|MarketManager » Envelope|
Plenums are not modeled separately in MarketManager, i.e. you should always enter the full "floor-to-floor" height for a wall in the wall data form.
We do model the plenum as it pertains to heat from lights or the type of short-circuiting of supply to return air that occurs in a ceiling bypass system. But for practical purposes, we consider the plenum as a perfectly isolated return duct.
If you want to model heat losses or gains from and to air ducts, you can do so in the Duct Item by entering length of ducts in unconditioned space. MarketManager considers the "delta-T" in this case to be *half* of the indoor-outdoor delta-T at that particular hour. There is no easy answer as to where the plenum fits in this simple model, though it could be considered somewhere between the extremes of "fully conditioned" and "unconditioned" spaces.
From everything we have heard, ignoring the plenum as MarketManager does is not a bad assumption. Think of it this way: heat or "coolth" lost from supply ducts traversing the plenum counteracts the lack of conditioning of the plenum. Studies that we have seen appear to confirm that the two effects are not far from canceling each other.
Otherwise feel free to contact our Tech Support staff at (805) 329-6565, or via email at firstname.lastname@example.org.