I have a two story home. As such, it's always hotter on the second floor than the first floor.

As advised by many articles and how-to videos out there, I usually close the dampers in the ducts leading to the first floor in the summer, so that the cold air from the air conditioner is forced to the upstairs vents and then sinks down to the lower level, and vice versa in the winter so the warm air rises.

I'm trying to get a straight answer on whether this is ok or bad. I've now seen many other articles that suggest you should always leave all dampers and vents open all the time because you could cause damage to your furnace by creating unexpected blockages.

But most of those articles seem to be focused around the idea of closing off airflow to a room in order to save on energy costs. That is not my goal. I'm not closing off any rooms by shutting doors or anything. I just want the air to start in one spot and naturally move up or down in order to maintain an even temperature throughout the house.

So should I be able to adjust those dampers and registers? Or should I be concerned that it will damage my furnace?

FYI, the house was a new build in 2012, with a high efficiency furnace and AC unit.


Even with the air filter out, it was still shutting off. Had a friend in the HVAC industry take a look. I wasn't there at the time, but according to my wife, he suggested that the furnace is too big for my house and that my duct work is smaller than it should be. So it was having trouble carrying the heated air away from the burners fast enough (which

She said that he turned down the gas valve a bit and increased the air flow (not sure how). Been a couple days and it hasn't shut off yet.

  • P.S. the reason for the question is that my furnace started acting up due to lack of airflow, so I'm thinking I closed too many dampers, but those how-to articles and videos don't really even seem to warn against this at all. So trying to figure out if I should be looking into other issues that may be restricting airflow.
    – Travesty3
    Commented Nov 15, 2018 at 4:32
  • 1
    What is the issue you're having that you think is caused by lack of air flow? Commented Nov 15, 2018 at 4:59
  • @HazardousGlitch The furnace would shut off and start back up about four times, each time giving the diagnostic code "A) Primary/Secondary limit switch open or B) Watchguard mode - limit switch open longer than 3 minutes", and then after about four times, it stays off with the diagnostic code "A) Watchguard - burners failed to ignite or B) lost flame sense 4 times in one hearing cycle." I tried cleaning the limit switches with sandpaper, no effect. The burners are definitely igniting.
    – Travesty3
    Commented Nov 16, 2018 at 3:17
  • As a test, I took out the air filter (which is new and clean, not dirty) and let it run without a filter, and it didn't shut off. I had a friend stop by who is an apprentice in HVAC and he suggested that I shouldn't close any dampers, but I don't fully trust his judgement because he hasn't been doing this for very long yet. We've now opened all the dampers and put the air filter back in and haven't seen any issues, but now of course it's hot upstairs and not as hot downstairs...
    – Travesty3
    Commented Nov 16, 2018 at 3:22

4 Answers 4


Too long for a comment. I agree that closing too many it can cause problems. When installing main trunks in multi story homes I like 60/40 in the main trunks in summer 60% upstairs for cooling and 40% downstairs. In winter 60% down and 40% up. Having these 2 simple marks on 2 dampers makes the seasonal adjustment quick and easy. By adjusting the main trunks each room on the level stays at the correct flow rate because the individual rooms were not messed with. The problem I have seen with closing down too many vents has been on electric furnaces more than gas in my experience but damage to both is caused by blocking the flow.

What happens with low flow, the elements over heat and trip the safety then they cool and the cycle starts over. This shortens the life of the elements and can cause the thermal snap switch to fail making it look like the element is bad. But other things happen with low air flow on gas units. Low air flow can cause the heat exchange / fire box to crack.

I have seen flex lines blown off and trunk lines split open when too many vents were closed. So these are the reason closing them too much is a bad idea.

Depending on how the duct work is routed you usually can reduce the air flow significantly but don't fully close them. This is when back pressure can start breaking things. Reduced airflow can also be caused by dirty air filters putting a larger load on the blower as Lee Sam mentioned.

So to recap, unless your system was designed to fully close zones keep each one open at least 10% and don't reduce the total flow below 90% unless you have a variable speed unit that can compensate. This will reduce wear and tear on the entire system.

  • Making sure I understand: the risk of the elements overheating is only applicable when the furnace is heating, right? So that only applies for, say, the winter. Otherwise, if we're running in cooling mode, then the elements wouldn't overhead with low air flow, correct? Yes, there are other risks, but not that one.
    – Rodrigo
    Commented May 23 at 18:54
  • 1
    Right the only chance of elements over heating is in the winter as long as your system is in the correct mode as it won’t be trying cool in winter if set up properly
    – Ed Beal
    Commented May 25 at 3:27

Yes, if you close too many dampers, you can overly restrict air flow which will burn out your furnace motor. However, what’s more likely is air flow will increase in the ducts/grills that have not been restricted. When this happens it could create a “whistling” noise because the ducts are too small for the amount of air being pushed down the duct.

When we design heating/cooling systems, we try to balance the air flow so more air can go where there is greatest heat loss/gain. Partially closing one damper will cause more air to flow to other ducts/grilles. So, yes, you can adjust your houses temperature in various rooms by changing (opening/closing) dampers.

  • This is what I was expecting as well. I didn't think that closing the downstairs dampers would cause enough blockage to cause any problems, as there shouldn't be much resistance to forcing all of that air to the upstairs registers instead. I think perhaps my issue is that I was closing the dampers all the way, and perhaps I should just close them halfway instead. I'll let it go with the dampers all the way open for a few more days to ensure the problem is resolved this way, and then I'll try closing a few downstairs dampers halfway and see what we get. Thanks for your response!
    – Travesty3
    Commented Nov 16, 2018 at 3:26

The system was designed to be used with all registers and ducts wide open. Any change in that will effect operation. Close too many dampers and you will start tripping out on high limit


A certain amount of balancing by using the dampers may be OK, but what you need in order to keep from damaging your furnace is the instrumentation to measure static pressure in your hot-air heating system.
Most hot-air furnaces I've seen document that any static pressure measurement that is over 0.5 inches is considered high, which means that the system has to work harder to push the air through the ducts. Check your furnace installation manual for acceptable static pressure limits.

Restrictions in the airflow (dirty filter, closed dampers etc.) will increase static pressure if you don't also lower the furnace blower speed to reduce airflow. But reducing airflow too much will cause the Heat Exchanger to get too hot, which can damage the (expensive) heat exchanger, and cause the furnace to shut down. So then you need to reduce the flow of gas to keep from overheating the heat exchanger.

Yes - it is a balancing act that requires special instrumentation to get right. Next time you get your HVAC folks in to do your annual maintenance, have them check the static pressures.
Chances are, you'll find that continually changing the dampers just isn't worth the hassle or the probable shorter life for your blower(s).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.