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Old 09-26-2019, 10:43 AM
 
Location: Formerly Pleasanton Ca, now in Marietta Ga
5,779 posts, read 4,297,868 times
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I live in a two story home. Upstairs has a separate ac from downstairs. I only use 2 out of the 4 bedrooms. I want to block the ac vents in the unused bedrooms. Is there any problems with this?
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Old 09-26-2019, 10:53 AM
 
73,586 posts, read 73,436,543 times
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Quote:
Originally Posted by aslowdodge View Post
I live in a two story home. Upstairs has a separate ac from downstairs. I only use 2 out of the 4 bedrooms. I want to block the ac vents in the unused bedrooms. Is there any problems with this?
You may not get enough airflow across the coil and the reduced load may ice up the coil ......when I was an hvac mechanic I used to see quite a few iced up coil service calls when customers closed down the grills in the basement
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Old 09-26-2019, 11:09 AM
 
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It won't save you enough (any?) money on A/C usage to be worth the trouble.
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Old 09-26-2019, 11:12 AM
 
Location: Somewhere in northern Alabama
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It isn't ideal. ACs are sized for the space they cool, and the dehumidifying ability will be disturbed. The reduction in airflow by 1/2 will slow air across the coil, add backpressure, and make the fan work harder. Overall efficiency will drop. The humidity in the closed off rooms could rise to the level where mold is more likely to form. If you are trying to save on cooling costs, you may find that you have just shifted costs and the amount you save is minuscule anyway.

Instead, consider partially closing the vents in those two rooms so there is still some airflow and dehumidifying and the unit is closer to operating under design specs. If your used rooms feel clammy instead of just cool you've cut airflow too much in the unused rooms.

With older ACs, the "on" time determines power use. Removing half the cooling load won't reduce on time by half.
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Old 09-26-2019, 11:31 AM
 
Location: The Triad (NC)
29,199 posts, read 63,457,573 times
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Quote:
Originally Posted by aslowdodge View Post
I live in a two story home. I only use 2 out of the 4 bedrooms.
Sell. Move into a home better suited to your use/needs.
Quote:
Upstairs has a separate ac from downstairs.
I want to block the ac vents in the unused bedrooms.

Is there any problems with this?
Yes. Doesn't save elec bill $$ and can damage the equipment.
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Old 09-26-2019, 01:03 PM
 
Location: Johns Creek, GA
12,846 posts, read 49,745,557 times
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Quote:
Originally Posted by aslowdodge View Post
I live in a two story home. Upstairs has a separate ac from downstairs. I only use 2 out of the 4 bedrooms. I want to block the ac vents in the unused bedrooms. Is there any problems with this?


The question would be "why?"
What are you wanting to achieve?
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Old 09-26-2019, 07:49 PM
 
Location: Formerly Pleasanton Ca, now in Marietta Ga
5,779 posts, read 4,297,868 times
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Thanks everyone. Two of my bedrooms are for storage only. The vents are closed, but they don't seal so while the rooms get warm, I can still feel the effects of the AC. Nothing ever got humid or mouldy.
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Old 09-26-2019, 11:30 PM
 
844 posts, read 251,316 times
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Quote:
Originally Posted by harry chickpea View Post
It isn't ideal. ACs are sized for the space they cool, and the dehumidifying ability will be disturbed. The reduction in airflow by 1/2 will slow air across the coil, add backpressure, and make the fan work harder. Overall efficiency will drop. The humidity in the closed off rooms could rise to the level where mold is more likely to form. If you are trying to save on cooling costs, you may find that you have just shifted costs and the amount you save is minuscule anyway.
Instead, consider partially closing the vents in those two rooms so there is still some airflow and dehumidifying and the unit is closer to operating under design specs. If your used rooms feel clammy instead of just cool you've cut airflow too much in the unused rooms.
With older ACs, the "on" time determines power use. Removing half the cooling load won't reduce on time by half.
Harry, in a simple residential system, wouldn't the fan still move significantly more than half the airflow, even though half the vents (in the unused rooms) were closed off? It seems like you'd just get more turns of air through the rooms that have open vents, the fan is simply going to be producing a slightly lower suction pressure and slightly higher head pressure, isn't it? Plus, if the OP only closes the supply registers and leaves the returns unobstructed, won't the system still pull a (admittedly much smaller than normal) amount of air through them, from under the closed bedroom doors? For example, I have one bedroom in my home that is used for storage, and have the supply vent closed in there. It isn't as cool in the summer or as warm in the winter as the other rooms in the house, but it is still "conditioned", I don't sense a lot of humidity in that room in the summer. I attribute this to the large bottom door gap (the room used to have carpeting, which has been removed), and the open return grille, plus some leakage through the closed supply vent. I've done this for years without issue, but have no way to measure the benefit - I assume that energy savings come from having a smaller delta T across the exterior walls of the home, so there is less heat transfer through them.

