# Door leading to basement "moves" when furnace turns on



## harpua728 (Jul 13, 2015)

I have a louvered bifold door leading to my basement (which I may chane to a solid door) and I've noticed that every time the gas furnace turns on, the door moves/shuts. I've been reading up on combustion, venting, air intake, etc and I want to make sure that this is somewhat normal.

The basement is unfinished and the windows have vents that are taped shut (by the previous owner) but are not air tight. The furnace is located underneath my fireplace and is vented into the chimney. However, I'm not sure if that's intake, exhaust, or both. The boiler room is enclosed by some cheap paneling and is not air tight.

I'm thinking about finishing my basement which would change things as I would replace the windows and do a better job insulating the entire area.

What do I need to take into consideration as it applies to proper fresh air for the furnace. Any help would be appreciated as I clearly know little to nothing about this.

If you need more info, or pictures, I'll me glad to provide them.

Thanks for your help


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## raylo32 (Nov 25, 2006)

The pros will chime in soon, but I can tell you that if the appliances draw air from the space they are installed in there are minimum volume requirements and your louvered door is probably required to meet the code by communicating with a larger space. Using a solid door in that situation could likely lead to an unsafe condition.


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## WhatRnsdownhill (Jan 13, 2016)

is it a hot air furnace or boiler with water in it?


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## harpua728 (Jul 13, 2015)

hot air furnace


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## WhatRnsdownhill (Jan 13, 2016)

just make sure they arent using that louvered door as your house return, being that its moving when the furnace turns on...


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## harpua728 (Jul 13, 2015)

Since most of this will be based on formulas I took some measurements.

My utility room has a 120,000 Btuh furnace and a 35,500 Btuh water heater. In a complete different section of my basement, I have a gas dryer (22,000 Btuh).

The furnace and water heater need 7,750 cubic feet of air intake (50x(120+35)). The boiler room is only 840 cubic feet but it's not entirely sealed off from the rest of the basement as there are big gaps high in the walls between that room and the rest of the basement. Plus, I keep that door open constantly. The ENTIRE basement measures only 3,780 cubic feet so even if the furnace is drawing air from the entire basement, it still needs more, and this is probably why I the door that leads to the basement keeps moving when the furnace turns on.

Is this condition considered safe? 

The basement windows have vents that are taped shut (pictured below). The previous owners did this but I never knew what the reason was. Are those vents needed for the furnace to operate properly? Even though they are taped shut they are definitely not fully sealed and air is still getting through.

Furthermore, if I finish the basement, I would replace these windows and do a better job of sealing/insulating the basement. May this cause problems as well?


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## harpua728 (Jul 13, 2015)

ShtRnsdownhill said:


> just make sure they arent using that louvered door as your house return, being that its moving when the furnace turns on...


I'm not sure what you mean by this but there are return vents throughout the house if that helps


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## WhatRnsdownhill (Jan 13, 2016)

ok first we are talking about 2 different things...combustion air is one and return air is 2nd... first the return air...the door could be moving because hot air furnaces push air out the heating registers in all the rooms, and usually in the hallway ceiling or wall there is a big grate where a filter can also go for the air that the big fan in the furnace pulls the air that is pushed out the registers...do you see anything like that big grate in the house?


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## harpua728 (Jul 13, 2015)

There is no big grate that I see in the house. I know some homes have this in the upstairs hallway ceiling but we don't. We just have return air vents spread out throughout the house.


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## beenthere (Oct 11, 2008)

If you finish the basement. You'll need to add combustion air intakes from the outside to your furnace room.


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## SPS-1 (Oct 21, 2008)

You say your basement is 3780 cubic feet --- so about 472 sq ft. How big is your house? I am no pro either but that sounds like a darned big furnace. 
Just the same, surprised just the draw of combustion air is moving a door.


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## harpua728 (Jul 13, 2015)

The house is 1,900 sq ft. The basement may be a little larger than I stated as I didn't factor in a large cedar closet.


