Guide to Zone Dampers for Heating & Cooling Air Duct Controls

  • ZONE DAMPER CONTROLS - CONTENTS. What is a zone damper and how do they work on heating or cooling ducts? - - Ductwork Zone Dampers & Airflow Controls - Hot Air Heat & Air Conditioning Troubleshooting & Repair Guide
  • POST a QUESTION or READ FAQs about how to find, adjust, & use automatic or manual HVAC air duct control s or zone dampers in heating & A/C ducts in order to balance heated or cooled air flow in buildings.

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Automatic or manual HVAC air duct airflow controls:

Here we explain both manual and automatic heating and air conditioning ductwork zone dampers & airflow controls used to control airflow through heating or cooling ductwork. We describe how to find sometimes hidden manual duct dampers, and which way to set the duct damper lever to increase or decrease airflow through various sections of the ductwork and the building.

We illustrate using air supply register controls to fine tune airflow in individual rooms or areas. We explain in detail automatic duct dampers and how they work. We also describe the use of individual airflow booster fans in the ductwork or in other locations to assure adequate cool or warm air supply in all areas.

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Ductwork Zone Dampers & Airflow Controls - Hot Air Heat Troubleshooting & Repair Guide

Manual Air Conditioning or Heating Duct Zone Controls

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Article Contents

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Manual heating or air conditioning zone dampers are mechanical doors that are manually (by hand) open or closed in order to control the amount of warm or cool air flowing to a particular room or area in a building. You can see two black manual duct damper handles on the white (asbestos-paper-covered) heating ducts in the photo at above left.

By totally or partially closing the damper door, the owner or occupants of the building can balance the flow of conditioned air into various areas.

But first you have to find all of the dampers - sometimes the ductwork or zone dampers are all located close to the furnace or air conditioner, but in some buildings there may be quite remote dampers.

So if air flow is too much or too little in some areas of the building, in addition to checking for crushed or disconnected air ducts, look for a stealth-damper whose location may not have been immediately obvious - this means looking on all accessible sides of all surfaces of the ductwork.

[Only a fool would have put a manual duct damper where you can't reach it, right?]

Because it can be tough for a new owner of an older home to even imagine, much less find where these duct air flow balancing controls are located, we include more photographs of what a manual duct air flow damper control lever might look like.

At below left our red arrow shows the direction of warm air movement out of a supply plenum and into ductwork, and our blue line shows the position of the duct damper - blocking most but not all of the airflow.

Shown in another example at above right, a manual zone damper or heating duct damper or cooling duct damper is particularly valuable in balancing air flow in buildings whose HVAC system includes long duct runs to some areas and short duct runs to other areas. This damper is in the at about 80% open position.

If we take no steps to balance the air flow among building areas, when the blower fan is running, the rooms closest to the blower will receive the most air flow and more distant rooms can be difficult to heat or cool.

How do I Set the Duct Damper Handle - in What Position?

In general, the handle of the duct airflow control lever or damper is parallel to the damper itself, so if the zone airflow control handle is across the duct it is closing off air flow.

That is, the position of the duct damper handle also indicates the position of the actual damping baffle inside the ductwork.

If the duct damper handle is parallel to the direction of the ductwork the damper is (probably) wide open (photo at left).

But if airflow control doesn't seem to work as predicted, some further checking is in order, since it's possible to mount the duct damper handle improperly.

The photo at left shows the duct damper handle turned parallel to the direction in which this old (asbestos-paper covered) heating duct runs, so this airflow damper is in the "open" position.

Using an Air Supply Register as a Heating or Cooling Air Flow Damper or Control

As we show here, an alternative to manual heating or cooling air zone dampers is the use of mechanical louvers that can be opened or closed at individual supply air registers.

But when an air duct serves multiple rooms in a building,

and when we wish that entire area to receive more or less air flow compared with other building areas,

it is simpler and more reliable to use a duct damper or zone damper to control the each heated or cooled area at once.

