Question about HVAC mod and design

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Old 07-21-14, 07:45 AM
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Question about HVAC mod and design

I have a 2 ton unit with a VS air handler. When I had it put in, I had extra air returns added. A single 14" with 25x30 grill was put in. I had them make that one a 16x15 and added an additional 16x25 grill with 12" return. Anyway, the system runs well, but with my pleated air filters in the air return, I notice that the unit 'speeds up' a little. Presumably, this is to compensate for a restrictive filter and maintain the CFM its programmed for. However, I do not notice any increase in the air flow that comes out of the registers. The register temps are exactly the same. Why?

Secondly, I am wondering if 'deepening' my returns so that they will accept the 4" deep pleated filters will increase the AC efficiency and lower costs to run it? Would this be an exercise in futility or would be actually be worth while?

To increase air flow I can:

1.) Deepen both returns so that they can accept a 4" deep Honeywell filter.

OR

2.) Change the 14" return back to a 25x30 and use the 1" filters instead. The framing is still there. It would be an easy mod.

Would this gain me anything or do I just have too much time on my hands?
 

Last edited by agdodge4x4; 07-21-14 at 08:15 AM.
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Old 07-21-14, 08:08 AM
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An ECM 2.3 variable speed motor will ramp up or down to deliver the desired cfm at various external static pressures.

Adding return air will allow the motor to ramp down and not work as hard to deliver the configured cfm. This will lower the amp draw of the motor and possibly increase the motor life but not increase cfm output since that motor measures its output.

Adding return air will increase cfm output with a standard psc motor.

I modified my grill to accept a 4.5 inch thick mechanical filter as well.
 
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Old 07-21-14, 09:03 AM
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If I modify my return to accept that 4.5" filter, how much space should be between the back of that filter and the duct board/duct?

Also, do I HAVE to use duct board for the back?
 
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Old 07-21-14, 10:40 AM
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I have a closet model furnace with wood a drywall for a return plenum. No ductboard.

I can't answer your question without more information about the application. A picture would help.
 
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Old 07-21-14, 10:55 AM
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Don't have pics right now, but I can explain. The return is framed into the wall. So, the wall is 2x4 studs with a grate on the drywall side. They put duct board on the back side of the 2x4's creating the return air cavity and their is a flex duct attached to that. This works great for 1" filters and there is a space of about 4" between the filter and the duct board.

That cavity is only 1 2x4 deep, so its not QUITE deep enough to be able to put this in:

Amazon.com: 16x25x5 MERV 10 Honeywell Grill Filter (2 Pack): Health & Personal Care

I cannot get the filter grate closed because the cavity is not 'deep' enough. My question is, what would the proper depth be such that this would fit properly. How much clearance should be behind this filter?

This filter is more designed to be used in return CHASES...and I don't really have that. So I need to create that space.
 
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Old 07-21-14, 08:08 PM
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Amazon.com: Customer Reviews: Honeywell F100F2002/U Media Filter Whole House Air Cleaner

You want more than 4 inches. Twice that.

16 x 25 x 4.5 Racks can be installed at the furnace.

The F100F2010 20x25x4.5 rack would be even better.
 
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Old 07-22-14, 07:40 AM
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I like the rack. But look at my air handler. The intake is 16x20 max...how do I make a 16x25 or 20x25 attach to that??

My unit is: FV4CN5002

It calls for Robot Check in the little slot already built into the air handler. So, that gives an idea of its size. Inlet is 16x20.

imgur: the simple image sharer

In this picture, as installed by a professional, you can see the 14" duct that is literally switchbacked and it comes immediately from the wall. You can see the insulation. The duct coming off of the top is the 12" duct.

How can I properly improve this for improved filtration while maintaining proper air flow and efficiency of the VS motor? Will your filter rack size work, if so, how do I do that?
 

Last edited by agdodge4x4; 07-22-14 at 10:25 AM.
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