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## jimn (Nov 13, 2010)

Furnace dies sound to large for a house that size unless you live in Churchill Manitoba . The furnace will draw combustion air but it should be enough to close a door. I would look for leaks in the return ducts or a blower door on the furnace not close properly


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## TheEplumber (Jul 20, 2010)

harpua728 said:


> I'm not sure what you mean by this but there are return vents throughout the house if that helps


Are you sure these are not supplies?
Where are the returns located?


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## veesubotee (Nov 22, 2008)

Don't know if this applies in your case, but FWI:

I had a great rush of air under my door to the basement when the heat was on. Using a 'tell-tale', I probed all of my basement return ducting and found a (hidden) big leak, which was promptly fixed and cured the problem.

V


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## harpua728 (Jul 13, 2015)

TheEplumber said:


> Are you sure these are not supplies?
> Where are the returns located?



They are on the walls about 6 inches off the floor. Same goes for the supply vents. They are both located on walls just off the floor. Some blow and some suck.


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## harpua728 (Jul 13, 2015)

veesubotee said:


> Don't know if this applies in your case, but FWI:
> 
> I had a great rush of air under my door to the basement when the heat was on. Using a 'tell-tale', I probed all of my basement return ducting and found a (hidden) big leak, which was promptly fixed and cured the problem.
> 
> V



Where was the leak and how did you find it?


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## windowguy (Jan 27, 2009)

beenthere said:


> If you finish the basement. You'll need to add combustion air intakes from the outside to your furnace room.


 Is this an absolute statement? are you saying every single basement that is "finished" needs outside air for combustion? Or did you just say that off the size of his mechanical room? My furnace was replaced and I am I not pulling in combustion air from outside my finished basement. Matter of fact I replaced the louvered door with a solid core door and added a "high vent" and "low vent" from the mechanical room to the main finished basement room with grates between the walls This all passed inspection. (yes I know not all inspectors are the truth). But I'm pretty sure I even read it on the board here a decade ago about the "high vent low vent" thing for air circulation.


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## Bud9051 (Nov 11, 2015)

One of the problems with these simplified guidelines is they don't address the real problem. You can have a huge basement that is sealed up very tight and thus has insufficient combustion air. Just because one approach works in one house doesn't mean it will work in another.

The idea of supplying outside combustion air for all combustion appliances is definitely good as it ensures the replacement air will not be sneaking through the many leaks around the house. Until we switch to sealed combustion appliances or eliminate the combustion inside our homes we should be providing an outside source for that air regardless of codes. In this the codes are definitely a minimum.

Bud


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## harpua728 (Jul 13, 2015)

If outside combustion air is brought into the boiler room, will it prevent the furnace from pulling through other leaks in the house if the house/basement is not tight? Basically, what I'm asking is will the furnace automatically pull all the combustion air from the outside supply line rather than using other sources?


On a side note, since the furnace is currently pulling air from other parts of the house, what is stopping it from pulling air from the water heater exhaust valve since that is technically an open supply line to fresh air (via the chimney)?


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## Bud9051 (Nov 11, 2015)

@harp "what is stopping it from pulling air from the water heater exhaust valve since that is technically an open supply line to fresh air (via the chimney)"
And that is part of the primary concern. If it is hard for the furnace to draw air, the basement pressure can become negative enough to not only draw air from the water heater when it isn't running, but it can also backdraft the flow when it is running. That creates the potential for combustion products being pulled into the living space, not good.

Providing a source for combustion air does not eliminate some of that air coming from the many leaks, but it reduces it. There are dedicated air sources that can be added to some furnaces, but not my specialty so cannot say which ones.

In my business, energy auditing, we have the ability to set the house to worst case, all exhaust appliances running, doors and windows closed or positioned for worst case, and then we measure that negative pressure in the combustion appliance zone (CAZ). We have reference standards that if met should prevent the furnace or water heater from backdrafting. Just assuming a basement of a certain area will provide sufficient air is not the best answer.

