Greetings
I installed a whole house fan for a customer Saturday. It was extremely windy that day, so when I ran it for the first time I couldn’t tell how effective it was. Well the customer called me tonight to say the fan didn’t seem to do anything. I told him I believed there wasn’t enough venting.
8 year old, 2500 sq ft house, hip roof, new timberline with ridge and soffit vents.
5700 cfm fan needs about 1000 sq in NFA
I figure about 35′ of ridge vent which for cobra ridge would be about 500 sq in NFA. I believe the soffit venting is comprised of several 8″x16″ screens, lets assume its balanced to the ridge vent in size.
They went with the ridge venting for appearance reasons, but aren’t adverse to some other form of additional static venting.
Am I correct in my diagnosis?
Whats the best way to solve this problem?
Thanks
Replies
What are the symptoms that the customer is complaining about? No drop in household temp? No perceived airflow increase?
If the airflow from fan is balanced to the exhaust venting area of the attic, and the house is still hot.... then maybe there is not enough insulation in the attic. Too much radiant heat passing thru the ceiling. Pump in some cellulose.
If you have maxed out on the ridge vent ability, then either reduce the fan cfm (doesnt solve problem) or make a bigger hole for exhaust. Mushroom powered vents on backside of roof (so not visible from street) would help. Or make a cupola on the roof.
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
You have to be carefull that your additional venting does not short circuit any "bottom to top" air flow patterns needed by the ridge vent.
If anything, I would think that additional vents at the bottom, feeding the ridge when the fan isn't running would be best. When the attic is over-pressurized by the fan, it will reverse the flow of course - but it won't matter as it will be flooded with much cooler air.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Thank you for your responses!
The customer said that there is no airflow when the fan is on. In fact he said more air was moving through the windows with the fan off. I checked on install and had him double check to make sure the fan was rotating the correct direction.
I like the idea of more soffit vents, since I really would rather not cut holes in his new roof. I would assume that having more soffit vent than ridge would not upset natural venting when the fan wasn't being used, is that correct?
There is an adequate amount of blow in insulation in the attic, It just occurred to me to check that the soffits aren't covered with it.
Thanks again.
Easy way to check the soffits and ridge venting for obstructions is to turn the fan on. You should feel air blowing out everywhere.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Better info-
The house is more like 3500 sq ft, typical mc mansion with 12' ceilings, lots of cubic ft anyway.
The fan is a 2 speed and rated for 1900 sq ft home, which is strange because a different brand fan at 6000 cfm was rated for 3000 sq ft home.
He said he had the windows open.
The ridge vent (cobra i'd think) was installed with the new roof a few months ago, so it shouldn't be plugged, but the soffit vents are original so I don't know their condition.
Maybe the expectations are wrong - from my understanding, one of the best uses of a house fan is to reduce the inside air temp in the early evening by replacing all the inside air (presumably hotter as the house has built up heat during the day) with cooler night air.
Just getting a breeze from the fan is kinda secondary - and if this fan is only rated for 1/2 the square feet it just might not have enough power to get what they want.
I don't know what size our fan is, but I'd guess maybe 3K CFM. And we never run it at more than half speed. Google and you'll find a lot of fans rated at 1500 or so that claim to be good for 3K sq ft.I doubt that a fan rated at over 5K is undersized.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Well, the poster said the one he bought was rated at 1900, but the house was close to 4000.
Different fans - different specs?
I suspect it's mostly based on what you have to sell. But certainly a 5K CFM fan should be enough to stir up the air, even in a house twice as big. If there really is essentially no air movement then there's something screwed up big time.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Probably - I think there is a big difference in cfm between the big, old metal winged fans and the new efficient ones which are basically big plastic computer fans.
Can't imagine getting a real breeze from those - but they will replace all the air in about 10 minutes and they're real quiet.
Yeah, I just did some surfing and found some fans that are about like ours. They run in the 4-5KCFM range. Ours, like I said, is never operated over half speed, and generally closer to 1/4 speed, and it works pretty well in our 2K sq ft house (though only 8-foot ceilings).I can see the argument that a 5K fan isn't oversized in a 3500 sq ft home, perhaps (though I;d certainly want a variable speed control on it), but I don't see that it's grossly undersized.I'm thinking that the roof vents are badly plugged, and fixing them may even eliminate the need/desire for the fan.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Most such fans will spec out the opening you need to exhaust that fan in terms of square feet of vent space. This ought not be debated--I bet the package materials has that spec.Regards, Scooter"I may be drunk, but you're crazy, and I'll be sober tomorrow." WC Fields, "Its a Gift" 1934
One I read was one sq ft per 750 cfm.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
> He said he had the windows open.Note that with any whole house fan some "tuning" of the windows is required. The windows closest to the fan should only be open a crack, with the ones farther away open more. This equalizes the flow.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Has anyone checked that the roof decking is indeed cut open PROPERLY (or even at all) under the ridge vent?
Quantum materiae materietur marmota monax si marmota monax materiam possit materiari?
that made me remember the homes Ive run accross with metal ridge running the entire length but no holes underneath. boy Ive seen some slop work, hilarious though.
> The fan is a 2 speed and rated for 1900 sq ft home, which is strange
> because a different brand fan at 6000 cfm was rated for 3000 sq ft
> home.Perhaps the manufacturer of the one you bought is more honest than the other guy?> He said he had the windows open.I think the key word here is "windows". If he expects a lot of air coming through each window when more than two or three of them are open, he'll need a much larger fan.> ... but the soffit vents are original so I don't know their condition.Many time homeowners will have insulation added, and many times this is done in such a way that these vents are blocked. Make sure that the vents aren't covered and that there's an air gap above the insulation so that air can flow from these vents.George Patterson, Patterson Handyman Service
I'm going to go back see for myself. I'll check the sheathing to make sure it was cut back, and drag out the big ladder to check the soffit vents (I hate the big ladder) since they are about 30' off the ground.
Then do some window tuning and see what happens.
Thanks again for all your input!!
I am guessing the fan is too small. Tamarack suggests 2 fans for that size house. http://www.wholehousefan.com/sizingchart.html
DanH-
By blowing out everywhere, I assume you mean light fixtures, etc.
I will have to have him check that.
Thanks again
Well, shouldn't blow out the light fixtures if they're properly sealed, but you should feel a good strong draft out of the soffit and ridge vents. If you don't then they're plugged."Cobra", IIRC, covers several different styles of vent. If what we're talking about is the stuff that looks like a plastic dish scrubbie, then it's possible for it to be nailed down too tightly (or, rather, for the shingles to be too tight on top of it), greatly reducing airflow.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I find it hard to believe that the existing venting isn't adequate. Our fan worked fine when all we had was three tin cans and six soffit vents.
Is he opening the windows?
And 5700 cfm sounds like enough to blow the shingles off. Hopefully there's a speed control on the thing -- 2000 cfm should be plenty for that house.
Ridge venting a Hip roof is a waste of time and materials, there will never be enough ridge to be effective. Low profile off ridge vents are the way to go. The GAF website has the chart you need to determine appropriate amounts of venting for the size of the home. If there isnt metal or vinyl soffits then they most likely dont have enough intake anyway.
I used to work for an IT call center. When people would call in with a computer problem we asked one question first the same one all the time. Is the power on?
As I understand the whole house fan it draws air from the under air side into the attic to cool it and draw in fresh air in the process? That being said I would ask one question to the home owner. Is a window open? I imagine that lack of airflow would be causing the problem. similar to a AC blower with a clogged filter. let me know.
by the time I learn what I need for this year I will have already forgotten what I learned last year.