HVAC ductwork question

Questions on HVAC ductwork in a multi-zone mini-split system (e.g., Mitsubishi City Multi). I am trying to minimize duct runs to help budget and energy efficiency. – I understand that the ductless interior units (wall mounted) are higher efficiency than ducted (ceiling concealed) – Is it also correct that SHORTER duct runs are always more efficient than longer duct runs, because there is a certain % energy loss per foot? – Is there a duct run that is too short (e.g. just enough to go from the unit to a grille (3 feet or so). I have heard that a very short duct run setup would be noisier? Does it have any negative performance / efficiency implications?

hawaiismurf

in Roofers 11 years and 5 months ago

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brucef | 11 years and 5 months ago

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August, I believe OP is probably asking about outflow, not return. And cooling not heat. OP, in regards to your 2nd and 3rd points, while technically true, they are minimal in the situations you are probably contemplating. If you were considering ducting 45′ from back to front, maybe, but no townhouse design should look like that anyway. While single evaporators are simpler and slightly more efficient, it is more in terms of how quickly a room is cooled, not comfort once the temps have stabilized.

Augustiner | 11 years and 5 months ago

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The reason why wall hung units might be considered more efficient is because they have 100% intake and exhaust in the same place. Ducted works best in bigger rooms because it´s easier to balance out. HVAC Heating is generally more recommendable for passive houses or more or less airtight homes (spray foam). The reason is that if the calculation is a bit off and you suck in more air in one place than you blow out in another, the air is being sucked in from wherever there´s the least resistance. In a non airtight house you might suck air partially through the cracks in the bricks. So you don´t circulate 100% of the inside air, and the system doesn´t work as great.