EnergyProfessional
Mechanical
- Jan 20, 2010
- 1,279
For VAV supply with ducted return the return ducts get balanced based on design flow. With that during design conditions each space should be relatively neutral to other spaces and we minimize infiltration and exfiltration (i.e. uncontrolled air paths). But as described here during partial load operation (99% of the time) each space is out of balance when it comes to supply/return.
The inherent problem is that the supply is pressure-independent, and the return is pressure-dependent. What is a good way to minimize or avoid this imbalance? It kind of defeats the purpose of ducted return vs. return plenum (I realize more decides on what is better, for now assume I need ducted return). I realize a good way is to use return airvalves, like in labs etc. But I don't want to add the cost and complexity.
Some ideas I have:
- design return duct for very little pressuredrop - with that the imbalances become less severe. Problem is cost and space requirement.
- Balance return dampers at a lower flow to simulate the everyday case. Problem is TAB contractor is barley able to balance per the book, they won't be able to do that properly. and during design days I may create large imbalances.
- Balance by design and size ducts to have all paths equal pressure drop. Avoid any balancing and call it a day. Problem is this is hard to correct later, but not needing (inaccurate and lying) balancers also is a good idea. this probably would require relatively large ducts. With what I see how balancers measure and balance, just not balancing at all may be more accurate.
The advantage of proper plenum return is, that we use an oversized duct that minimizes pressure drop and therefore the differences in pressuredrop during different conditions are even smaller. Hence they don't need (and can't) be balanced. But how do we implement this advantage in ducted return?
The inherent problem is that the supply is pressure-independent, and the return is pressure-dependent. What is a good way to minimize or avoid this imbalance? It kind of defeats the purpose of ducted return vs. return plenum (I realize more decides on what is better, for now assume I need ducted return). I realize a good way is to use return airvalves, like in labs etc. But I don't want to add the cost and complexity.
Some ideas I have:
- design return duct for very little pressuredrop - with that the imbalances become less severe. Problem is cost and space requirement.
- Balance return dampers at a lower flow to simulate the everyday case. Problem is TAB contractor is barley able to balance per the book, they won't be able to do that properly. and during design days I may create large imbalances.
- Balance by design and size ducts to have all paths equal pressure drop. Avoid any balancing and call it a day. Problem is this is hard to correct later, but not needing (inaccurate and lying) balancers also is a good idea. this probably would require relatively large ducts. With what I see how balancers measure and balance, just not balancing at all may be more accurate.
The advantage of proper plenum return is, that we use an oversized duct that minimizes pressure drop and therefore the differences in pressuredrop during different conditions are even smaller. Hence they don't need (and can't) be balanced. But how do we implement this advantage in ducted return?