Greetings:
I am planning to install a whole house fan to help cool my in process 44×32 gambrel roof house. The floor plan is VERY open and I have a 25′ cathedral ceiling and upstairs bedrooms/bath. I am planning to install 2 or 3 of the smaller 1650 CFM or so fans as they will fit in my gable ends between my scissor truss webs (assumed to be venting straight outside). The roof trusses are true clear span scissors. I plan to build a “box” in the “attic” and install the fans in the boxes. I plan to vent the fans directly outside through shutters. Would it be better to blow into the “attic” as opposed to outside? I have a 40′ ridge vent and also much soffit venting. I’d think blowing directly outside would be preferred. I also plan to build the “boxes” so the panels can be unscrewed from the inside, permitting access to the “attic”, albeit for someone smaller than me to climb through the scissor truss webbing!
Any comments would be great. Thanks much!
Replies
Here it is the fifth day after your post and no answer!
I've lurked long enough............................here we go.
Have you calculated the volume of your house?
How does that compare with the needed turnover volume and your selection of the number and size of attic fans?
You describe a gambriel roof but then move to scissor trusses..the info is not structurally obvious...please clarify.
How much "attic "space is there.?..actual cross section dimensions of the scissors or gambriel trusses would be helpful.
"Climbing through the webbing" is not a realistic concept unless these trusses have very large spans and/or large differences in top and bottom chord pitches.
Blowing the exhaust into the attic and letting it bleed out through the perforated soffit panals is a reality, ONLY IF you can calculate the free air flow capacity of the soffit and allow for adequate loss of flow due to dust, dirt, and lint accummulation in the perforations.
Lastly, exterior shutters are unsightly pieces of hardware that are seldom weather tight. Heat loss will be apparent unless you provide for additional cold weather seals and insulation. I would also recomend an interior catch pan for blown in precipitation.
All in all you have presented quite a conundrum.........but this is a beginning.
.................Iron Helix
Thanks Iron...Let me explain more. The trusses are clear span 32' gambrel scissor trusses. Therefore there is a total 32' clearspan across the building, no wall at the pitch break which is typical. I ballooned framed so I have a 2' knee wall upstairs that the trusses sit on. The truss pitch is outside: 19/12 and 6/12 and inside 12/12 and 5/12. The trusses measure 4' from the inside peak to the outside ridge peak. From the top plate of the wall to the inside peak is 12' and hence 16' from top plate to ridge peak. They are on 2' centers...2x6 top and bottom chords and 2x4 webbing. Just for kicks: They arrived in 2 pieces and I had to bring them together...3/4 ply on both sides of the truss and 20 3/4 bolts, 5 in each chord. Yes, 20 3/4" bolts. They came out very nicely and I'd highly recommend it for someone that really likes gambrel roof pitches.
5/8" plywood sheathing, 2' overhang made with lookouts on both gable ends. Put in a ridge vent using the (want to say cobra) rolled "fiber mesh" stuff. Used the supplied nails carefully to get the correct install. This ridge vent is 40' long and about 3" wide. In the soffits, I installed 8 16"x8" vents along each soffit. So there's 14 sq ft. in the soffits (that's alot I think) and 10 sq. ft in the ridge vent. OK, so assume there's actually on 1/3 of this as free area (which seems safe I'd think and rather conservative) and I have 24/3=8 sq ft. "free and clear".
Building footprint is 44x32. Downstairs ceiling height is 10' (this building has the potential of turning into a wonderful workshop someday if I build the full scribe log home...) and upstairs is the slanted gambrel roof. It's 14' from the subfloor of second floor to peak. So I assume an average wall height of 15' with a 44x32 building footprint...to obtain about 21,000 CF. So, a 3000 CFM fan would give me an exchange in about 7 minutes.
Rule of thumb I have read is 750 CFM requires 1sq. ft. of clear venting. So with 3000 CFM (one change every 7 minutes) I need 3000/750= 4 sq. ft of venting. So I'd think I have plenty of venting in the attic.
Now, the "attic" is defined by the scissor trusses and does not have anywhere near the volume that a nominal 44x32 foot flat ceiling attic would have. But I also have considerably more venting than the rule of thumb.
Phew...that was long, but hopefully very clear. I do agree, I am not excited about having external louvres for direct outside venting. Initially I thought I'd frame a box around the fan and blow directly outside...but when thinking about the weather leaks as you mentioned and the appearance, I wondered if the more typical venting straight into the attic would work.
Attic will have R-38 fiberglass insulation. I will staple up the vent soffit "protector" sleeves, etc.
I'll have to scan some photos and then try to attach them here...It has been a wonderful experience and I think has come out very well. Nice site 1000' back into the woods.
Thanks for your help and again, sorry for the novel.
Bruce
Them there sound like some serious trusses - I'd really enjoy seeing some pics of them if you have any.
Most of the field spliced trusses I've been involved with over the years have been assembled wrong - Makes me kind of leery of them. Hopefully you followed the instructions you got carefully.....
Thanks for the whole story..................but I'm still pondering your original questions about to vent in or out.
If the rule of thumb is 750 cfm per sq ft of vent then we can expect 12.5 cu ft to pass throught the 1 sq ft every second which translates to an 8 mile an our breeze at each opening and a pressurized attic space.
I really think you will have better performance if you could build a nice gable end vent with a water tray that would compliment your house design and place the automatic shutters inside that cosmetic louver.
Regards....................................Iron Helix