Bud


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## SeniorSitizen (Sep 10, 2012)

If we're talking bout combustion air, look up air fuel ratio and you'll see that providing combustion air isn't going to suck the walls in until the house collapses.:surprise:

If talking blower on a central unit that's different matter concerning pressure differentials in the house that cause doors to relocate slightly.


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## beenthere (Oct 11, 2008)

windowguy said:


> Is this an absolute statement? are you saying every single basement that is "finished" needs outside air for combustion? Or did you just say that off the size of his mechanical room? My furnace was replaced and I am I not pulling in combustion air from outside my finished basement. Matter of fact I replaced the louvered door with a solid core door and added a "high vent" and "low vent" from the mechanical room to the main finished basement room with grates between the walls This all passed inspection. (yes I know not all inspectors are the truth). But I'm pretty sure I even read it on the board here a decade ago about the "high vent low vent" thing for air circulation.



Withe the size of his basement, and the size of his furnace and water heater. Either need outdoor air for combustion. Or needs to be taken from the upstairs/first floor. Which would then have a lot more unconditioned air being drawn into it. making it drier, and colder feeling.


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## beenthere (Oct 11, 2008)

harpua728 said:


> If outside combustion air is brought into the boiler room, will it prevent the furnace from pulling through other leaks in the house if the house/basement is not tight? Basically, what I'm asking is will the furnace automatically pull all the combustion air from the outside supply line rather than using other sources?
> 
> If you put a solid door on, and the room is sealed. Yes, it will reduce a lot of your infiltration to the rest o the house.
> 
> On a side note, since the furnace is currently pulling air from other parts of the house, what is stopping it from pulling air from the water heater exhaust valve since that is technically an open supply line to fresh air (via the chimney)?


It could be pulling from the water heater. Hopefully, the chimney has enough draft to prevent that. But we don't know that it isn't back drafting the water heater. Is there rust on top of the water heater.


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## harpua728 (Jul 13, 2015)

beenthere said:


> It could be pulling from the water heater. Hopefully, the chimney has enough draft to prevent that. But we don't know that it isn't back drafting the water heater. Is there rust on top of the water heater.


No, there is no rust on top of the water heater.


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## beenthere (Oct 11, 2008)

harpua728 said:


> No, there is no rust on top of the water heater.


Thats a good sign. Not a guaranty of no back drafting, but still a good sign.


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## harpua728 (Jul 13, 2015)

beenthere said:


> Thats a good sign. Not a guaranty of no back drafting, but still a good sign.



I've actually been able to find a home energy audit that was done on my house a few years ago. Obviously things can change over time if there's any issues with the ducts but the combustion and CO output reading were all fine.


That doesn't answer my primary question(s) of whether or not the basement door closing (I can feel the air being sucked in under the door) is "normal" or what issues I may have if I finish the basement.


Assuming everything is working just fine at the moment, I assume that finishing the basement and insulating the walls and rim joints would diminish some of the fresh air that is being sucked in and that may cause problems.


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## Bud9051 (Nov 11, 2015)

Let's separate the combustion air from the fan circulating air inside the home.
Does the door move when the furnace turns on or when the fan turns on? In normal operation the furnace fires first and then a few seconds later when the plenum is up to temperature the fan comes on. If the door moves with the fan, then your problem is balancing the supply and return air, which can also involve leaks or a disconnected duct somewhere outside the envelope.

Watch for the timing and let us know.

Bud
Note, those combustion and CO readings were probably measuring what goes up the chimney, not the CAZ pressure (combustion appliance zone).


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## harpua728 (Jul 13, 2015)

Bud9051 said:


> Let's separate the combustion air from the fan circulating air inside the home.
> Does the door move when the furnace turns on or when the fan turns on? In normal operation the furnace fires first and then a few seconds later when the plenum is up to temperature the fan comes on. If the door moves with the fan, then your problem is balancing the supply and return air, which can also involve leaks or a disconnected duct somewhere outside the envelope.
> 
> Watch for the timing and let us know.
> ...



