Good idea, not “ideal” in my field but it would work, you can also install a back draft damper on the bath exhaust run. It would help with any smells running back to the bathroom.
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Fans in series fight each-other.
Two exhaust fans in parallel will out-flow the same two exhaust fans in series, provided you give them enough intake area.
Two exhaust fans in parallel will out-flow the same one intake fan in series with one exhaust fan (basically same flow-losses as running two exhaust fans in series), provided you give them enough intake area.
Three exhaust fans in parallel will outflow the same two exhaust fans in series or parallel & an intake fan, provided you give them enough intake area.
Also keep in mind that if you see the tent sucking in, the exhaust fan is telling you that it can flow more, but you need to give it more intake area for it to be able to do so.
Practically speaking, if you need two or three 4" exhaust fans in parallel to provide enough flow for your tent even after you dial them in for their max flow, you probably should have started with the one 8" exhaust fan in the first place, & provide it with enough intake area to reach static pressure (max flow).
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^^^
One other thing I wanted to touch on was the exhaust tubing to the attic. You can minimize losses there by stepping up the size of the tubing as compared to the fan exhaust outlet size. 8" fan connected to an 8"-to-12" vent step-up connector, connected to 12" tube for example.
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Not sure if you can see but this is what my tent looks like now currently 79.5 degrees. With lights on
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Ok, but you’re running three fans now, & still leaving a lot of flow on the table because they are in series & don’t have enough intake area. If you run fans in series, you’re creating flow restrictions, even though you will probably see minimal flow increases. But I can take those same fans & run them in parallel, & the flow-increase will be a multiple of how many fans I’m running. But if I want them to reach their max flow at 100% power, I have to make sure the intake area is enough. If I run fans in parallel but I skip tuning the intake area & it is too small, then that is still a flow restriction & the tent will want to suck in. I would also need to provide enough exhaust area through individual pipes for each fan, or a very large single that I could Y into. Practically speaking,
seems kind of silly to do all of that. Why didn’t I start with a single exhaust fan powerful enough to do the job?
Possibly you could get to the same place you are now by running two exhaust fans in parallel, & providing them with enough intake area to be able to reach their max flow at 100% power. They would need their own exhaust tubes or a much bigger single to “Y” into at that point, so, practically speaking you would probably just want to start with one exhaust fan that is powerful enough, then match or over-size the exhaust tube diameter, & then give the tent enough intake area so the fan can reach max flow at 100% power (tent won’t be pulling a vacuum anymore).
You can also free up some more flow by straightening out those bends in the exhaust.
Also you want to consider fan design. An 8" axial “computer” style fan won’t do it. They stop flowing as soon as they see any negaative pressure. You need a true centrifugal blower or a hybrid centrifugal/axial blower. Difference there is that the hybrids will outflow the centrifugals of the same size at static pressure & to a point as negative pressure increases. Then the true centrifugal starts to beat the hybrid as negative pressure continues to build, because the flow of the true centrifugal will taper off linearly as negative pressure builds, while the hybrid falls off a cliff at a certain point. So if you have a lot of unavoidable restrictions, the true centrifugal might be better in that case, where a more free-flowing setup might do better with a hybrid of the same size.
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Also, just to throw in a little math, the surface area of a 6 inch circle is over twice as big as 2, 4 inch circles. An 8 inch circle is aprox 4 times bigger than a 4 inch circle.
Doesn’t mean as much when talking about fan power, but when talking about ductwork moving air, it matters quite a bit.
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@Bluedream1 wondering how long the duct run to the attic is. Also if you know how hot your lung room usually gets at its worst. Also wondering which exact 4" fans you have, looking at max power draw.
The vent tube is about 5 feet to celing. The 4 inch fans are visison 195 cfm fans. Lung room at its hottest get 83 f. But i been running dehumidifier on smart mode so it get about 80 max. Dont know the power draw but its not much at all.
Ok assuming 5" to the ceiling means it ends there, that is a pretty short run. You should be able to go with something like an 8" hybrid fan, like Vortex S-Line or AC Infinity has I think three different 8" hybrids available. If it was something like a 20’+ run to the end of the tube, then I’d say go full centrifugal like Vortex VTX. Any of those are going to pull around a couple amps at 100%.
Do you have the Vivosun D4? Those don’t pull a lot of power, I think less than a quarter amp at full power. I can say that mine died in less than a year. Motor bearing/bushing failure. Luckily it was not a primary exhaust fan, I just use it as an ‘anti-humidity’ exhaust fan on a separate controller. It’s not very powerful & loses a lot of flow due to the shorty carbon filter I have on it. But it’s enough to drop the RH without affecting the temperature too much. Actually works out well. I replaced the dead fan with the same thing because it was $22. Personally for my primary heat extraction fans I go Vortex because of the build quality & the 10 year warranty. Also, they publish flow vs. pressure maps. ACis aren’t that much cheaper when they only have a 2 year warranty, & they & most of the others don’t publish flow vs. pressure maps. If you shop around you can find deals on the Vortex. But if what you have gets you though for now, then run it. You can always line two or three of them up in parallel (side-by-side) as exhaust fans if you need more flow, just give them enough intake area, & short straight exhaust tube runs for best flow.
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