zoned system question


Old 08-21-06, 03:02 PM
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zoned system question

I'm finishing my basement and have talked to two HVAC contractors about having the basement zoned separatley from the first floor (the second floor has it's own system). They both recommended that a new trunkline be added for the basement supplies. This makes sense to me since just tapping off the 1st floor trunkline will reduce the airflow to all the registers and compromise performance. However, as for the returns in the basement, they both said it's fine just tap off the 1st floor return trunkline. Doesn't the same arguement apply to the return side of the system? Just wondering, for example, that if in the winter time say that the first floor calls for heat. Will it not run as efficently as before since the airflow in the returns will not be as strong?
Old 08-21-06, 08:35 PM
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I am not a fan of zoning forced-air systems except when the zoning is used to keep a single room at a lower temperature than the main thermostat setting. Large-scale zoning of forced-air systems introduces several problems that rarely are adaquately addressed.

Return-air ducts should have a cross-sectional area 1-1/4 to 1-1/2 times the cross-sectional area of the supply ducts that they serve. In other words, the total cross-sectional area of all of the return ducts at the furnace should be 1-1/4 to 1-1/2 times the cross-sectional area of the supply ducts being served by the furnace. If you need an additional supply trunk duct from the furnace outlet then most likely you also need a corresponding RA duct back to the furnace. Undersized return air ducts are common, and a common cause of poor performance in forced air systems.
Old 08-22-06, 08:50 PM
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Red face

Me too No way would I put a zone set up in for the 1st floor and the basement. We run all basement outlets to near the outside walls. Put a return in the cold air drop. You want it low in the basement . If need be cut some off the bottom of all doors down there ,for a return out of rooms. Also in winter you want more heat down there .But this will go up to the floor above and help out up there. Summer just close the registers down a little down there. You did right with a unit for upstairs and one for down. zones dont work in a home . They do work for large Comm. jobs.


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