Messages and Online Forum Q&A

Re: soffitted supply and returns

From: info@builderswebsource.com
Category: HVAC
Remote Name: 64.75.140.51
Date: 12 Jul 2001
Time: 02:01 PM

Comments

In order to properly compute the size of supply and return ducts (as well as the size of the heat pump), it is important to know the overall thermal loss in BTU/hr. Several factors, including the number, size and type of windows, orientation, wall r-value, thermal mass, house volume, etc. will determine the overall heat loss/gain. Furthermore, total temperature differentials are important factors in sizing an HVAC system based on expected indoor temperatures relative to peak and average outdoor temperatures.

Generally, however, it's better to keep supply and return ducts at different vertical levels in order to promote movement of air. If the supply and return ducts are both up high in the soffit, then air will tend to recirculate near the top of the ceiling, but do little to pull air from below -- where people are. Depending on whether your area is generally "hotter" during most of the year (requiring cooling)....or cooler (requiring heating) might dictate whether the supply or return should be near the ceiling. Cool air drops and warm air rises. For example, for room heating, it's most efficient to release the warm air near the floor surface, with the return air duct near the ceiling. For air conditioning, the opposite it true. Of course, most residential structures use a common set of ducts for heating and cooling which means you need to compromise.

We recommend that you work with a local HVAC company that can help ensure your design is efficient and optimal given your physical constraints. If you buy the parts from a local company, they will often assist you in the design at little or no additional charge.

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