OUR PROSPECTIVE CONSULTANT, a mechanical engineer, has observed that our restrooms have exhaust fans yet no clear way to replace the exhausted air. He believes that in order to approve our permit for expansion restrooms, the City might well require a dedicated source for intake air. (HVAC people call it make-up air, presumably because it “makes up” for the lost air that the exhaust fan is removing from the room.)
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| A typical restroom exhaust fan |
This raises the question: What is the best way to supply make-up air to our restrooms?
This question will eventually apply also to the basement restrooms, since they are likewise slated for eventual expansion. So let’s think about a comprehensive solution that will work for them, as well.
Below the jump, I offer a couple of cautions about one option that the architect has mentioned as being fairly normal. Then I outline a custom alternative that takes advantage of our building’s unique configuration [revised 9/10].…
The doorway approach
Our architect says that one typical way to supply make-up air to restrooms is to shorten the doors, making a 2-inch gap at the bottom. Then air is drawn in through the doorway, passing under the door.
The architect quickly pointed out a potential problem with that approach in our case. Namely, we would then need to find a way to get make-up air into both the lobby and the stairwell, in order to supply it to the women’s room and the men’s room, respectively. Yet if a door to the outside is not open (which is not a reliable enough answer to likely satisfy the Building Dept.), providing that intake might be a significant challenge.
I can think of another potential problem: Sounds from inside the women’s restroom might pass all-too-readily through the gap under the door and into the lobby. People who are in the lobby might hear things like the flushing of toilets, chit-chat, and possibly more personal “restroom noises.” I.e., the gap might not be conducive to privacy, not to mention the decorum of the sanctuary’s lobby.
So let me suggest another approach. The whole idea obviously needs refinement by a mechanical engineer, but hopefully the following speculation will give them something to chew on.…
The combination-and-switcheroo approach
I assume that the ideal source of make-up air is a roof vent, where there is minimal interference with noise transmission and other complications. So in what follows, I explore what can be done with our existing roof vents.
Women’s restrooms
There is a women’s room both on the main floor and on the ground floor. Each one has its own exhaust duct that vents to the roof.
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| Exhaust vent for basement women’s room |
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| Exhaust vent for main-floor women’s room |
Yet those vents appear to have excess capacity, which maybe we can use to create a channel for make-up air, as follows.
The basement women’s room has an initial flexible aluminum exhaust duct of 4” diameter (center of first photo) before eventually feeding into a brown, rigid steel vertical exhaust duct that is 7” in diameter (on right in second photo).
That is, this airway’s cross-section is 3 times larger where it passes through a plenum in the wall of the main-floor women’s room than when it exits the basement women’s restroom (0.087 sq ft versus 0.27 sq ft).
Therefore it seems that the 7” pipe in question is oversized for the basement restroom’s exhaust needs. So perhaps it could be tapped for ventilation of the main-floor restroom, as well—maybe by cutting open a hole, installing a register and a one-way damper, and moving the exhaust fan into that pipe. The idea is that one pipe could serve to exhaust both women’s restrooms, one on each floor.
If so, then the existing exhaust duct/vent for the main-floor women’s room would become available to be repurposed as an intake source. In other words, we would reverse the flow in that pipe.
Men’s restrooms
The situation with the two men’s rooms is similar (since they are likewise relatively close to each other), although perhaps not quite as straightforward.
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| Rooftop vents on the north side, view looking east |
The basement men’s room exhaust duct runs through a rectangular plenum that is inside the small storage room of the mezzanine. (The basement’s water heater exhaust duct likewise runs through that plenum.) The main-floor men’s room exhaust duct runs inside close to the wall between the mezzanine and the sanctuary.
Ducts above the mezzanine,
view looking north from the center access opening
Supply for the basement restrooms
A similar approach may be possible for supplying air to the basement restrooms. The same plenum in the wall of the main-floor women’s room also contains a 36”x10” rectangular, rigid aluminum kitchen exhaust duct, which likewise vents to the roof. (In the first photo, it’s behind the round brown pipe.)
Its cross-section at the plenum base is 2.5 sq ft. Yet according to a CaptiveAire mechanical engineer, the minimum necessary cross-section for both intake and exhaust in the kitchen (to meet the code requirement for commercial exhaust hoods) is 1.84 sq ft. This implies that the difference, which is 0.66 sq ft, is potentially available to supply make-up air to the downstairs restrooms—which is more than enough. (About 0.2 sq ft seems to be enough for that purpose.)
In saying this, I am assuming that the existing large duct would be replaced by three smaller side-by-side ducts, each with its own function:
- Kitchen exhaust air
- Kitchen make-up air
- Restroom make-up air
To meet code, the kitchen exhaust needs to be powered by a rooftop fan. Even so, the City might not allow make-up air vents to be located so close to the kitchen exhaust. That’s another potential design problem!
Lastly, how would that supply air get across the building to the downstairs men’s room? There is a fairly easy answer: Via the enormous attic space that is both below the lobby (main floor) and above the kitchen/central corridor/men’s room (basement floor). The air would move freely through that space; no duct pipe would be needed.
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