I’ve read enough entries to know that leaving out the details doesn’t always work, but I’m going to try it, and see how far I get. Without explaining all the whys of my situation, it’s the implementation I need help with:
I’m trying to vent the moisture from our bathroom. I’ve already purchased and installed a Fantech RVF 4 exterior mounted remote centrifugal fan/ventilator. I have installed these before, and this is not the problem. My question is about the intake grill for the fan. (By the way the fan moves 124cfm) Due to space constraints everywhere in this particular instance, I’ve been contemplating using a manifold type intake. What I’ve thought of is a piece of possibly 2″ pvc pipe, about 3-4′ long, with small holes drilled along it. This pipe would then turn through the wall near the ceiling and in a chase change up to the 4″ diameter duct I have to run to the fan. What I don’t know are the ins and outs of the amount of “intake space” I need to make (number of holes and diameter), given what diameter of pvc I pick, to efficiently utilized the capacity of the fan, and at the same time, not make a big noise producing manifold. I’m thinking I will also need to incorporate a type of access or “clean-out” in this manifold for codensation or cleaning, not a problem, I will incorp.
Replies
It sound like it might be noisy. how about two of these manifolds if space is a constraint? Just have the two 2" manifolds go to a 4" wye. That way the fan breathes enough and you lower the noise in the bathroom.
Great thoughts, and thank you. What I'm looking for though is the knowledge you seem to have, and maybe take for granted? That more manifold means less noise, etc. I'm mostly a carpenter, although my work in the past 6-7 years took me a lot closer to mechanical things within homes and buildings, probably just close enough to be dangerous though, you know, knowing enough to get into this stuff, without knowing all the background to properly engineer something. So, if you, or anyone else, could explain a few of these basic dynamics, I'd appreciate it.
Something in my brain says I need to be carefull not too make a manifold too big either, or I will be limiting the fan's effectiveness?
Think of it this way, if you had just the open end of that 4 inch duct at the ceiling the fan would be satisfied and not trying to suck too much air thru too small of an opening. Assuming of course that the fan is not supposed to be serving two 4 inch fan ducts from 2 separate rooms.
Taking a 4 inch duct and making the equivalent volume using 2 separate ducts you'd need 2 pieces of duct of nearly 3 inch diameter each (2.828 inches).
Just the area of the duct opening is Pi * radius * radius.
4" duct == 2 pieces of ???
Pi * r1 * r1 == 2 pieces * Pi * r2 * r2
3.14159 * 2 * 2 == 2 * 3.14159 * r2 * r2
2 == r2 * r2 take sqrt of both sides
1.414 == r2
2.828 == diameter, each, of needed 2 pieces
You can never get enough volume thru a two inch PVC pipe as required by that 4" fan without some serious noise and too much load on the fan itself. And that is with an open ended pipe. If you cap the end and just drill some holes in the sidewall you now have a noisy flute!
If you must stick with 2 inch PVC, use 3 lengths together. To get creative and what might look better, cut thin slots along their lengths but not within a few inches from either end. You'll need enough slots to get enough free open area equivalent to the 4 inch duct requirements. Real fancy would be to cut wider slots where the individual tubes would butt up against eachother thus forming a kinda corrugated plenum.
HTH
Phil
Well, there's noise and then there's pressure drop.
Noise first: 124 cfm in 4" pipe moves at 1420 fpm or 16 mph. Think of an open car window at 16 mph. Not much noise. Pull that same airflow through 1/2 the diameter (1/4 the area) and it will go 64 mph. Your open car window at 64 mph DOES make noise.
Pressure drop also increases tremendously as the velocity increases. So much so that you wouldn't get 124 cfm - the fan can't pressure enough pressure/vacuum to move the same amount of air through the smaller pipe. It will only move 40-50 cfm through a 2" pipe which will leaves the smells and moisture in the bathroom for longer.
