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I need to vent a sauna room (~200F) with an inlet duct that will open below the heater and an outlet that will open near the ceiling at the opposite side of the room. The ducts will be small (2"). Drilling through the studs I'd like to make the holes small (.75") to preserve structural integrity. The only air pressure here will be that provided by the temperature of the air itself, from the outside (30-90F), to the inside (~200F).

Are there any downsides to the small bottlenecks through which the air is going to flow? I'd think the air would just accelerate through the bottleneck and other than that everything else would be the same. Do I need to drill bigger holes?

Thanks

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  • I'm no expert on this, but...if the air is flowing just based on temperature gradient, as opposed to a fan, I think the air will flow based on the smallest holes. Which means you have a 3/4" natural flow (which is going to be slow to start with) air source for the sauna. Doesn't sound like enough to me. Related: What kind of heater?
    – manassehkatz-Moving 2 Codidact
    Dec 9, 2018 at 4:16
  • it's a sauna heater lol. it heats rocks to > 200F and the inlet air vent will be next-ish to it.
    – Walrus the Cat
    Dec 9, 2018 at 4:24
  • I mean "what fuel source does the heater use?" If it is electric then I'm not concerned as far as the heater ("only" issue is having enough air for people to breathe). But if it is combustion (e.g., oil or gas) then there is a very real issue of making sure you have enough air for safe & complete combustion and for removal of CO/CO2.
    – manassehkatz-Moving 2 Codidact
    Dec 9, 2018 at 4:33
  • yeah it's electric. can the temperature differentials provide enough pressure for meaningful ventilation?
    – Walrus the Cat
    Dec 9, 2018 at 4:39
  • I don't know. I recommend following John Canon's suggestions.
    – manassehkatz-Moving 2 Codidact
    Dec 9, 2018 at 5:03

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Small-duct systems normally use a fan that provides high-pressure, not high-volume. Air will not move through those small holes without a fan unless there is a pressure differential, or a strong vertical convection caused by heat.
Use firring strips to build out the wall, and to get a better duct size and design. Then add an exhaust fan where the warm air exits the structure.

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  • the pressure differential comes from the temperature differential no? is the 2" duct size enough by itself?
    – Walrus the Cat
    Dec 9, 2018 at 4:26
  • Not without some type of fan. Temperature difference by itself does not cause pressure differential. You would get movement only if the heat source was at the bottom of a vertical duct. The resulting convection would occur if warm air could easily enter at the bottom, and exit easily at the top. Imagine a fireplace and chimney. To get a good flow, the fire must go for a few minutes to warm the column of air and get things moving. And that's with a large heat source and a wide-open vertical duct (chimney.) A 6-inch computer fan (12 volts) will move a lot of air when connected as an extractor.
    – John Canon
    Dec 9, 2018 at 4:52
  • at these dimensions should i use fans on both the inlet and outlet?
    – Walrus the Cat
    Dec 9, 2018 at 5:44
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    A single exhaust fan (quiet, medium airflow) should be fine. The fan will exert a slight negative pressure inside the cavity, so do some caulking to improve the integrity of the duct space. If there is an inlet and outlet in the same room you have a recirculating system. If you exit the air from the room, it is an exhaust system. If you exhaust the room, add a small relief air grille into the door.
    – John Canon
    Dec 9, 2018 at 17:37

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