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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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.
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.
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 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.
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.
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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
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. InspectAPedia.com 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:
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
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.
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.
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.
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)
- 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
Where mechanical ventilation rather than passive (windows) venting is required, the total ventilation air flow rate requirement is estimated by
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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Frequently Asked Questions (FAQs)
Question: troubleshooting air leaks in a Brivis 30IN with 3 zones
(July 11, 2011) Toni said:
Just had a Brivis 30IN installed with 3 zones. All was fine until heavy winds went up the flue pipe.
Toni I don't have a quite clear image of what's going on with your system but heavy winds going up a flue pipe (a chimney??) - how did wind get into the flue pipe bottom to go up?
And in any case there should be absolutely no connection beween a flue pipe (that vents heating system combustion exhaust) and air ducts that carry building air.
In a problem installation, if return air for an air handler is being drawn right at the furnace and the furnace is running (heating mode) there is a risk of drawing flue gases and hot air into the ductwork - very dangerous.
If by hot air coming out of a cold air supply duct the system is certainly not working properly in cooling mode - you need an HVAC service tech to do some diagnosis.
Question: can I over-ride automatic duct dampers?
(July 27, 2011) fred said:
i have some room with little to no aire (hot or cool) is ti possible to manually operate the automatic dampers?
Fred you should be able to manually operate a duct damper by disconnecting its power or the entire drive motor shaft, and manually pushing and fixing the damper in an open position. This is not however a step that I recommend before first doing some troubleshooting.
For example if your inadequate air supply in heating and cooling modes is caused by some other duct defect (leaks, disconnected sections, dirty air filter, dirty blower) you'd be better off finding and fixing that problem.
(July 29, 2011) sherry said:
I am having a sound like a fan blade hitting something. The sound is coming out of the return air register. It only happens when the air cuts off after reaching the correct temperature. It's not a lot of noise just a slight scraping sound. What can this be and how
Sherry, if you are hearing a mechanical sound like a fan blade hitting something I'd inspect the blower assembly fan - sound might be transmitted into ductwork in either or both directions. It's possible that a bad fan bearing, pulley, or belt (not all systems use a drive belt) can wobble or make noise only at start-up or shut down and not at full run speed.
If the sound is not regular, I tend not to suspect rotating parts as a source and might look for something else loose in the duct system.
Question: Individual zone dampers are not coordinating or responding properly
(Aug 8, 2011) Julia said:
I have 3 zone damper ducts. The dampers appear to be responding to the thermostats, each one closes, opens upon change in temp setting. However, they don't seem to be "coordinating". i.e., if upstairs thermostat kicks in, the all the other zones continue to receive air as well. Consequently, in order to keep my upstairs cool downstairs is icy cold. I've had the repairman out, he keeps telling me I need to have the whole furnace rewired, however, I don't see how this is necessary when the dampers are responding to the thermostats individually.
Julia SOP would be that any individual duct zone damper opens and closes individually in response to just its thermostat, and any one or more of these same thermostats can call for heating or cooling (if both are provided).
If someone did not wire up your system correctly (or perhaps if there is a control board in the system that we don't know about) you'd have the symptom you describe. You need a service call to examine and fix the control wiring.
Normally individual zone dampers are each wired to an individual room or area thermostat. So I would not expect them to all open or close at the same time; rather the dampers open or close in response to the thermostat setting.
When the technician says "re-wiring" is needed, perhaps she or he means that your thermostats and zone dampers and heating and cooling equipment controls were not properly wired in the first place. But a "complete re-wire" is a bit much work - first let's determine where the problem lies: it could be one or two simple connections.
Question: humming zone damper problem
(Jan 12, 2012) Dave said:
I have an automatic air conditioning damper in my ceiling that 'hums' or vibrates when that zone is switched on. I suspect that there is a harmonic vibration in the damper or the damper control from the airflow. Is this a common thing? Should I be able to fix it by removing/inspecting and/or cleaning the damper?
Dave, a humming zone damper first makes me think that a low voltage transformer is bad; possible other problems could be a failed zone damper motor, control, or a jammed damper mechanism. See LOW VOLTAGE TRANSFORMER TEST
Question: effects of water on fire dampers in HVAC duct systems
(June 5, 2011) hafiz said:
what happens when water gets in contact with fire dampers?
If you were asking what happens if there is a fire and water is used to extinguish it, at that point the fire dampers should have already closed the HVAC ducts to help resist fire spread through the duct system. In that case soaking HVAC controls is moot: the ducts are already closed and the fire damage repair will have to include repair and replacement of HVAC components as needed.
Question: how to add dampers on old AC system
(Aug 8, 2012) NW said:
How do I install dampers on an old AC system in a building with no visible controls on the large or small ducts and where the upstairs receives constant flow as a result and the downstairs remains static?
NW an easy approach if there is no access to ductwork is to install supply registers that include an operable control to close the supply openings in each room.
Where ducts are accessible it's usually trivial to cut the ductwork to install a manual duct damper.
If you want to go to automatic duct damper or zone damper systems you'll need to locate an accessible portion of ductwork at the start of individual heating or cooling zones, then add an automatic zone damper, wiring it to a room-mounted thermostat in each zone.
