Negative pressure in my furnace room

[Ray K.]

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

[Gary R. Lloyd]

Seal the ducts. Pay particular attention to the return ducts.

Unblock the openings.

You're welcome. :)

Gary

HVACR Trouble Shooting Books/Software

[Tim or Marty Shephard]

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.....

[Gary R. Lloyd]

How does restricting return air make a house positive?

Gary

HVACR Trouble Shooting Books/Software

[Gary R. Lloyd]

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

[Tim]

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

[Peter Swinson]

[profft]

It won't, but it will cause the furnace to run hot

[Gary R. Lloyd]

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

Gary

HVACR Trouble Shooting Books/Software

[profft]

It wouldn't but again it would cause a overhot furnace, wouldn't do the blower wheel much good either

[donald girod]

Look. If the furnace is pulling a vacuum in the room in which it is enclosed, obviously the air return for the blower is open to that room somehow (which it obviously should not be). I would not be surprised if there were a substantial opening in the cold air drop. I don't think small leaks in the ductwork would move enough air to produce the described effect (closing doors etc.)

The furnace in a house I own was installed by somebody who forgot to cut open a piece of tin between the main air return duct and a couple of branch return ducts. As a result there was almost no ductwork allocated to return air in the upstairs. When the furnace didn't work, somebody cut a hole in the cold air drop in the basement and put in a register. 90% of the return air for the furnace came thru this register, and it would slam the door at the top of the basement stairs. The people lived with this for 17 years before I bought the house and discovered the problem, and got out my tin snips.

[Tim or Marty Shephard]

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.

[Joseph]

Could it be positive pressure in the house, since the garage door is not a remedy to the pressure problem? If not this is a serious hazard and should be looked at by a service tech. right away...

Joseph

[bill]

Fella I don't know who you are but you don't know what you are talking about. Fenestration is windows.

[Tim or Marty Shephard]

You're right. I wanted to be thinking infiltration / exfiltration.

[Gary R. Lloyd]

The return duct is under negative pressure.There is a leak in the return duct. When you open the garage door, air is sucked into that leak, pulling the garage into negative pressure. Where does that extra air go? Into the house. Does this give you positive pressure in the house? Positively.

Gary

HVACR Trouble Shooting Books/Software

[Joseph]

If the return is open in the attic space or what ever, it could be only the house that is pressurized, no?

Joseph

[bill]

That'll work. : )

[modervador]

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."

%mod%

[Gary R. Lloyd]

Stop right there and you've got it.

Gary

HVACR Trouble Shooting Books/Software