The door closes when the fan turns on, which is about 30 seconds after I hear the furnace fire up.


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## yuri (Nov 29, 2008)

If you have AC turn the heat to OFF and turn the fan to ON. That will start the fan only. If the door moves then it is because of the fan not the heat/burner.

As a rule of thumb if you have that many BTU's (around 150,000) you should have a 6" combustion air pipe. That will allow you to seal the furnace room. Need to check that in your gas code book or whatever the local authorites require. May need 7".

Then you need to find if there are any large air leaks in your return ducts which are causing the door to move.

You should ALSO have 2 new CO detectors. One by your bedroom and the other at the top of the stairs from the basement. Any CO leaking will rise up the stairs and get caught/alarm there and the one by your bedroom can save your life if the first one fails.

If you seal the furnace room you CANNOT take any return air from it or it can downdraft the chimney or suck fumes from it. I have seen where they put a return air vent on the opposite side of the furnace room wall but in the return duct. The wall seperates the room. You have to be 3 feet away from the front of the furnace or more depending on your local codes.


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## Bud9051 (Nov 11, 2015)

@harpua "The door closes when the fan turns on, which is about 30 seconds after I hear the furnace fire up."
That sounds like the problem is NOT the combustion air, although, as Yuri said, when you build a furnace room you will still need a combustion air source.

But, the door issue is related to your fan. What is currently in the basement for supply and return vents or openings? Do any of your ducts run through your attic?

Let's also confirm, the air being sucked under the door is from the house to the basement?

And, how many return vents and supply vents are there in the main house. Also, when the fan is running, feel each to see if all are moving a noticeable amount of air, in or out. If any are weak, they are suspect for leaks or being disconnected.

Bud


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## harpua728 (Jul 13, 2015)

Bud9051 said:


> @*harp* ua "The door closes when the fan turns on, which is about 30 seconds after I hear the furnace fire up."
> That sounds like the problem is NOT the combustion air, although, as Yuri said, when you build a furnace room you will still need a combustion air source.
> 
> But, the door issue is related to your fan. What is currently in the basement for supply and return vents or openings?
> ...



Thanks


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## Bud9051 (Nov 11, 2015)

@Bud "Let's also confirm, the air being sucked under the door is from the house to the basement?"

*@ harpua "Can you please clarify what you mean by this?"

*I want to be sure which direction the air is going when you say sucked under the door. I don't know which way your door swings so I can't be sure if it is closing from air coming up the stairs or blowing down the stairs*.

Also, has there been any air sealing done on the ducts? You would see foil tape on all of the seams or a mastic material gooped all over those seams.*

*Bud*
*
*


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## harpua728 (Jul 13, 2015)

Bud9051 said:


> @*Bud* "Let's also confirm, the air being sucked under the door is from the house to the basement?"
> 
> *@ harpua "Can you please clarify what you mean by this?"
> 
> ...



Gotcha. The air is being sucked into the basement. The basement door opens out into the upstairs hallway and is being sucked shut when the fan turns on.


The ducts at the furnace are taped but I wouldn't call it tight. However, I don't feel much of an airflow in either direction where the "loose" tape is.


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## Bud9051 (Nov 11, 2015)

OK, there are a couple of possibilities and a few more questions.
If your duct system is pulling air from outside the house then the house is being pressurized by the supply air and the basement is somewhat outside (all of the leaks). One common source for this is what we call panned ducts, where they use a joist bay or stud cavity instead of running actual metal ducts. Those cavities can have varied paths to the attic or outside. This is usually only on the return side as metal ducts are required on the supply side. Go investigating in the basement to see if you can identify where all return ducts go. Also, check the filter area to see if it is open to the basement. Sometimes they are not well sealed and return air flow from the basement will will result in supply air from upstairs needing to replace it.

As a note, supply air is easier to detect than return air, an incense stick would provide some smoke to help see air flow.

