Negative pressure in my furnace room

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My high-efficiency gas furnace is located in the laundry/furnace room, which has three doors: one to the kitchen, one to the garage, and one to outside. With the blower running, if I open the garage or kitchen doors, I can feel a definite resistance until the door is open more than about 2 inches. If I move the door to within 3/4" inch of closed, the lower pressure in the laundry room will move to toward closed position, even though these doors are heavy, solid doors.
It seems like the return duct is either too small or obstructed. Any other possibilities?
So other details, which may be helpful:
The ducts within the furnace room are sheet metal. Once they pass above the ceiling into the unconditioned attic, they are 1" duct board.
There is also a gas water heater and gas clothes dryer in the laundry room. No problems with either.
Before installing the high-efficiency furnace there were openings near the ceiling and the floor in the wall between the laundry room and the garage to provide combustion air to the old one. The new furnace gets its air directly from the outside through a dedicated pipe. So I blocked the openings to the garage to prevent cool winter air from entering the furnace room. I could unblock them, but this will not solve the problem, because even if I open the door to the garage first, I can still feel resistance when I open the kitchen door (with the garage doors to the outside closed).
Thanks for your comments. If you post more than two days after this date, please also email me; fix my address by removing xxx.
Ray
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wrote:

Seal the ducts. Pay particular attention to the return ducts.
Unblock the openings.
You're welcome. :)
Gary
HVACR Trouble Shooting Books/Software http://www.techmethod.com
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On Fri, 03 Oct 2003 21:17:30 GMT, snipped-for-privacy@gatecom.com (Gary R. Lloyd) wrote:

Correction. The openings should be to the outside, not the garage. These provide combustion air for the water heater and gas drier.
Gary
HVACR Trouble Shooting Books/Software http://www.techmethod.com
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Assuming doors open into the kitchen... Ducts are often laid out a little restricted on the the return side, this leaves the house a little positive, in an attempt to prevent air from entering the home from leaks. Sounds pretty normal to me.....

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On Fri, 3 Oct 2003 17:20:50 -0400, "Tim or Marty Shephard"

How does restricting return air make a house positive?
Gary
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It won't, but it will cause the furnace to run hot
wrote:

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If the air handler is out of the space/envelope, and the return air system is just slightly more restrictive than the supply (typical in my homes, many outlets, few returns) the same air flow will not occur until the pressure in the house rises just enough to overcome the slight extra resistance in the return. Same flows, different pressure drops. Done right, leaves the house slightly positive. Ashrae uses the term fenestration for air leakage that is prevented by this effect.
wrote:

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Fella I don't know who you are but you don't know what you are talking about. Fenestration is windows.
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You're right. I wanted to be thinking infiltration / exfiltration.
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That'll work. : )
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So he's saying running a supply and return at .05" is a "good" thing according to ASHRAE ???
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Correction to fenestration. He said he mean't infiltration and I didn't go back and reread the corrected thread. What he he is saying is yet to be determined I think.
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Then why not connect an outside air inlet to the return duct?
wrote in message >

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Then you don't have the furnace in the utility room, like the OP has.
Whether the air handler is inside or outside the envelope, if the ductwork is sealed properly, then an undersized return is just a restriction in the total circuit. The return operates a bit below atmospheric pressure where it enters the fan box, and the undersized return will have a stronger negative pressure, but the envelope knows only the pressure at the supply and return grates and these pressures will be in balance as long as air does not enter or leave the system through leakage in the ductwork.
Note: positive envelope pressure can be created by adding a proper outside air intake to the return and not relying on leakage. Consider this to be a "controlled leak."
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On Fri, 3 Oct 2003 21:53:53 -0400, "Tim or Marty Shephard"

Stop right there and you've got it.

Gary
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wrote:

A restiction on any side of the fan will reduce *total* flow across the fan. The imbalance in the furnace room is caused by air being taken away without being replaced. It can only be taken away through exaust systems (dryer) or the suction side of the furnace (return air plenum) IMO.
Making a large opening in the return duct within the furnace room will cause a strong negative. The dryer will also rob air from the space.
Kevin
Kevin
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I'm still waiting for you to explain this further. You said "same flows, different pressure drops." This means that there is no net flow into or out of the envelope. If there is no net flow, there is no pressure difference between the inside and outside of the envelope. So the house cannot be "slightly positive." If the house was slightly positive, then there would be a net flow out from the house. Where does the air to make up for that outward flow? You aren't just creating it inside the more restrictive return, are you? No, you must be introducing air into the return from outside the house. By throttling the return upstream of the makeup air inlet, you create a pressure drop (slight vacuum) that helps draw fresh outside air into the return. If there's a leak in the return in the furnace room, it will depressurize the room.
If you simply have a smaller return than supply, you can't get a positive pressure in the house. In the extreme, the return is completly choked off with zero flow, thus there is no flow in the supply either, so no positive pressure in the house. You can't get positive pressure unless you continuously supply makeup air to the system.
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modervador wrote:

insignificant source of leakage.
You can't have positive pressure unless you take in more air than leaks out. Restrictions in you internal system don't enter into the equation.
Boden

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hmm how about the simple things first... Is the Air filter totaly clogged?

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On Sat, 04 Oct 2003 00:02:48 GMT, "Tim"

How would that cause a negative pressure in the furnace room?
Gary
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