Derek Cole explains why closing vents to redirect heat or cooling backfires: it raises static pressure and forces your blower to work harder without delivering more air. A duct sized for 100 CFM can't deliver more no matter how many vents you close, like a garden hose on a fire hydrant. Use dampers and zoning instead.
Do not close your vents or registers in one part of the house to redirect cool or warm air to another part of the house. Just about any HVAC technician you talk to is going to tell you exactly that.
Never ever close the vents off in your house. All right, so a friend sent this to me, uh, this past weekend, and the gentleman goes on to state how, um, mainly it's a static pressure issue. I think he measured his static pressure and it goes up like maybe 1 inch of water column when closes all the vents. Yes, that pressure is a factor in not closing your vents. That's true, because you're causing more air to go back to the blower and this working harder to push it out. But mainly, that is one factor depending on your home. Now, in his home, it wasn't that big of an issue, but mainly the issue is You're trying to redirect— people close vents because they're trying to redirect air to a place they're not at to a place they are at. Uh, your duct system does not work in that fashion. So a 6-inch run of duct in a room is made for 100 CFM of air. It can only hold 100 CFM of air. So if you close the vents off in the the room next to it or the room down the hall, more air is not going to come through the 100 CFM vent that only holds 100 CFM. It's not going to hold 125 CFM. Does that make sense? It's similar to putting a garden hose on a fire hydrant, okay? And expecting the same result if you had a fire hose on the fire hydrant. No, you have a garden hose on the fire hydrant. It's not going to suddenly spit out all the water that is there because it's a garden hose. It's only made to handle what it's made to handle.
That's like putting a— some folks say a bigger unit is better. Well, if you put a 5-ton unit on a 3-ton duct system, you only have a 3-ton duct system. So you can put a 5-ton unit, but you're still only getting 3 tons worth of air in the home. Does that make sense? It's not going to do extra because you have a bigger unit. It's not going to do extra because you want to close vents and redirect the air somewhere else. It doesn't work that way. It might work in your car, close the vent next to it, and it moves, you know, 2 inches over and blows a little harder. It's not gonna work that way in your home. So if the goal is to close vents to redirect air, then we need to size it properly, go through the home. You probably need zoning so you can control dampers and control airflow versus trying to control it at the grill, which is not going to work.
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