Adding HVAC register - should be simple...but maybe it isn't.

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J Blow

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South Dakota
Seems like the more I read the more conflicting information I get.

I think this is simple but just want to run it by someone. My basement was unfinished but had heating/cooling registers roughed in for the basement - in 3 spots. It should work perfectly fine as those 3 spots are 2 bedrooms and the family room. The only other room (besides utility room) is a bathroom. I think I need to add a register in there. I could probably get by without it as it's heavily insulated but probably not the smartest move.

Above the bathroom, there's duct work coming from the plenum that feeds a walk in closet above in the master bedroom. It seems it's a perfect scenario where reducing a little flow wouldn't cause much problem. Can I simply splice into that lead and feed the bathroom? What about return air? Of course the bathroom does have a vent/fan, too. Seems it?



In the trades
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Duct systems need to be designed for the relative air flow needs for heating or cooling. The ACCA Manual-D duct design specifications work pretty well.

There are a couple of problems with just hacking in to support the heating/cooling needs of basement zones. The heat gain/loss characteristics of above-grade rooms are quite different from those of a basement (even a walk out basement), so getting the temperatures to balance reasonably is impossible. Tweaked to perfection for one particular day's conditions, it can be a bit off the next day, and WAY off by the next month.

Opening up the flow to a new room in the basement robs flow to the down-stream rooms of the above grade rooms, altering the temperature balance for all rooms, not just the down-stream rooms (like the walk-in closet.)

All supply duct registers need a correspondingly high cross section of return path, whether ducts, door-cuts, jump ducts, etc to an appropriately sized return register. If you don't the resulting room-to-room pressure differences drives outdoor air infiltration skyward whenever the air handler is running, resulting in higher heating energy use in the winter, and an extreme drying effect on the indoor air due to that infiltration. A tight house with tight well designed duct systems will never need added humidity in most houses, and will stay at comfortable/healthy humidity levels just on the moisture delivered by cooking/bathing/breathing behavior of the occupants.

Getting to the "right" solution for any heating/cooling problem starts with calculating the loads (Manual-J, or an I=B=R type calculation). The heating loads of basements tend to be pretty low, especially if insulated, and the cooling loads even lower. If there's a budget, it may make more sense to skip the cooling for the basement that it probably doesn't really need, and to run a small hydronic heating loop off the hot water heater than to start hacking on ducts and screwing with the balance of the whole system only to achieve a less-than-satisfactory result in the basement. With it's own heat source & system the basement can then run as it's own zone, with very reasonable room-to-room temperature balance & control.


Retired Defense Industry Engineer xxx
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New England
Some of it would depend on how well the current system is designed.
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