Closing off an AC vent

Submitted by flyingGoatPenis t3_zpa1mk in DIY

I need to close off a flexible AC line. I combined two rooms by knocking the wall between them down, and the room is small enough that one register is plenty. I’m having subfloor redone so I can remove the extra register as well. To close off the flex line, do I just need to cut it and seal it as tightly as possible with tape?

More permanently, I’ll try to remove the line entirely, but for a temp fix until I’m able to do that, would this work, or is there a better fix?

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BxMxK t1_j0rpvpc wrote

You also need to make sure the rest of the ductwork can handle the increase in pressure from closing off one of the ducts.

I've seen more than a few main ducts burst open because people thought they could just cover a register or two in rooms they weren't using and didn't take into account that the blower was still trying to move the same volume of air through less openings.

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monthos t1_j0sbro4 wrote

After working in telecom/IT for so long now I decided if I ever build or renovate a house I am putting in a home equivalent of a BAS system.

Multi stage heating and cooling, PWM controlled supply fan, dampers to limit airflow from the air handler all so each zone can be set to different temps, but keeping the static duct pressure in the optimal range.

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Gr3yGhost t1_j0se9gn wrote

My house is built like this. Built in 2018. The HVAC has 3 zones, 1 for the master, 1 for the living room, and 1 for the 2 lesser bedrooms on the other side of the house.

It works great when I'm just chillin in my office/game room and it gets toasty, I can just close the other 2 dampers so that only the game room is fed all the cold air I am requesting. And because I'm only cooling a small room thats about 350sq-ft at most, it cools it off pretty damn quick. Also helps that each zone has its own intake vent also

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Aurum555 t1_j0t7xqr wrote

This only works with a well insulated house otherwise you end up just spending extra money for your system to fight itself,that said I'm insanely jealous, and want to reinsulate my e tire house so I can even dream of doing something like that.

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Semanticss t1_j0s84eg wrote

Agreed. Closing just one duct has tripped my blower fan in the past from the change in air pressure.

Might need a professional for this. But honestly, best thing to do is leave the vent. I don’t see any reason to remove it other than the slight inconvenience of furniture positioning.

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MattsAwesomeStuff t1_j0ufg6k wrote

> Closing just one duct has tripped my blower fan in the past from the change in air pressure.

That's not how motor load on a fan works.

The more you block it up, the lower power the fan consumes.

This is counterintuitive to some people, but is a fact.

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WKS01 t1_j0v6gpa wrote

That is not entirely true, it depends on the type of fan being used. In a constant torque motor the fan would increase amp draw as it attempts to overcome the higher external static pressure.

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MattsAwesomeStuff t1_j0vgyqm wrote

> In a constant torque motor the fan would increase amp draw as it attempts to overcome the higher external static pressure.

... is that the type of fan used in whole-house HVAC systems?

... no.

[Edited to add, see below, in some cases, actually yes]

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WKS01 t1_j0vlqkl wrote

There are Manufacturers that do use constant torque motors in residential furnaces though. Trane S8X2 S8X1 and S8B1, Carrier Comfort and Performance series, York TM9Y, Bryant Preferred and Legacy Line are just a few that use constant torque motors for the blower fan. Basically every Manufacturer offers a lineup that consists of different series that use VCA (Variable Speed Constant Airflow), VCT (Variable Speed Constant Torque) and FCT (Fixed Speed Constant Torque).

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skippingstone t1_j0tb3w7 wrote

Is there a way to change the blower settings?

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BxMxK t1_j0tjk9x wrote

Really depends on the unit.

Multi-zone units typically have variable speed motors.

Single zone units usually have single speed motors. Some have blowers mounted directly on the motor shaft. Some are belt driven. With a belt drive you have a little flexibility to swap pulleys to adjust he cfm.

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pLeThOrAx t1_j0tqrog wrote

Do you mean swap the pulleys themselves/change the gear ratio?

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shoeish t1_j0ssi50 wrote

That square footage needed two ducts before and the lack of a wall won't change much.

Leave the second duct and pinch them down with with the damper on the diffusers if needed.

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T1Demon t1_j0t565g wrote

Not necessarily. If they were two separate rooms they would have had two separate vents regardless of square footage or one room would have no heat/ac. The two small bedrooms in my house have 1 vent each, but my bonus room is bigger than the 2 of them combined and does fine with just 1.

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flyingGoatPenis OP t1_j0tyxbz wrote

Yeah it’s two bathrooms. They combine for less than 50 sq feet. My master bedroom has over 100 and one 1 vent

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ithaqua34 t1_j0rp3ca wrote

You could just use a cap of the proper diameter for the flex to seal it (using tape and panduit strap). Best way would be to pull the flex out and seal the hole in the ductwork with a sheet metal patch.

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D4greatness t1_j0tzt8l wrote

What everyone seems to be missing here is it isn’t about how many vents, it’s about how many cfm. 1 vent could be 100cfm another could be more or less. Before you add or remove ductwork you need to know the requirements of your particular house and hvac system . A property installed system has a balance that is critical for functionality and efficiency Find a good hvac contractor that can do this math before you remove it.

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flyingGoatPenis OP t1_j0u0irv wrote

Yeah, most of the vents in my house are basically the same. Same size flex, same flow, everything. We’re not talking a huge house or a huge unit. 1000sq feet and a 1.5 ton unit

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D4greatness t1_j0xkrqw wrote

1.5 ton needs 600 cfm to work properly. Assuming you have 6” flex and 4x10 or so vents that’s roughly 100 cfm per vent. This is very rule of thumbish as those number can vary a bit depending on many factors If you still have at least 6 vents remaining it should be ok but I would recommend leaving it as your still working with the same amount of sq footage. That room might suffer a bit with only one vent. You could also upsize the remaining vent to compensate.

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WKS01 t1_j0v44t4 wrote

Since you say it is a small bathroom originally you would be most likely fine to block the vent off with no real impact. If I had to guess that vent gets maybe 30-50 cfm and is probably less than a 5” duct. Your blower fan will not blow up, your system will adjust itself to the removed duct and the small amount of air will redistribute. Pressure increase would be negligible. While this is not the “proper” way it will work. Best way to block off the run is get as close to the main trunk as you can and use a cap. Second best is to at least get away from the flex duct and on the hard duct again where you can use a cap, I would attempt to avoid just zip tying that off.

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knowitallz t1_j0v8uqz wrote

I would just leave it.

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rutheman4me2 t1_j0sfqz3 wrote

35 and up is now considered high risk pregnancy ! Time is a ticking.

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