Hello and thank you in advance for your help.
I have a two story house. Each floor has its own independent forced air system. The house also has two attics, one on the front and one on the back.
When the outside temperature drops below 50F and while the system is completely off, I can feel a draft of cold air from all the vents located towards the back of the house. The colder the temperature outside, the more air comes in. As soon as outside is 51F, the air stops coming from the vents. This happens on both floors but only on the vents located towards the back. The vents on the front of the house have no problem at all on either floor. Therefore, the problem doesn't seem to be specific to a duct and/or system (1st floor vs 2nd floor), it is more about back vs front of the house. I must say that the vents having the issues either have their duct running through the back attic or are very close to the back attic (which some times makes me wonder if there is a problem with the back attic). Also, something similar happens during the summer with the same vents (if it gets hot enough outside, the same vents start pushing hot air when the system is off).
I had an energy company come and seal with mastic all the register boots around the house to prevent leakage between the boot and the ceiling. This didnt help at all. I then removed a vent and put my hand inside the duct to test where exactly the air was coming from. The cold air came up from the duct itself, not from the boot connection.
My obvious question is: why do I get the air from the vents when the system is off? Is this "dirty" air coming from outside the house, through the attic, then into the duct and finally into my living space? I don't think this cold air is coming from outside of the house and here is why. I can have the heat off all day and on a very cold day (below the 50F threshold) and while I can feel the cold air coming through the vents, I also see that the humidity inside the house will not increase at all even though the humidity outside is 85%. If the cold air were coming from outside, then the humidity would go up, correct? My main suspicion is that the air coming from the vents is air that is being circulated from the returns back into the vents and it feels cold because the duct exposed to the attic is cooling the air before it goes down through the vent. But, if this is the case and it is just air circulation, why does this happen only with certain vents and not with all of the house?
Again, thank you all for your help.
I have a two story house. Each floor has its own independent forced air system. The house also has two attics, one on the front and one on the back.
When the outside temperature drops below 50F and while the system is completely off, I can feel a draft of cold air from all the vents located towards the back of the house. The colder the temperature outside, the more air comes in. As soon as outside is 51F, the air stops coming from the vents. This happens on both floors but only on the vents located towards the back. The vents on the front of the house have no problem at all on either floor. Therefore, the problem doesn't seem to be specific to a duct and/or system (1st floor vs 2nd floor), it is more about back vs front of the house. I must say that the vents having the issues either have their duct running through the back attic or are very close to the back attic (which some times makes me wonder if there is a problem with the back attic). Also, something similar happens during the summer with the same vents (if it gets hot enough outside, the same vents start pushing hot air when the system is off).
I had an energy company come and seal with mastic all the register boots around the house to prevent leakage between the boot and the ceiling. This didnt help at all. I then removed a vent and put my hand inside the duct to test where exactly the air was coming from. The cold air came up from the duct itself, not from the boot connection.
My obvious question is: why do I get the air from the vents when the system is off? Is this "dirty" air coming from outside the house, through the attic, then into the duct and finally into my living space? I don't think this cold air is coming from outside of the house and here is why. I can have the heat off all day and on a very cold day (below the 50F threshold) and while I can feel the cold air coming through the vents, I also see that the humidity inside the house will not increase at all even though the humidity outside is 85%. If the cold air were coming from outside, then the humidity would go up, correct? My main suspicion is that the air coming from the vents is air that is being circulated from the returns back into the vents and it feels cold because the duct exposed to the attic is cooling the air before it goes down through the vent. But, if this is the case and it is just air circulation, why does this happen only with certain vents and not with all of the house?
Again, thank you all for your help.
source https://hvac-talk.com/vbb/showthread.php?2226154-Air-flowing-through-vents-when-system-is-off&goto=newpost
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