As a first-cut design, keep the cross-sectional area constant. If necking down from 4", use two 3" pipes or four 2" pipes. Use smooth transitions - DWV fittings are much better than supply piping fittings.
When taken to an extreme (really small holes) edge effects start to dominate. For instance, 64 1/4" holes have the same cross-section as one 2" pipe. But those holes have 50" of edges whereas the 2" pipe has 6" of edges. So I would use about 150 1/4" holes to "manifold" into a 2" pipe.
Actually, I would drill fewer, bigger holes - maybe thirty or forty 1/2" holes to "manifold" a 2" pipe. Because 150 seems like a lot of holes to drill.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Yes, as Phil says, the first approximation would be to maintain the same cross-sectional area throughout the setup, keeping in mind that pies are square.
Thanks to all of you in regards to this one. I'll look at it again in the morning with a fresh cup of coffee, instead of Friday evening after a long week, with my second beer in hand. All of the different ideas are great. If it helps make the creative juices flow, read on, I'll try to explain more of my situation, and see if anyone has a better idea.
Given:
SMALLLLL house, in order to have ceiling heights appearing more than 6.5', we eliminated the ceiling surface across the bottoms of all joists for second floor.
Bathroom, 6.5'x6.5', only one in house, for two, full bath, clawfoot tub with oval shower curtain ring and shower riser.
Because CF tub is 4-5 inches off floor, joists in bathroom were headed off (they ran perpendicular to long axis of tub) a "well" only this time overhead was created for a space that now contains the shower curtain ring (and my head) while showering.
Problem:
Vent cannot be boxed in, in the 5.5" space between floor joists, since this is now open air space of the room (especially directly over shower). Therefore vent was located remotely, with intent of running a 4" duct into the bathroom sidewall (interior wall/chase).
The best area to remove moist air from is directly above the shower, since this area is pretty much a box 5.5" deep, the length and width of the tub, with a the shower curtain ring about 2" above the bottom of this "box", the best area is the 3.5" remaining in height.
A 4" duct, takes a circular trim grill that is about 6.5" in diameter.
A 4" hole out of the 5.5" sidewall of the "box" doesn't quite line up well with anything.
Therefore:
I've considered rimming the "box" with pvc pipe, drilled with a series of holes, which would pass through the 5.5"dimension sidewall easily, and in the chase area connect up with 4" vent.
Have I lost anyone? Have I fed imaginations? Let me know. This forum is a great place. Thanks a bunch.
Scott
You lost me, but this strikes me as much ado about nothing. In such a tiny bathroom, the air sucked through a 4" duct anywhere in the room should do a fine job. I don't think it would be worth the trouble to try to get it over the shower. Just bring the pipe in as high in the wall as possible and put a grill on the wall. Install a timer on the fan to encourage reugular use.
Al Mollitor, Sharon MA
You could have a rectangular galvanized vent "body" in that 5.5" area that then transitions to a 4" round duct. Or two rectangles that then transition to round 3" and then "Y"`d to 4" duct. Put a nice looking rust-proof grill over it.
Have you considered using the various pieces-parts of sawdust vacuuming systems to create your ducting needs?
IMHO, the PVC pipe route will look kinda low-budget compared to grills, more suited to a bathhouse down at the public beach. Just my opinion.
Yeah, I can't quite follow what you're describing either, but I'd be tempted to use the space above the ceiling to fashion a plenum (using sheet metal or drywall to box off the space between joists). Drill holes in the joists (in the middle, less than 1/3 the depth of the joists) to allow the adjacent joist spaces to "communicate". Then you can put in any number of vents.
Thanks all again, this forum is so great for brainstorming, and in this case getting me out of the rut I'd put myself in re-engineering and thinking about PVC. I had once early on thought about the rectangular, transitioning to round behind the wall, and think I'll go that way,
Thanks again everyone, someday, when I find the right drawing package, I'll be able to get my ideas visually accross, I think that would help.
Scott