Negative pressure in my furnace room

[Gary R. Lloyd]

Is the garage pressurized, too?

Gary

HVACR Trouble Shooting Books/Software

[zero]

So he's saying running a supply and return at .05" is a "good" = thing according to ASHRAE ???

zero

[Joseph]

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?

Joseph

[Gary R. Lloyd]

The impression I got from his first paragraph is that the furnace room is negative in relation to both the house and the garage.

Gary

HVACR Trouble Shooting Books/Software

[bill]

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.

[Ray K.]

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 something out.

Thanks to all for your replies. Never expected so many.

Ray

[Oscar_lives]

Then why not connect an outside air inlet to the return duct?

[Kevin]

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.

Kevin

[Kevin]

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

[modervador]

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.

%mod%

[el]

Fenestration is nothing more than the leakage around windows, a not 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