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
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...
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
HVACR Trouble Shooting Books/Software
The garage is not pressurized at all, that's the point. If the wet room
had a negative pressure in it why wouldn't all three doors act exactly the
same way? The impression I got from the op is that it is strong only on the
door to the house.
Maybe I am missing something here, I guess I'd like to Know if any other
exterior doors exhibit the same pressure effect?
Yes, that's right. I feel resistance when I open the door to the either
the garage or the kitchen. (Both doors swing away from the furnace room.
) Even if I open the garage door first, there is still resistance when I
open the kitchen door. This is surprising, because the two vehicle doors
in the garage are not particularly well sealed to the outside world.
The safety issue someone raise because I blocked the combusion air
openings for the dryer and water heater is a good point that I will fix,
once this low-pressure problem is resolved.
Donald mentioned an opening somewhere in the return duct within the
furnace room. So I looked closer. All the sheet metal is new, (less than
three years, installed with the new furnace). All looks perfect.
However, when I removed the humidistat for the Aprilaire 560, and looked
at the junction between a horizontal duct at the ceiling and the
vertical duct bringing the return air to the furnace, I could see light
at the two rear corners. That's where the leaks are. The band that is
supposed to surround the junction isn't properly installed. It will be
tough to fix, as that junction is only two inches from the wall, and I
can't get closer to it than 24 inches, because of the location of the
water heater on one side and the furnace on the other. But I will figure
Thanks to all for your replies. Never expected so many.
Be certain that ther is actually not a return air grille in the furnace
room. This will cause a negative pressure especially if there is no supply
register to the same space. Another poster suggests to seal the return air
plenum in the furnace room for the same reasons.
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