Since all seams and junctions on your ducts should be sealed, while you are investigating you could also do some sealing, I like foil tape less mess. 

Where you noticed any return vents that were weak, check attic and basement to look for unintended openings. If you look through those return grills and don't see a metal duct, then they are using the cavity and electrical wires and plumbing may have left holes to the outside.

Bud


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## yuri (Nov 29, 2008)

You need an equal amount of supply air and return air. If the return for the bedroom is blocked off by a closet then you are starving for return. Then the furnace fans sucks harder and is taking it from some leaks in the return duct downstairs. Sounds like you have a badly unbalanced system. That can be hard on the furnace as it may be starving for air and runnng too hot. Add some more return vents upstairs and deal with the blocked one.


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## harpua728 (Jul 13, 2015)

Bud9051 said:


> OK, there are a couple of possibilities and a few more questions.
> If your duct system is pulling air from outside the house then the house is being pressurized by the supply air and the basement is somewhat outside (all of the leaks). One common source for this is what we call panned ducts, where they use a joist bay or stud cavity instead of running actual metal ducts. Those cavities can have varied paths to the attic or outside. This is usually only on the return side as metal ducts are required on the supply side. Go investigating in the basement to see if you can identify where all return ducts go. Also, check the filter area to see if it is open to the basement. Sometimes they are not well sealed and return air flow from the basement will will result in supply air from upstairs needing to replace it.
> 
> As a note, supply air is easier to detect than return air, an incense stick would provide some smoke to help see air flow.
> ...



Thanks again, Bud. I'll need to look around and get back to you.


I believe all the return ducts are metal but that is based on what I see in the basement. I can't see what happens once it runs up the walls so it could be going through a cavity like you said but I wouldn't be able to confirm that easily. 


What do you mean by the following: " Also, check the filter area to see if it is open to the basement. Sometimes they are not well sealed and return air flow from the basement will will result in supply air from upstairs needing to replace it."


What do you mean is the filter area open to the basement?


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## Bud9051 (Nov 11, 2015)

Your air filter is usually in the return path and the filter has to be removable. If that opening is not sealed, covered with something, then the fan can draw in air it shouldn't. I've seen magnetic sheets covering the opening on some, but my exposure to hvac is limited.

The easy place to see the return duct to see if it is metal is behind the grill. You may be able to see through it or remove the grill and look inside.

Where this is headed is, as yuri said, a balance problem between supply and return. In some cases where there is only one central return the problem started with the duct design. You have said you have several return vents so I'm thinking an installation problem rather than design. although few systems provide sufficient return capacity.

As a quick test, uncover that supply register downstairs and you might see less reaction from the door. You will still want to check as the supply only in the basement should not be needed, but it may demonstrate the pressure issue.

Smoke stick and foil tape for your pick up list . I love making to-do lists for others. 

Bud


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## harpua728 (Jul 13, 2015)

Bud9051 said:


> Your air filter is usually in the return path and the filter has to be removable. If that opening is not sealed, covered with something, then the fan can draw in air it shouldn't. I've seen magnetic sheets covering the opening on some, but my exposure to hvac is limited.
> 
> The easy place to see the return duct to see if it is metal is behind the grill. You may be able to see through it or remove the grill and look inside.
> 
> ...



I have a Trane air filtration system. It recently stopped working (no power for some reason) but nonetheless there is still a filter in there to catch dust/particles. The filters are enclosed within the "box" so I would consider this to be sealed. 


As for the basement supply vent, if I uncover it, wouldn't the system have a greater imbalance as I would be adding a supply vent when I need to add return vents?


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## Bud9051 (Nov 11, 2015)

You need more return capacity upstairs to match the excess supply that is pushing the door closed. Just testing by uncovering the supply downstairs, but adding supply down there will reduce the effects of the excess pressure upstairs. My suggestion to test this way may just be confusing the issue as we really want to determine IF there is an installation problem, disconnected duct or unknown major leak, and get that fixed. If all searching fails to find a problem, then we move to balancing the existing ducts.

