I think my return grill is too small


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Old 08-11-12, 02:34 PM
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I think my return grill is too small

I recently bought a house that was built in 1968. It has a 19 year old 100k BTU 80% furnace that I am replacing. I am also adding air conditioning at the same time. I have multiple proposals, calling for both 4 and 5 tons of cooling.

My concern is that my return air grill is only 300 square inches. The contractor that I've had the most communication with does not seem concerned. I'm thinking it needs to be much larger, 800 to 1000 square inches. We have a second home where the return grills are 500 square inches and 800 square inches for a 2.5 and 3.5 ton A/C respectively. I've also seen CFM/pressure calculations that support the 200 square inch per ton rule of thumb.

The furnace is just about on the other side of the wall from where the return grill is located, so it doesn't seem like major work to enlarge it, so I'm not sure why the contractor would want to omit it from the proposal.
 
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Old 08-12-12, 06:30 AM
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300 is way to small. 25x25 would be better. 2 20x20 would be better. 22 - 24 inch round duct would be great
 
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Old 08-15-12, 07:13 AM
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The return grille is not really the determining factor of your return air. The duct size is the controlling factor. The square inches of return duct cross section must at least match the square inches of return opening on the new equipment. Get your contractor to insert "return air ducting to match new equipment." in his bid. He is then on the hook to make sure the ducting is adequate. Any responsible contractor will have no problem adding this note to his proposal. If the equipment requires more return, he can add another return or rework the existing to comply.
 
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Old 08-15-12, 05:26 PM
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Perhaps I need to clarify - I care about the return grill because that's where the filters are installed. I'd like to be able to use MERV 10 filters without creating excessive static pressure.

I've since spoken to the contractor and asked if they can either increase the size of the return grill so I can use bigger/multiple filters (they can) or install a whole-house filter and not use a filter at the return grill (they can). They are going to take another look at how the air enters the furnace and make a recommendation to me.
 
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Old 08-18-12, 03:24 AM
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Yes indeed, if you choose to use the smaller micron filters on your system, you must have very large surface area filters as this type of filter will clog much faster and create air starvation to the return. If you do go with the high micron filters, make certain to check them OFTEN to prevent this situation. If you go with a whole house electrostatic type filtration system, it is still smart to use the cheap fiberglass (low restriction) filters at your return grilles to prevent an insect, or pet hair etc, from getting to your central filter system. Then make certain to get completely schooled on maintenance for the central filtration unit.
 
 

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