My suggestion to the OP would have been to partially close the supply vents, then monitor the two rooms and close the vents further as needed or tolerated if no problems occur - but don't want to contradict people who work in the industry. I've always assumed that if it were harmful to the system, then supply registers would not be adjustable - it's simply a way to control comfort in individual rooms in the absence of zone controls.
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Old Yesterday, 01:13 AM
 
73,586 posts, read 73,436,543 times
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Quote:
Originally Posted by Curly Q. Bobalink View Post
Harry, in a simple residential system, wouldn't the fan still move significantly more than half the airflow, even though half the vents (in the unused rooms) were closed off? It seems like you'd just get more turns of air through the rooms that have open vents, the fan is simply going to be producing a slightly lower suction pressure and slightly higher head pressure, isn't it? Plus, if the OP only closes the supply registers and leaves the returns unobstructed, won't the system still pull a (admittedly much smaller than normal) amount of air through them, from under the closed bedroom doors? For example, I have one bedroom in my home that is used for storage, and have the supply vent closed in there. It isn't as cool in the summer or as warm in the winter as the other rooms in the house, but it is still "conditioned", I don't sense a lot of humidity in that room in the summer. I attribute this to the large bottom door gap (the room used to have carpeting, which has been removed), and the open return grille, plus some leakage through the closed supply vent. I've done this for years without issue, but have no way to measure the benefit - I assume that energy savings come from having a smaller delta T across the exterior walls of the home, so there is less heat transfer through them.

My suggestion to the OP would have been to partially close the supply vents, then monitor the two rooms and close the vents further as needed or tolerated if no problems occur - but don't want to contradict people who work in the industry. I've always assumed that if it were harmful to the system, then supply registers would not be adjustable - it's simply a way to control comfort in individual rooms in the absence of zone controls.
it rarely works where you move enough air through the remaining vents because their size would be to small to compensate ... the registers are adjustable to allow balancing the airflow not turning rooms on and off
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Old Yesterday, 06:57 AM
 
Location: Somewhere in northern Alabama
18,214 posts, read 55,028,955 times
Reputation: 31329
Quote:
Originally Posted by Curly Q. Bobalink View Post
Harry, in a simple residential system, wouldn't the fan still move significantly more than half the airflow, even though half the vents (in the unused rooms) were closed off? It seems like you'd just get more turns of air through the rooms that have open vents, the fan is simply going to be producing a slightly lower suction pressure and slightly higher head pressure, isn't it? Plus, if the OP only closes the supply registers and leaves the returns unobstructed, won't the system still pull a (admittedly much smaller than normal) amount of air through them, from under the closed bedroom doors? For example, I have one bedroom in my home that is used for storage, and have the supply vent closed in there. It isn't as cool in the summer or as warm in the winter as the other rooms in the house, but it is still "conditioned", I don't sense a lot of humidity in that room in the summer. I attribute this to the large bottom door gap (the room used to have carpeting, which has been removed), and the open return grille, plus some leakage through the closed supply vent. I've done this for years without issue, but have no way to measure the benefit - I assume that energy savings come from having a smaller delta T across the exterior walls of the home, so there is less heat transfer through them.

My suggestion to the OP would have been to partially close the supply vents, then monitor the two rooms and close the vents further as needed or tolerated if no problems occur - but don't want to contradict people who work in the industry. I've always assumed that if it were harmful to the system, then supply registers would not be adjustable - it's simply a way to control comfort in individual rooms in the absence of zone controls.
As mj pointed out, the primary reason for adjustable supply vents is balancing... although serious balancing is done with the ductwork itself by proper sizing and the occasional in-duct damper.

There are a bunch of factors involved in sizing and design, and there are always compromises. Home systems are standardized and somewhat cookie cutter. (If you ever get the chance to see the AC/heating in an old movie palace, those are amazing, with tunnels you can walk in, silent anemostats, HUGE filter rooms, and the such.)

Your general thinking is reasonable, but would benefit from considering more of the factors. Any duct is going to have some resistance to airflow. A fan is not a pump. As it gets more heavily loaded, it won't push as much air. You can completely block a plenum from a fan and the fan will still rotate (and heat up) whereas a pump will completely stall. Air at different pressures AND/OR humidity levels or temperatures will tend to naturally diffuse and mix if there is any air movement. AC motors have odd characteristics as they are increasingly loaded. They have a balancing speed based on the design and voltage frequency, unlike DC motors. They really like to operate at the loading they were designed for, and efficiency/performance fall off rapidly outside of that. Those are just a few of the other factors.

To directly respond - will the fan move sig. more than 1/2 airflow? Maybe. It will also run hotter and with less efficiency. Suction and head pressure generally refer to the refrigerant and compressor/scroll, but to answer; pressure in the ducts will be significantly more unless the fan is very inefficient to start with. Intake "suction" will be somewhat reduced.

On the bottom line, how much is saved by closing vents in a room varies, but the savings with AC systems is smaller than most people expect. How to put this... ...
Imagine you own a muscle car and there is a gas shortage. You can save money and gas by taking the spare tire out and leaving it at home, but the engine is still the dominant factor.
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