Ducts too small to support unit?

Old 01-10-06, 04:25 AM
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Ducts too small to support unit?

My house was built in 1971 in the Nashvile TN area with a 3-1/2 ton unit that was sized for the main level that is 2,000 square feet. The Heat Pump is a single package unit. The feed line and return duct sizes are both 12 x 20 and the Filter opening for return air is 20 x 30. In 1993 I put in a 4 ton unit - nobody mentioned anything about ducts at the time. Thus unit , a Trane/American Standard WCC0048 compressor failed recently so I will need to replace the Heat Pump and am considering one of the 13 Seer models that are now available.

Most of the contracters say the duct sizes of 12 x 20 will work on a 4 ton unit (1600 CFM). but one contracter (he seems to know what he is talking about) say the Main and return ducsts need to be replaced and sized for a 4 ton unit that will handle a flow of 1600 CFM at the correct static pressure. As I am ablle to determine, the 12 x 20 duct size is only capable of 1300 CFM.

The original 3-1/2 ton unit did not seem to heat or cool the house adequately so that is why I moved to a 4 ton unitn originally.

The house has also has two rooms in the basement that total 800 Square feet so that is why I am leaning towards 4 tons although the contractor says that an additional return and additional ducts would need to be installed for the basement - If I want to Heat/Cool it eficiently. Cooling has never been a problem in this area - only Heating. I can live with external heaters for that matter.

1. Considering the ducts feed and rethrn ducts are 12 x 20 and the filter is 20 x 30 will this work affectively with a 4 Ton 13 Seer unit ?

Last edited by ddwoodbiry; 01-10-06 at 12:37 PM.
Old 01-10-06, 04:11 PM
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Your contractor is correct. Static pressure is very important. While the 12x20 duct may be large enough, I believe what he is saying is that the ducts leading into this duct are not sized correctly. With a heat pump it is not as critical as a gas or oil furnace but should still be close to design as possible for efficiency.

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