Automatic Heating or Air Conditioning Zone Damper Controls

What is an automatic duct damper and how do they work?

Automatic heating or air conditioning zone dampers are mechanical doors that open or shut under thermostatic control to provide individual area or heating or cooling zone control in buildings served by a single warm air heater or central air conditioner.

In response to individual room thermostats the damper opens to permit conditioned air to flow into that room or building area when needed.

An automatic duct damper is usually controlled by a room thermostat, which opens or closes a metal baffle inside of a warm air (or cool air) heating (or cooling) duct in order to provide multiple heating zone control in a building.

The duct damper is located in an air duct and blocks the flow of warm or cool air into building rooms. It has nothing to do with venting flue gases up a chimney.

Our photo shows a motorized zone damper or air duct zone control produced by TrolATemp® a producer of automatic heating system controls. This unit is an automatic opposed-blade duct damper.

Here is a commercial automatic air supply register control which contains a booster fan as well.

You can spot an automatic zone damper on duct work by noticing that there is an external motor mounted on the ducts in one or more locations.

When heating or cooling are not needed in that building area or zone, the duct damper remains in the closed position.

Our photo shows a commercial duct damper in a New York City office building. In this case the damper is controlling airflow out of a supply duct into a single ceiling supply plenum and register.

Automatic Fire Damper in Ductwork and how they work

The automatic fire/smoke damper operates (usually) by automatically closing a door or doors within the damper on the detection of a temperature increase or by operation of a separate smoke detector.

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Our photo at at left shows a pair of open fire damper doors in the ceiling of a commercial building in New York City.

In event of a fire these doors are intended to be automatically closed. Similar dampers may be installed right in HVAC ducts where the ducts pass between different building areas.

Details about automatic fire and smoke dampers are

Adding a Heating or Cooling Zone to an Existing Air Conditioning System

Reader Question: I want to use my existing system to add A/C to the basement

I'm wanting to use my already installed central heat and air unit for my house to supply AC to my basement. I will not need to heat the basement, only cool it. There is no thermostat down there and no vents. What is the best way to do this?

Note: AC might be required in the basement even in winter months when I would need to heat living area above the basement. I'm looking for stable temps all year around in the basement. I live in the Deep South so winters aren't very harsh here.

Thanks for any advice/suggestions you may have! - J.B. 6/16/2014

If the unit you have has enough capacity you should be able to add ductwork to send supply air and take return air from the basement area. To be more sophisticated you could add a second cooling zone with its own thermostat.

Watch out. But if you think you will need to be heating some areas while cooling and dehumidifying others, your single central heating/cooling system won't work, as it won't like being asked to be in both heating and cooling mode simultaneously.

If that problem confronts you, that is if you need to sometimes heat one part of your home while cooling and dehumidifying another, I would think about using a separate system, perhaps a heat pump, so that you can separately heat or cool the basement while cooing or heating the upstairs, or vice versa.

see PORTABLE ROOM AIR CONDITIONERS for these alternative approaches.

You need to bring in your local HVAC technician to get some more accurate advice that is informed by your home, requirements, and the equipment you have already installed.

Reader follow-up: why do I need return air for my HVAC System?

Thanks for your prompt response! Sounds like I need a 2 zone board, 2 zone damper, and a thermostat for the basement. The AC unit is 2.5 tons, which is plenty to supply both the living area and basement based on measurements.

I'm a little confused when you mentioned "return air". Why would I need to return air somewhere. Could I not just dump cool air into the basement without running a return? Please explain what you mean by return air?

Reply: examples of bad HVAC design when adding a heating or cooling zone


You could indeed just "dump cool air into the basement" but this is usually unnecessarily expensive and it is also rather likely to be ineffective at dehumidifying that space.

When we are trying to push warm or cool air into a space through supply ducts and registers, we need also to provide ductwork and at least one central return-air register for air from the same space to return back to the air handler that is doing the heating or cooling.

