If a duct system is designed to operate with ducts open, then it's counter productive to close them.

If a duct system is designed to open and close various ducts at various times, then it makes little difference as there are variances in air pressure that is 'designed' into the system.

The bigger the duct, the more volume of air you are shutting off and forcing to go else where. So to say it's ok is misleading... there are no two ducts alike... length, diameter, bends, type of material used... hard pipe, flex, duct board etc.

Saying it's ok and won't matter much when it could drive pressure up and blow your ductwork apart... not saying it will, but the risk is there.

Because what you do to your system is dependent on how it was designed to operate.

If your duct system was just slapped in any old way, just grab a duct and put it here, grab another duct and put it there... then why would it matter if you closed off a duct.

The keyword is 'design' or lack thereof....

There is always a reason to 'why' you want to close a duct off. If that reason is that another area is not comfortable, then closing a duct is rarely anything that will do much good. But people many times look at the cost involved to correct it and then decide to just settle for the cheaper option of closing a duct and then living with whatever that result gives them.

Thanks for info. This was regarding friends new build. They like a cooler house and were wondering what to do about their elderly mother who will be living with them, who wants a much warmer room. The ductwork is already in. Observed at the time by her husband. Insulated. Long runs as it is a large home. When you mention 'system'- are there certain types of HVAC? Thx

I'm not an HVAC expert, just a homeowner. But in my experience, every ducted HVAC system needs some amount of experimentation to achieve "balance" to have even temperatures from room to room within the home. To balance, I mean that some louvers need to be more open, while others need to be less open. Which need what is a trial and error process.

As an example, the air opening closest to the blower is going to get more airflow than the one farthest away. Without reducing the louver in that vent, the room will get too much heating and too much cooling, compared to the rest of the house. Depending on the location of the elderly mother's room, playing with the louvers may not be enough, she may need some kind of supplemental electric heater for her room.

You can close vents to prevent air flow to a room, but you can over do it if you close too many. This will help keep the temperature lower. Sometimes duct work has manual dampers which can be closed off to reduce air flow. You have look to see if the duct work has these.

I agree with Austin's answer,,,,,but then something very misleading was thrown into the conversation. It is not necessary nor recommended homeowners open and close vents to balance their system. If a newly installed system cause's one or more rooms to be uncomfortable,the installer should be called back to properly install the system. In your friend's case,the system is probably installed as it should be but now require's unusual alteration. As an act of goodwill,installer might very well come out when things are slow (not now during record winter storms) to adjust the system. Even if your friend must pay a service call,they should have it done by a pro rather than risk unwanted problems. 25% of service calls and replacements in Texas are directly attributed to fire ants and unwise actions by homeowners.

"It is not necessary nor recommended homeowners open and close vents to balance their system."

That's not what my HVAC contractor (who last year put in two completely new systems for me and externally sealed and insulated sheet metal ducts) told me. But let's play this along - what do you suggest as the remedy to deal with the typical situation of too much air/too little air from one room to another?

EJ Fudd,I thought A. Air covered it and went as far as saying so but here it is in different words. Unfortunatly the typical system barly has adequate airflow. Closing a vent often diminishes efficiency and capacity of system. The remedy is to install another duct at a strategic location then furnish homeowner brief instructions on manipulating airflow at subject vent and auxiliary vent. Ideally,customer request's oversize ducts with dampers when system is installed so that if use of rooms later change it's a simple matter of adjusting dampers. With the latter arrangement,technician request's occupant keep a log of temperatures in various rooms then advise tech of where and how much they want change. Tech can set dampers or furnish instructions for opening/closing vents. If the truth be known,the typical tech isn't 100% qualified to deal with the afore mentioned. I have often called on ventilation specialists to design ductwork for new installations. Fact is I have realized there was a problem with existing systems and asked for help.

From the OP: When you mention 'system'- are there certain types of HVAC? Thx

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I believe you are referring to this statement I said : Because what you do to your system is dependent on how it was designed to operate.

You have 'systems' when referring to your HVAC as a whole system -systems within the system. So in this context above I was referring to your duct system as this was the context of this thread. With in the 'system' header of a HVAC system you can have the equipment, the duct system, the zone system: 'If any more than one zone per HVAC system' or multiple units that cover more than one zone or area of the structure, the electrical system, the condensate drain system, the refrigerant system or refrigerant circuits -'line set'.

Each one of these 'systems' is important. Not to mention the design or layout of such systems.

If you have a duct that is 5 feet long sits close to equipment... and you have a duct that is 25 feet long. Which room would you want to be in on a hot day or cold day for that matter?

Answer: It depends on where the thermostat is LOCATED. Another system within 'THE SYSTEM'.

You can balance the system until your eyes spin and fall out of your head of the above situation. You will never overcome this 'DESIGN' limitation... unless you change the design... or outdoor ambient is 65-70 degrees Fahrenheit.

