercial applications.

As long as the makeup (fresh) air AND the return (recycled) air both pass through the main filter, that would be normal and OK for almost all residential applications. If the return air does NOT pass through the intake filter, then you would need this filter or else the inside air would pass back into the ducts without filtration, distributing the household dust in the air throughout the house.

A return intake filter, like a 4 inch fiber filter or a pair of 2 inch filters (some they can be changed while system is running), is commonly used in commercial applications where a lot of dust is generated - like in workshops where woodworking or fibreglass work or sanding is going on, in buildings where products are sandblasted, etc. Of course, the ducting and fan are sized for the greater friction loss.

Is there a possibility this 1 inch filter was installed originally becuase the homeowner had a woodworking shop in the basement or garage - or maybe they had someone in the house with allergies, and used a double-filtration system to reduce that ?

Your HVAC contractor can do an assessment of your system in a few hours to determine if static pressures are appropriate in various parts of the system, or if this is reducing your airflow and possibly overloading the blower, although in modern systems the "squirrel cage" type of blower used is not as susceptible to that as a fan. However, extra friction on the return intake side can result in the unit drawing more makeup air than normal to compensate, which is bad for your energy efficiency.

Answered 7 years ago by LCD

0
Votes

Makeup aiis not common by any means in residential homes. It is extremely unlikely that this home has a makeup air system.


Static pressure can be taken in minutes, all that is important , is what is the static at the intake and discharge of the indoor unit,

Source: www.bayareacool.com

Answered 7 years ago by BayAreaAC

0
Votes

I stand corrected by BayAreaAC - I live in an area where extremely tight houses are common, and energy standards for local and state loans are very high, so newer systems are required to have makeup air in addition to recirculating interior air.

In older systems and most parts of the country, the "makeup air" just infiltrates into the house under doors, through the chimney, etc.

Answered 7 years ago by LCD

0
Votes

I agree with BayAreaAC comments - if you do decide to remove the extra filter, if it is just a slip-in type without a sealed door be sure to duct tape over the slot it goes in. Double-layer, so it does not creep and get sucked into the opening over time. Otherwise you will be letting air in at that point, which will reduce the negative air pressure in the rest of the return ducts, cutting down on whole-house air circulation.

Answered 7 years ago by LCD

0
Votes

You stated that, "I also have a regular 1" filter downstairs at the return air intake." I would think that is a remote location and the "intake" is a Grille in the wall/floor/ceiling. If so reomovfin it should not cause any issues.

Source: www.bayareacool.com

Answered 7 years ago by BayAreaAC

1
Vote

I am glad to see that hvac contractors' are starting to come around when it comes to educating customers’ on required/design (static) pressure in the duct system. However, the loss of pressure or the "added" stress that was referenced in previous comments is discussed in these posts. No one, however, makes note of the fact that it is quite possible the 4" filter could itself be highly restrictive to airflow in this system and should be checked for excessive pressure loss. Otherwise, this whole discussion is academic and the customer does not get the benefit of seeing the equipment operate at its rated efficiencies—true energy savings.

Answered 7 years ago by Stans HVAC

0
Votes

BayAreaAC - You said "I would think that is a remote location and the "intake" is a Grille in the wall/floor/ceiling."


That is exactly the situation.


Thanks to all for your great feedback!! This was extremely helpful.

Answered 7 years ago by Guest_93891501

0
Votes

Thanks for getting back to us with the update. Sounds like you have a great AC contractor.

Source: www.bayareacool.com

Answered 7 years ago by BayAreaAC




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