Question: Should I use one large HVAC system with zone dampers or individual HVAC systems?
(Sept 15, 2012) Anthony said:
I'm going to be installing forced hot air and central ac in my home. I want to have a two zone system. Should I use
There are both installation cost and operating costs to consider.
I'm not smart enough to give a sure answer to your question as the building and ductwork layout and requirements of your home are not given. It is certainly less expensive to install one heater and zone dampers than to buy two heating systems. On the other hand, building layout and usage and size might argue for two smaller heating systems.
In hot water (not hot air) heating systems there is an argument for installing a cascade of smaller boilers to heat large buildings so that we don't run a big (less efficient) boiler unnecessarily. But I'm not sure how that theory translates to air handlers.
But often for efficiency and also effectiveness (in pushing warm or cool air) in larger and multi-story homes we see two heating/cooling systems installed, one in an attic for upper floor space and one in basement for lower building floors. You pay more for the equipment and earn that back over time in operating cost savings.
Question: how to write reports
(Nov 29, 2012) MOHAN said:
I have problem how to write the reports Is there anyway to catch it ASAP
Mohan, for help with learning to report defects in air condtioning systems see DEFECTS LIST - AIR CONDITIONING -
or for heating systems see
Question: how to seal ceiling air ducts for winter
(Dec 21, 2012) cold said:
Besides paying $20 per cover, is there a more cost effective or simple way to seal my ceiling a/c ducts for the winter to prevents heat loss?
I should add that they are circular, 14", and have twist knob coming down to shut damper.
Cold, first try just closing the registers. But if they are leaky, there are simple magnetic covers that you can apply, or for a cheap, ugly solution, use foil or plastic wrap.
Question: damper door closing when the Lennox pulse furnace stops
(Jan 27, 2013) Lee said:
I hear what sounds like a damper door close after my Lennox pulse has run it's cycle. Is this normal?
Lee if your system uses duct dampers for zone control it would be normal to hear the damper close at the end of a heating or cooling cycle.
Question: green lights on the zone controls - dampers open?
(Feb 1, 2014) Anonymous said:
When both thermostats are off, are all three zone lights supposed to be green (dampers open)?
When you are not calling for heat, I'd expect heating zones to be closed.
But if you can tell me brand and model number we can look up your particular devices and comment further.
Question: warm air blowing out of cooling vents
(June 20, 2014) 3 zone airconditioning said:
We have a fairly new home with 3 zone air conditioning for the 3 levels being the upstairs with 3 rooms, the main floor, and the finished basement. Our son recently moved back in the warm weather and finds it very warm upstairs where his room is. Our bedroom on the main floor is not as warm, and comfortable with the windows opened. When the air is only on upstairs, the air conditioning comes through the vents on the main floor even though our zone is off. Is this normal, or was it installed improperly?
Air blowing out of a cooling zone that is turned off sounds like a problem worth investigating and correcting. Certainlu im a zoned air conditioning system if you are sure that thermostats are not calling for cooling in two of the zones but that the 3rd zone is calling for cooling, then cool air should come only out of the active zone's duct system. The problem may be improper duct routing, improper damper location, or something else we've not considered.
Question: why is there no cooling in my damper-controlled room?
(Aug 16, 2014) Anonymous said:
If you've no air flow then the damper is not opening, OR there is another failure such as the blower fan not running or ducts that have become disconnected.
Also see AIR FLOW IMPROVEMENT, HVAC
Question: when to replace a zone damper system
(Sept 10, 2014) Nancy said:
Nancy that's not necessary unless the zone damper is not working properly.
But it would make sense to inspect the state of the whole duct system when putting in a new furnace, less the investment be wasted.
Question: what is the proper lever position on a Weathermaker Furnace
(Nov 15, 2014) Anonymous said:
I'm not sure what lever you are asking about. But if you want heat you need the zone damper or duct damper in the heating duct to be in an open position; just how far open might vary if manual zone dampers are being used to balance air supply among different building areas.
Question: how to check duct damper operation
(Dec 26, 2014) Maurice said:
Heating zone dampers, discussed in the article above are manual or automatic. In either case one needs to observe that the damper blade moves to either open or close the ductwork in response to its operation.
There can be a hidden failure: if the damper blade comes loose from the rod that operates it then the operating mechanism may continue to move but the damper does not - and might be left in any position between open and closed. One might notice unexpected presence or absence of air flow out of the appropriate supply registers, or ultimately one might need to inspect the duct interior by partial disassembly.
Question: using a variable speed fan to blow heating air to a lower floor
10 Feb 2015 Dean Becker said:
Indeed it's a bit difficult to push warm air down into a cold space (warm air we know wants to rise) - which would mean you may need to run the fan on its higher speed.
I don't understand your HVAC guy's advice (but then he may be very smart but not a good communicator), unless he is saying that the variable speed fan, when in "ON" Mode, is running at a low speed and that on AUTO it will run at a higher speed. In any event it ought to be possible to set the fan to run at its highest speed on a call for heat - which makes me think perhaps the controls are not wired or set properly.
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