I'm not sure why my comments attracted such off-topic philosophizing but most people are dealing with existing houses with existing ducts, not new ones.

With a given system and without undertaking more work, uneven temperatures can be balanced to some extent by adjusting vent louvers. That's why they're there. It doesn't take pontificating to see the point. Yes, closing off too many vents entirely isn't a good idea but I didn't suggest doing that at all.

When an HVAC system has an air flow problem, isn't it usually because of inadequate return capacity and too small or constricted ducts? I know that flex duct, used because it's cheaper and quicker to install, has more friction to slow air flow and is prone to kink when not installed properly. Many furnace and air handler models have adjustable blower speeds and that can be another approach to address flow concerns - but that should be done by a qualified HVAC tech. Too much noise is a sign the speed is higher than the ducts want to handle.

It's true existing houses with existing ducts presents challenges to one degree or another. If you have a two story structure as an example... the duct work with no access will likely remain until the house is torn down.

Uneven temperature problems related to duct work can not be resolved by balancing a system that is unbalanced from the start. So let me explain this:

The above statement has a whole lot of fine print like: depends on climate, depends on location of zones or areas being conditioned, design of system etc.

So you're not going to hear me say 'you absolutely can not do it' in all circumstances. It varies from one climate to the next and what you decide you can live with or not.

To tell you what I mean as it relates to air conditioning: (heating has it's own problems)

If you have a room on west side, windows on this side make it even more of a challenge. The heat gain this room will have will vary, but will always be the warmer part of the structure.

How do you balance a duct system in which the HVAC equipment is on other side of the house when certain areas of the structure have varying loads?

It's common knowledge that you can have a 10 degree or more difference from one side of house to the other. If it's 75 degrees on one side it could be 85 on other side.

The thermostat in this example is on the side in which it's 75 degrees. The thermostat in this example does not know it is 85 degrees on other side of the house. If you lower the thermostat the 75 degree side becomes too cold and in many cases the warmer side is still too warm.

You can not over come this design flaw with air balancing. The system will always be out of balance.... ironically it only really shows up when you need it the most >>> during temperature extremes of the climate in which you live.

So it's not the problem I make it out to be in 'some' climates nor is it a problem all the time as in climate extremes and ultimately it comes down to what you can or can not live with.

If the sizing and ducting were properly done using recommended load calculations, any HVAC system will struggle when the outside temperature is at or beyond an extreme for its location - whether much hotter than normal or much colder than normal. And parts of the house with more sun exposure (for A/C purposes, as an example) do need more air flow. But a 10 degree difference in parts of a house on a typical day isn't normal, nor to be expected, and suggests there are many problems. Yes, with the HVAC system and maybe with the house itself too. The location of the thermostat isn't causing the temperature difference.

Thx all for your opinions! The story: two different homes involved. #1 home in construction- elderly mother will be living with them. She prefers a much warmer room. #2 home also is multigeneration home. When closing off a vent in a room while hvac guy doing yearly upkeep on hvac he said closing off the vent raised the pressure significantly - stressing the hvac. Because #1home is under construction that owner is worried closing vents will raise the pressure of the system, possibly causing damage to new system eventually.

Yes, anything is possible so if anything this merely underlines the point that no two HVAC systems are alike. What works for one system will not work for another. There are too many variables to suggest you could close off a vent and get it to work without causing some other problem as it relates to pressure within your duct system.

The location of the thermostat isn't causing the temperature difference.

That's right, the location of the thermostat is the area in which the thermostat decides based on temperature whether the HVAC system will cycle on or off. Beyond 10 feet from this thermostat location... you're on your own.

A 10 degree difference from one side of house to other is very normal with single HVAC system design. Especially when HVAC system and thermostat are located on opposite side of the structure.

In some cases this is fixable, in some cases the hurdles are big... not to mention the expense.

The other (not thought of side) of this is people only have one thermostat in the home... they have no idea the 5, 6, 7, 10 degree or more difference even exists. (until it's unbearable.... when do you think this is?)

There is a temperature difference in ALL homes (from one side to the other or from one level to another)... how much the difference it is depends on a long list of variables.

No two homes are alike in this regard.

Thank you for the 'candid' discussion.

In #1 home and #2 home the intention is to create a temperature difference. Most of the discussion here has been to decrease temperature differences that are caused by poor duct work design.

Closing 1, 2 or 3 registers in a house that has 20 supply registers is not going to raise the pressure "significantly" in my opinion. I sometimes close two of the three registers in my master bedroom and the static pressure reading is still within an acceptable range. I guess I should do an expert an close an close registers one at a time to see if how much the static pressure changes with each register closing.

If it is not too late I suggest dampers in the supply line be installed in home #1 so air flow can be regulated to the cooler and warmer parts of the house. The installer should take a static pressure readings with dampers opened and closed. This is something the typical new home HVAC installer will do, so the homeowner should make this request.

