Looking at rigid ducting for when I redo our bathroom fan installation, and the new fan housing has a 4" outlet but the roof vent uses a 6" connection. Which will allow me to move more air overall, adapting the duct to 6" immediately upon exciting the fan housing or doing it right at the roof vent? Sound levels are a concern, but not as important as moving as much air as I can.
Looking at rigid ducting for when I redo our bathroom fan installation, and the new fan housing has a 4" outlet but the roof vent uses a 6" connection. Which will allow me to move more air overall, adapting the duct to 6" immediately upon exciting the fan housing or doing it right at the roof vent? Sound levels are a concern, but not as important as moving as much air as I can.
Why rigid ducting? I am going with semi-rigid so I don't have to deal with angles and stuff.
Simple math shows a cross section of 12.57 sq in for a 4" and 28.27 sq in for a 6". I would think the adapter right at the housing would move more air, but it may reduce the effective suction as well.
I have been thinking about this a lot, as we're replacing the 50 cfm 3.5 sonne fan (4") with a 110 cfm 0.3 sonne (6"). My roofer will be over this week to install the 6" roof vent for me.
That's what has me puzzled. Using the same math you did to calculate the cross sections, the 6" duct will allow more than double the air to pass through it assuming the same velocity between it and the 4".
I had considered semi rigid, but I want to wrap it in sound insulation since our bathroom is right next to baby girl's room (no one wants to wake up a 2 y/o that finally went to sleep after two hours of screaming) and I would imagine the rigid would support the added weight better than semi rigid.
I had considered semi rigid, but I want to wrap it in sound insulation since our bathroom is right next to baby girl's room (no one wants to wake up a 2 y/o that finally went to sleep after two hours of screaming) and I would imagine the rigid would support the added weight better than semi rigid.
Perhaps. The semi rigid I bought is attached with large hose clams and it's always possible to put a couple of self tappers thru the duct and roof vent flange. Just sayin.
Run 6" all the way. Newer, larger cfm use 6" ducts. That way you only do it once. Get a whisper fan model if you are worried about noise. Or insulate the wall. If you have a horizontal run, make sure the duct is pitched. And yeah, use rigid if you can. Seal the joints with duct sealant, not duct tape. For where I live, I insulate the last 10'
I prefer rigid ducting just because it is smoother on the inside, but semi-rigid is fine as well. You can buy a 10'x4" sleeve of insulation that can go over the duct as well.
I prefer rigid ducting just because it is smoother on the inside, but semi-rigid is fine as well. You can buy a 10'x4" sleeve of insulation that can go over the duct as well.
That's a good point with the rigid. When you mention a sleeve do you mean one meant for rigid or a flexible one meant for semi rigid?
PA raises a good point about maintaining higher velocity through 4" pipe though. I did forget to mention early on that the fan I'm installing is a 110CFM and I'm estimating about 10' total ducting length from the fan to the roof.
That is funny about the air getting stuck in the pipe. It is not a vehicle exhaust system where you lose low end power.
Believe me, the air will find its way out. As long as it leave the bathroom, do you care what happens to it.
A 6" duct 10' long may have the same flow as a 4" 1' long pipe.
Yeah I don't want that hot moist air blowing right into a pile of attic insulation, which the dumbfxxx that installed the current POS Walmart Special did. Along with exposed electrical wiring and no junction box installed. Oh it was there, just tossed into an adjacent pile of insulation. Moron probably looked at it, said "I don't have time for this electrical safety crap. I gotta get back downstairs to drink a Budweiser and fxxx my sister" chucked it aside and proceeded to enjoy the rest of his day drinking Clydesdale urine and hittin it Lannister style.
Have I mentioned before that I have ZERO tolerance for incompetence, especially when it results in a flagrant disregard for other peoples' safety?
I had a boss years ago who had a favorite expression-he applied it according to the situation, but it was "It's a Flow thing"
I might take exception to PA's comment about the pipe being larger--if the pipe is larger, the flow capacity will increase(note--not the velocity-which is not what you want--but the flow). If you have a 100 cfm blower and try to jam the output through a 1" pipe-your velocity may be high, but you will not be moving much air(a compressible fluid). Change the output pipe to 6" and you will move lots of air(and the velocity will be much lower). As a side note-the power to run the fan will increase in this case also, as it is doing more work.
I might take exception to PA's comment about the pipe being larger--if the pipe is larger, the flow capacity will increase(note--not the velocity-which is not what you want--but the flow). If you have a 100 cfm blower and try to jam the output through a 1" pipe-your velocity may be high, but you will not be moving much air(a compressible fluid). Change the output pipe to 6" and you will move lots of air(and the velocity will be much lower). As a side note-the power to run the fan will increase in this case also, as it is doing more work.
As explained to me by an HVAC engineer at one point, there is a point where a fan will not be able to "push" against the static air in the duct. As you say, the velocity decreases as the duct size increases - and it basically reaches the point where it stalls. You reach this point sooner the larger the duct is in relation to the fan. For a smaller sized fan, it is better to put it on the outer end of the duct and have it "pull" the air than "push" it.
Someone in the industry could explain this much more elegantly than my basic description.
The fan units also have the recommended duct run, sizing and type listed in the installation manual...But who reads those? Some plans will also specify what type of duct to run as do some cities.
I will try to use rigid duct wherever possible, as the mooth interior has dramatically better flow compared to the flexible ducts corrugated interior.
Here in SoCal, we have the Green inspectors that check all the fans for duct sizing, dampers and seals. So your choice may be made for you?
I learned quick that sleeping/quiet baby = happy parents.
She's already entering what we're referring to as the Pazuzu Stage (ie: the terrible two's). Damn it seems like time started moving exponentially faster since she was born.
Doug, as far as I know we don't have the Eco Nazis here. Just seasonal burn bans but those are more from a public safety perspective I think.
PA--you are correct in your assessment--kinda like opening the discharge valve on a centrifugal pump--at some point, the fan will not longer INCREASE air flow no matter how large the exhaust--but it will still move air(this is based on the fan/motor capacity). Yes the fan will move max flow if it has near zero discharge pressure(such as the configuration you suggest-at the end of the pipe)
Now to figure out how to adapt this to Spook's issue-----maybe remount the fan near the exhaust?
