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Door leading to basement "moves" when furnace turns on

23K views 53 replies 13 participants last post by  veesubotee 
#1 ·
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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#18 ·
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.
 
#40 ·
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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#48 ·
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.
 
#2 ·
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.
 
#6 ·
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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#8 ·
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?
 
#13 ·
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
 
#19 ·
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
 
#20 ·
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)?
 
#24 ·
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.
 
#21 ·
@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
 
#22 ·
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.
 
#28 · (Edited)
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).
 
#30 · (Edited)
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.
 
#31 ·
@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
 
#32 ·
@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?


The basement only has one supply vent and no return vents. That supply vent is shut since the basement is not finished and we are never down there unless we are doing laundry. If we finish the basement we may open up that vent but I am not sure if it will be enough to warm up the area. Furthermore, I assume it would take away some of the air being blown upstairs.


Do any of your ducts run through your attic?


No

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


Can you please clarify what you mean by this?

And, how many return vents and supply vents are there in the main house.


Each bedroom has one supply and one return, although the return in the master bedroom is blocked off by a built in closet. Kitchen has one supply vent and the dining room has one supply and one return. Center hall has one supply and one return and living room has 2 supply and one return. Each of the 2 bathrooms have supply vents as well as exhaust vents that empty into the attic. I have blocked these off the prevent moisture/mold build up in the attic (not the best ventilation up there).


In total, this means that there are 8 supplies and 5 returns if I don't count the bathrooms.


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.


The master bedroom and kitchen supply vents are noticeable stronger than the rest and the guest bedroom vent is noticeable weaker than the rest. The supply vent in the main hall and in the dining room are also a bit weaker.

Bud

Thanks
 
#33 ·
@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


 
#34 ·
@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

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.
 
#35 ·
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
 
#37 ·
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?
 
#36 ·
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.
 
#38 ·
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
 
#39 ·
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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