If closing a duct or two doesn't increase the static pressure of the system, then what is the sense in closing the duct? What effect will it have? What are you really accomplishing?

You have to think more about the reason why you are doing what you are doing... what is the purpose?

If the duct you are closing is 10 feet, 15 feet, 25 feet long... if you close it at the vent register... the air that is trying to go out this vent doesn't know that you closed the vent.

So what does it do? Each and every time you turn the system on... the duct situation highlighted above: will inflate this duct and the air is forced to back up and turn around. This situation does more than just pressure alone it creates system turbulence.

System turbulence in comparison to your lungs could be like you going thru having the hiccups. You can still breathe, you won't die... but after awhile it's extremely irritating and annoying.

This doesn't mean you can't close a vent, it only means that what it is actually accomplishing isn't what you probably think.

Sometimes, I visit a home and find out that the problem the homeowner is having with their system is because of what they are doing to it. Cue the music 'killing me softly' LOL.

The original question was about closing a register, not a duct. The purpose was to make a room cooler during the heating season.

I have manual dampers I adjust accordingly depending on season. I also have clocks with temperature reading so I know what each room is at. If I notice a room is cold or hot I adjust.

I completely close the dampers to the basement during the cooling season, so it doesnt get uncomfortably cold down there with the AC running. During heating season I open up everything and if a room gets too hot I close the damper halfway.

It isnt hard to figure out, they are accessible and labeled which runs go where.

To answer your question, yes closing one damper increases pressure to the others. Your fan speed setting will supply a certain amount of CFM dependant on pressure. If the pressure and CFM is too high then you will get noticeable duct noises. It can also put extra stress on your blower motor.

It is better to control flow to registers with a damper near its origination point instead of closing at the register.

I completely close the dampers to the basement during the cooling season, so it doesnt get uncomfortably cold down there with the AC running. During heating season I open up everything and if a room gets too hot I close the damper halfway.

Sure, some locations this probably works pretty well. But for my climate a home owner is driven insane just having to switch from heat to cool on the thermostat in what seems like a continual battle for 2-4 months in my climate. (Katy, Texas)

We don't have basements here... the Gulf of Mexico would probably find a way in....

What is a shoulder season?

Climates are not the same nation wide. Say what?

In my location (Katy, Texas) winter is often times short, can last two weeks and then go back to summer like conditions. Can you imagine having to readjust manual dampers every two weeks, two days, 24 hours as the weather changes?

Realize many times I post things as they relate to MY climate. This forum board is a nation wide board so it really pays no dividends to make blanket statements that probably will only work for you.

A shoulder season is the season of my climate in between the dire hot of summer and between the dire cold of winter. (winter here [Katy, Texas] is typically very short, can last a few days, a few hours, only to turn hot again for a short time then repeat this process over and over for 2-4 months.)

You dont have basements, so why are you griping about how I do it? Its a non-issue how I do it and my comfort level is fine. It is not my fault if some homeowners are clueless enough to figure out their heating and cooling situation.

Troll elsewhere. My situation covers at minimum half the nation. Your situation covers like one fourth.

If you knew anything about basements, which you dont, you would understand that people that run AC with basements and dont adjust their registers will have super cold basements. Most houses I have visted or had a short stay at during summer, their basement was like 60 degrees vs what was set upstairs at like 72-74.

Obviously I am posting what works for me... that goes without saying. Thank you captain obvious.

"Realize many times I post things as they relate to MY climate."

Yes, and as they relate to your experience, Austin. Both are obviously behind most of what you say (as it would be for anyone), and I'm sure you understand that that background limits the usefulness of many of your comments. Most people who visit here don't live in houses build on slabs in proximity to the Gulf of Mexico.

Vlth, basements are very rare in the Southwest and in most of California (where I live).

I would still like to find the answer to my original question... I am asking if closing vents can cause some form of stress on the hvac unit itself- not the runs. This was a comment by a respected hvac company in Gainesville. I will be having yearly 'checkup' on my hvac and will also question my hvac person regarding same. Thank you. This is not a thread about arguing who does/doesn't have a basement or how lazy someone is/isn't. This is a question about numbers and how it does/doesn't stress a unit.

You guys stop hiding the ball. You do not need specs on the system to answer yes or no,all systems are created equal,tell Ed and Steve the truth so they can tell their people the truth. My answer is yes.

Whatever makes you happy. I was only raising the concerns of what was attempting to be done.

'If what you are doing, doesn't result in what you want 100% of the time or at least 50% of the time so you don't drive yourself crazy (recalculating - readjusting)... then there is little point to do it.'

50% of the time merely means you will have to change it / adjust it back at some point.

I stand by everything I said in this thread. Knock yourself out, do as you wish.

PS: I was born in Wisconsin. I am well aware of what a basement is. Just as much as I am aware of what a crawl space is... hence the reason I moved to the south so I would never be forced to crawl thru nasty crawl spaces.