If we don't provide a return air path to the air handler then we have designed a system that either won't cool or heat well because it has trouble pushing

air into the space (think of blowing up a balloon), OR we have designed the most expensive possible cooling or heating system because it is pushing conditioned air just in one direction: into the conditioned space.

A "one-way" air movement design means that the heating or cooling system is forever taking supply air from some other source and trying to bring it to the temperature called-for by the thermostat in the conditioned space.

For an example of a really bad design and just the case that you are describing, imagine if you were heating the upstairs (by some other means of heat) while trying to cool the downstairs (which I must say is an odd situation but one that you describe).

And imagine that your air handler is taking heated air from upstairs, sending it through an air conditioner to cool it down, then blowing it into the basement. The air source is warm - we will never successfully dehumidify the cooled air conditioned space, and we are spending as much money as possible by always taking the heated air from area A and cooling it but sending it to area B.

A more efficient and effective design circulates the air from area B (Basement) repeatedly through the air handler where it is cooled and dehumidified.

And as your reply didn't mention it I must repeat this warning

Watch out. your single central heating/cooling system won't work to simultaneously heat one area while cooling another, as it won't operate in both heating and cooling mode simultaneously.

How to Push Warm or Cool Air to Where You Need More of It

Reader Question: how to add a thermostat control to move heat to other rooms

Do you do a thermostat that i can put in my room when it gets to hot it will turn a fan on to take the heat into other rooms? - K.H.

If you are asking if we can sell you a heating part the answer is no, sorry, but we do not sell anything. provides building and environmental diagnostic and repair information. In order to absolutely assure our readers that we write and report without bias we do not sell any products nor do we have any business or financial relationships that could create such conflicts of interest.

But we can tell you what you might want to install to address your heat distribution improvement question:

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with your heat or heating distribution system - something that might change how you want to "fix" a problem of uneven heat in the building. That said If you are unable to turn down the heat in a room that is too hot (by partly closing a radiator control valve, by installing a thermostatically operated individual radiator valve, or partly closing an air supply register)

You could indeed direct warm air from one room into another that is adjoining by the following procedure:

    Purchase an air register booster fan - this is a fan-operated through-wall ventilating register - this looks like an warm air duct register cover that includes a small electric fan. Mount the fan assembly in an opening cut near the ceiling on the wall between two rooms - one that is too warm and a second that is too cool. These systems often work using low-voltage motors and controls, powered by a separate power transformer and low voltage wiring - easy and less costly to run than a new 120V circuit. In wiring the fan in this wall opening, install a thermostat that will switch the fan on or off as a function of room temperature. Alternatively, some air register booster fans include a remote control that will operate the register booster for you with no additional wiring.

With that set-up, when the overheated room reaches the set temperature on the new wall thermostat, that switch will turn on the electric fan that will begin moving warm air from the too-hot room to the too cool room. When the room temperature in the "too hot" room falls, the new wall thermostat will turn off the air moving fan.

Several companies make these air-moving registers and they come in a variety of sizes and colors such as

Reader Question: Stuck Fresh Air Damper Control

Reader question: 6/30/2014 Michael said: How common is it for a damper to get stuck?

I disconnected it from the motor controller and I noticed the rod that adjusts the damper was not moving. and when I forced it to move the rod to open it just now spins in a 360. but I never heard the dampers open..

it's a Trane H&V system from the early 60's

Michael I have not found statistics on damper failure, and I pose that the failure in "stuck" mode would vary widely by damper type. Most often a motorized damper failure to open or shut is probably a motor or control failure. Forcing a damper to move if the motor is jammed might break the drive shaft or gears, leaving the damper flopping freely but no longer under motor (or manual control arm) control.

At this point I'd take a closer look at the control, motor, and connections by removing and inspecting the assembly. Let me know what you find, or use our CONTACT link to send me some photos and I can comment further.

Reader follow-up:

Hi I sent the email, the motor damper is fine but someone that opens and close the dampers have bound up and so the dampers are closed.