Bud


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## windowguy (Jan 27, 2009)

beenthere said:


> Withe the size of his basement, and the size of his furnace and water heater. Either need outdoor air for combustion. Or needs to be taken from the upstairs/first floor. Which would then have a lot more unconditioned air being drawn into it. making it drier, and colder feeling.


 am I reading this right? "...which would then have a lot more unconditioned air being drawn into it making it drier and colder feeling" (like somehow the condition of combustion air matters?, that one I'm skeptical on.. its combustion air)

OR

are you talking about unconditioned air being drawn into the FURNACE or you mean unconditioned air being drawn into the first floor (negative pressure pulling more outside air in to the first floor.. but I'm skeptical on that.. combustion air flow causing negative pressure on first floor?")

please clarify for me, I am not drawing outside air for my furnace in finished basement so I care to really know about this. 

if a furnace is pulling outside air its always going to be unconditioned air.


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## yuri (Nov 29, 2008)

What goes out of the furnace must come back in easily.

Ideally you don't want your house to be in a negative pressure as cold air/dirt can be sucked in and is not efficient. Also can be dangerous if the chimney downdrafts.

You also don't want a positive pressure as it forces your heat and AC out.

Balancing is very tricky but you don't want to alter the original duct setup by having blocked return vents. Supply is not so bad as the fan will build up pressure to overcome blocked or shut off registers. Return are worse and there is less "pressure".


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## yuri (Nov 29, 2008)

Ideally you want the combustion air to go straight to the furnace and that is why we have sealed combustion high efficiency furnaces. That way you are burning cold "free" unheated air and not the air you paid good $$ for when heating your house.

If the furnace room is sealed it can take the combustion air from outside with the proper pipe. However the room can be cold. Ideally if you have a old furnace that is on it's way out and needs replacing then a high efficiency is better unless you live in a warm area like Texas.

If you are burning house air then it is humid air and has to enter the house from around yours doors/windows/cracks etc to be replenished. That air is dry and some people don't want it drying out the house. Combustion air from outside is free and does not deplete your moisture.


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## user_12345a (Nov 23, 2014)

Returns the way they're done with panned off joist bays are very leaky and there's nothing much you can do about it. You can offset it by increasing supply.

If the furnace/water heater are venting fine and not backdrafting, I say leave it alone. 

Basements are also by their very nature are under negative pressure due to the natural stack effect even with no furnace with leaky returns.


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## beenthere (Oct 11, 2008)

harpua728 said:


> I've actually been able to find a home energy audit that was done on my house a few years ago. Obviously things can change over time if there's any issues with the ducts but the combustion and CO output reading were all fine.
> 
> 
> That doesn't answer my primary question(s) of whether or not the basement door closing (I can feel the air being sucked in under the door) is "normal" or what issues I may have if I finish the basement.
> ...


The door closing is a sign that your return air ducts leak in the basement, and that you probably don't have enough return air from the first floor/areas above the basement.

Insulating and drywalling the walls will reduce air infiltration from the outside.


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## beenthere (Oct 11, 2008)

windowguy said:


> am I reading this right? "...which would then have a lot more unconditioned air being drawn into it making it drier and colder feeling" (like somehow the condition of combustion air matters?, that one I'm skeptical on.. its combustion air)
> 
> Combustion air drawn through the house/occupied areas has to effect that part of teh houe and people. Since it is now the air the people are in.
> 
> ...


When you make a sealed mechanical room for the furnace. The combustion air is only drawn into the furnace room. Not into the rest of the house, or finished basement.


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## windowguy (Jan 27, 2009)

okay I think I got it.. the opening at the top of the furnace (where the PVC pipe would go to pull outside air in) which I don't have the pipe.. its just sucking in basement air:

so what that's doing is actually sucking in "conditioned" air.. and its actually even air that my whole house humidifier is producing. its taking that "good" air and drying it out again through the cycle. verryyyyy interesting.. 

so its not just about "do you have enough FLOW to properly operate combustion?" its also about "what condition is the air you are tossing back into the furnace.."


