Duct air flow control damper lever (C) InspectAPedia.comHVAC Duct Zone Damper FAQs
Diagnostic questions & answers on HVAC zone dampers

  • ZONE DAMPER FAQs - CONTENTS: FAQs questions & answers about HVAC duct zone dampers and how they work on heating or cooling ducts
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Automatic or manual HVAC air duct airflow zone control FAQs:

Questions & answers about diagnosing & repairing both manual and automatic heating and air conditioning ductwork zone dampers & airflow controls used to control airflow through heating or cooling ductwork. These zone damper questions and replies help diagnose & repair problems with both automatic duct dampers and manually operated duct air flow controls as well as the use of individual airflow booster fans in the ductwork.

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Ductwork Zone Dampers & Airflow Control FAQs

A motorized duct zone damper (C) Daniel Friedman

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.
In one zone there is now a hot air seepage. Been told this is normal?
Is it. It's not a lot of hot air but enough to notice it;'s there.
Been told the tape / foam contracts?
Have no idea if this is true as it worked fine before.
Unit had error 68 58 48 then stayed at 49
Also now slight hum in the machine which wasn't there before the wind.
Any help appreciated.


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
do I correct it? The heat has been over 100 degrees for over 30 days here in texas and
the air has run almost continuously. The hottest month is just starting. I have two ivalids in the house and cannot have the air off. Please can you tell me what I should look for or how to fix the problem. I can't afford to have the air conditioner people come out to fix and have them jack me. I would like to know they are telling me the truth. We have never had this problem before. Thanks so much. Appreciate anything you could do to help.


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?


When water, say from a plumbing leak, wets motorized dampers intended to close ductwork during a fire, I'd be worried that electrical components have been damaged, shorted, or corroded. You wouldn't notice a thing, since the fire damper is normally not doing anything. But in the event of a fire it might not work. I'd check with the manufacturer about what they recommend you do; most likely they'll want the equipment inspected and tested.

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
one larger unit with automatic dampers or two smaller units? What's more effieient?



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.
:Besides paying $20 per cover, is there a more cost effective or simple way to seal my a/c ducts for the winter to prevents heat loss?


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:
why does is my damper controlled room not cooling?


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.


Question: when to replace a zone damper system

(Sept 10, 2014) Nancy said:
Should I replace zone damper system when I replace furnace?


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:
In what position should I have the lever on my Weathermaker8000 furnace during the winter?


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:
How to check if damper operates properly?



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:
I just built a new 2200 sq. ft ranch house. I had 6 runs ran downstairs and my furnace is a variable speed and I have the fan on all the time which is recommended. The only heat that goes to the downstairs is when the heat clicks on upstairs so it is always about 4-5 degrees cooler downstairs. My friend has a second zone downstairs and it is cozy warm and makes you want to be downstairs. My hvac guy said that if I wanted a second zone, I would have to put my furnace fan on auto and couldn't use my variable speed motor to have fan on all the time. Is that true? We are going to put a fireplace downstairs and maybe instead of putting a second zone downstairs we would put a really good put out lots of heat fireplace downstairs instead of the second zone. But it takes for ever for the basement to heat up with the fireplace and not to mention you don't always want the fireplace on if you are gong to go down stairs for a short period of time. What is your recommendations?



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.

Question: Hot air blowing out of wrong ductwork

(Mar 13, 2015) Jack G said:
I have a single air handler heating two different parts of the house. The basement and first floor. both zones have independent thermostats. No problems in the beginning when it was installed. Now, out of the blue, when we raise the thermostat heat in the basement, hot air blows into the first floor instead of the basement. The thermostats were set for 64 degrees in the basement and 61 degrees in the first floor. First floor temperature is 68 degrees while the temperature in the basement is 62 degrees. The Handler is Lennox and the boiler is Mclain. Thanks.


I think I'd be looking for a control board failure or cross-connected thermostat wires.

Question: hurricane loud noises in duct work

(July 20, 2015) Bruce Savik said:
My heat pump supplies air to two zones. One of the zones is only a small room with three registers. The heat pump is a new 5 ton Carrier Performance unit with dual stage compressor and variable air handler. The problem is when only the zone for the small room calls for air the air flow sounds like a hurricane. Each zone is controlled by a motorized damper. Any suggestions.


I'd ask the HVAC tech to look for an air leak at a motorized damper (or however else you're controlling the two zones) and also to check the actual air velocity. It may be excessive.


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