So here is the story I noticed the arm for the damper from the motor control was not locked down on the rod for the damper, so for testing I disconnected the rod and tried to manually turn the damper by hand and I noticed the damper would not move in ether direction.

I tried to get the damper to move by using some pliers by turning it clockwise and all the rod does now is just spin in circles, and I never heard the dampers move at all.

I was trying to get the damper to open because the church at night is 86 F inside and under 67F at night so cooler air inside would be nice.

Reply: stuck fresh air damper vs zone damper: same repair procedure

Inside the ductwork the damper typically combines a flat metallic blade shaped to match the cross section of the duct work with a rod that is fixed to the center of the blade. Your photo shows the end of that operating rod.

Corrosion, a bent damper blade, or loss of connection of the damper blade to the rod can leave the damper stuck and the rod spinning freely.

Since the problem is not that the damper motor is broken but rather that the damper itself doesn't seem to move, the repair is most likely going to require opening the ductwork and repairing or replacing the damper assembly.

Your damper and grille are not a zone damper (which opens or closes hot or cool air to a building area) but rather a motorized damper that controls the admission of outdoor air into a building's HVAC system. This is a VENTILATION, SUPPLY-ONLY system design.

However the damper operating principles are the same except that outdoor or makeup air supply dampers are often controlled not by a thermostat and motor but by a manual cable or lever that allows the HVAC technician to leave the fresh air supply damper in a set position.

Reader follow-up:

hank you Daniel for adding the info to help others as the church does not have A/C right now and only 200A 120/208V service we are having a issue finding if the church is able to have A/C the issue is the amount of people square feet and A/C tons needed.

The size of the area that needs cooling the most is 4,153 sq. ft with a 27' high ceiling and 400 people.

I can provide a photo of the area, it's pretty much split into 3 blower units and I can't find any info on the trane units at all.


you'll see a BTU sizing chart, though it does not go up to 4200 sq ft. - you can extrapolate.

But a more intelligent design for a large space with a high ceiling takes more into consideration:

- the ceiling height (as you know)

- the distribution of locations of both supply air and return air ducting and its sizing

- the requirement for variation in cooling depending on occupancy level (more people during holidays for example)

- CFM cubic feet per minute movement by one or multiple (staged) air handlers

- introduction of makeup air is also an interesting topic. I measured the CO2 level at a New York synagogue during services and found that the CO2 level climbed steadily over the period of religious services (which may have explained people dozing during the Rabbi's sermon). The building had no fresh air introduction system at all.

Typical rules of thumb for fresh air ventilation in buildings require

15 CFM per person OR 0.15 CFM/sq ft

Where mechanical ventilation rather than passive (windows) venting is required, the total ventilation air flow rate requirement is estimated by

CFM - cubic feet per minute of air movement = Room volume (in cubic feet) x (Number of air changes per hour required) / 60

As you've already learned by the stuck damper, motorized dampers need to be inspected & maintained - very often I enter a large building where the building super thinks the heating & cooling are working just fine only to discover by entering spaces where no on has been for some time that the system has been snarled up for years. I find disconnected air ducts cooling the attic instead of the occupied space, jammed dampers, even air handlers that are simply not running.

A thoughtful design will inspect the whole building, discuss its occupancy schedule and level, and will evaluate the insulation, windows in number and type, air leakage, and might even perform a blower door test. The more accurately you understand the building's heat gain in hot weather and heat loss in cold weather the more effective and economical can be the HVAC design.

I infer from our correspondence that there was an automated or manually-set fresh air introduction system - though it may have been only for heating.

You will want to know the details about the equipment already installed: location, capacity, economy, features as well as to consider add-on or retrofit vs other cooling options (rooftop or outdoor split system units might be adequate and less costly)

An "Economizer" uses outdoor air ventilation to cool the indoor space at lower cost than using a refrigeration-operated system and works when outdoor air is at or below 72F.

Continue reading at FIRE & SMOKE DAMPERS, AUTOMATIC r select a topic from the More Reading links shown below.

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