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## SPS-1 (Oct 21, 2008)

Its not exactly drying out your conditioned air. Its taking conditioned air for combustion and exhausting it to outdoors. But for every cubic foot of conditioned air that is used for combustion and exhausted, another cubic foot of unconditioned outdoor air is sucked (through the many gaps and cracks in your house envelope) into your house.


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## veesubotee (Nov 22, 2008)

harpua728 said:


> Where was the leak and how did you find it?


My 'telltale' was a dowel with a one inch strip of facial tissue fixed to the end. With this setup, I was able to probe every inch of my return ductwork. The leak was so severe, that the flag straightened out well before I was at the leak.

The leak was between the ceiling panning and the return trunk (about 1/2" gap x 14" wide). Since there was just a few inches clearance between the trunk and ceiling, I used the same carpet padding as mentioned in 'vibration' thread. 

While I was at it, I found leakage at the bottom of my Aprilaire filter housing. 

V


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## harpua728 (Jul 13, 2015)

The main trunk that comes out of my furnace is insulated and wrapped with old foil tape. When the heat is on, can feel heat blowing straight down from one side of the trunk where the tape has come loose. Will taping this up make a noticeable difference?


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## veesubotee (Nov 22, 2008)

*An FYI on 'leakage'*

With regard to my post regarding return duct leakage--what turned me on to this was a tip I learned in one of the HVAC forums which may be useful as a quick determinant of leakage.

If the dwelling has an (unheated) attached garage with a doorway leading to the garage, use a tell tale (a string works nicely) placed by the door jam and barely crack open the door. If the string pulls in, you have a return leak; outward, in the supply. The degree of movement will be an indication of the degree of leakage.

V


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## beenthere (Oct 11, 2008)

harpua728 said:


> The main trunk that comes out of my furnace is insulated and wrapped with old foil tape. When the heat is on, can feel heat blowing straight down from one side of the trunk where the tape has come loose. Will taping this up make a noticeable difference?


Bet way is to remove the isulation, and seal the duct. And then reinsulate the duct.


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## SeniorSitizen (Sep 10, 2012)

veesubotee said:


> With regard to my post regarding return duct leakage--what turned me on to this was a tip I learned in one of the HVAC forums which may be useful as a quick determinant of leakage.
> 
> If the dwelling has an (unheated) attached garage with a doorway leading to the garage, use a tell tale (a string works nicely) placed by the door jam and barely crack open the door. If the string pulls in, you have a return leak; outward, in the supply. The degree of movement will be an indication of the degree of leakage.
> 
> V


One must take natural flow of temperature differential into consideration using the string method.

With the HVAC unit blower off, slightly open the garage door described with a temp. difference between the two areas of say for example 30°F. At mid levels of the opening, minimal air flow can be detected. 
As we change elevation to lower, the cold air from the garage enters the warmer area and vise versa as the elevation from mid level is raised. As the temperature differential increases the velocity increases and can usually be felt with our bare hand and certainly observed with a smoke stick.

The same happens in an open stairwell with a temperature difference between the 2 levels.


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## veesubotee (Nov 22, 2008)

SeniorSitizen said:


> One must take natural flow of temperature differential into consideration using the string method.
> 
> With the HVAC unit blower off, slightly open the garage door described with a temp. difference between the two areas of say for example 30°F. At mid levels of the opening, minimal air flow can be detected.
> As we change elevation to lower, the cold air from the garage enters the warmer area and vise versa as the elevation from mid level is raised. As the temperature differential increases the velocity increases and can usually be felt with our bare hand and certainly observed with a smoke stick.
> ...


The test was performed with the blower on. I can't remember when (season) I did the test, but the result was visibly striking.

As a member of another forum has said, "heat flows to cold". Well, I can tell you that at most anytime of the year, opening the door gets a rush of cool/cold air coming in.

V


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