Ventilation questions

Hey everyone, anyone have some answers to these ventilation questions?? 1. If you use a system with a passive intake, what can you do to prevent rodents and cold air from coming through? On the exhaust, I know you can buy something with flaps... is there something you can use on the intake? 2. I'm confused about the size of the duct work.... The fan I'm looking at is 8", but I'd much prefer to put only 4" holes in the side of my house. Large duct work and registers help keep the noise down, but does it matter if the point where it exits or enters the outside is only 4"? 3. If i'm using a fan to exhaust air, and no fan on the intake duct, would I make these ducts the same size? Or should the intake duct be larger? Thanks!

Tom, you asked:

1. If you use a system with a passive intake, what can you do to prevent rodents and cold air from coming through? On the exhaust, I know you can buy something with flaps... is there something you can use on the intake?

Any fresh air duct open to outside air can have a rodent/insect scrren install at the outside face - this screen should be 1/4" min. As far as cold air goes - when the system is not in use - the barometric damper reuiered to maintain proper pressure within the space will be adequate to stop the flow of cold air.

2. I'm confused about the size of the duct work.... The fan I'm looking at is 8", but I'd much prefer to put only 4" holes in the side of my house. Large duct work and registers help keep the noise down, but does it matter if the point where it exits or enters the outside is only 4"?

Actually - that would depend on the capacity of the fan as far as static pressure goes. SMACNA (SHEET METAL AND AIR CONDITIONING CONTRACTORS' NATIONAL ASSOCIATION) only allows for a reduction of 25% of the cross-sectional area of the duct for short distances. You're talking about reducting the duct size by 75% - which will create huge static pressure increases in the duct - which will tranlate to the totasl load on the fan. If the fan is capable of handing those loads you would probably be fine. But remember - the minute you increase the static pressure - you decrease the fan capacity while increasing fan noise - look at the fan specs: BROAN FAN – MODEL L100 CFM/SONES – AT STATIC PRESSURES (Ps-inches of H20) …………...................……..(0.0”)(.10”)(.125”)(.250”)(.375”)(0.5”)(.625”)(.75”)(.875”)(1.0”)NOM…AMPS…WATTS ……………...................…….Ps......PS……Ps….....Ps…...…Ps…......Ps……Ps…..Ps…..Ps…..PS…...RPM…@60hZ Model # CFM Hor….…….136.....115..109…...93….....80…....65….…44…..12………….......…...640……1.1……….87 .. . .. SONES Hor…......….0.5…..0.8….0.9…...1.3…....1.8…...2.3……3.0….3.2 Pay attention to just how drastically the fan loses flow as the static pressue rises. [quote]3. If i'm using a fan to exhaust air, and no fan on the intake duct, would I make these ducts the same size? Or should the intake duct be larger?[/quote I would make them the same size. Sincerely, Rod]

Thanks Rod! Is this what you mean by barometric damper?? I can get this locally at a home depot and I think it comes as 6" or 8" http://www.homedepot.com/webapp/wcs/sto ... =100396420 It looks like it just opens and closes depending on whether it's on or off (so I guess I'd wire it into the same switch that turns the fan on). Does it have to be an electric damper? Do I need a Barometric damper on both ducts? Also, this is the fan I found locally in 6 or 8" http://www.suncourt.com/Inductors.html (The DB206 and DB208). They recommended the 208 to me. My room is approx 1375 cubic feet. What do you think? Would this be a good fan? I'm having trouble interpretting the specs, can you tell from what they provide if I could get away with 4" holes to the outside? Maybe with the 6" fan/damper/ducting so it's only a 33% reduction? If that would still provide adequate ventilation?

TomM wrote:

Thanks Rod! Is this what you mean by barometric damper?? I can get this locally at a home depot and I think it comes as 6" or 8" http://www.homedepot.com/webapp/wcs/sto ... =100396420 It looks like it just opens and closes depending on whether it's on or off (so I guess I'd wire it into the same switch that turns the fan on). Does it have to be an electric damper?

That is NOT a barometric damper - a barometric damper does not have a motor - it operates strictly on the barometric pressure within the space. This is a barometric damper: http://www.famcomfg.com/PDFOutput/prdspec.pdf

Do I need a Barometric damper on both ducts?

No - you need one on the inlet - the outlet should be a spring released door that opens (against the spring) when the fan comes on - just like you see for dryer exhausts.

Also, this is the fan I found locally in 6 or 8" http://www.suncourt.com/Inductors.html (The DB206 and DB208). They recommended the 208 to me. My room is approx 1375 cubic feet. What do you think? Would this be a good fan? I'm having trouble interpretting the specs, can you tell from what they provide if I could get away with 4" holes to the outside? Maybe with the 6" fan/damper/ducting so it's only a 33% reduction? If that would still provide adequate ventilation?

First off - you need to brush up on your math skills - a 4" round is a reduction of 44.45% of the volume (28.2743 s.i.) of a 6" round. who are "they? Rod

Hi Tom Sorry to jump on the thread but I am also in the quest of improving my knowledge in this area

Do I need a Barometric damper on both ducts? No - you need one on the inlet - the outlet should be a spring released door that opens (against the spring) when the fan comes on - just like you see for dryer exhausts.

Rod, So that i'm clear in my understanding, on page 123 of your book, there is a diagram depicting a static relief barometirc damper which looks to be on the supply side and is used to presumably exhaust to the outside when pressure within the system becomes too positive ? Is the fact that Tom is using a fan to exhaust air the reason to use a Barometric damper on the Inlet ? as fyi, my potential HVAC supplier suggests that I provide a fan on the inlet. He also suggested avoiding Barometric relief, however, i've searched on this topic here and read page 122 time and time again and my conclusion is that it is critical. Reading Tom's thread about where he is planning to put the fan (ie in the exhaust) has made me stop and think...yet again... Thanks Tom and Rod for this thread Regards John

johnp wrote:

Rod, So that i'm clear in my understanding, on page 123 of your book, there is a diagram depicting a static relief barometirc damper which looks to be on the supply side and is used to presumably exhaust to the outside when pressure within the system becomes too positive ? Is the fact that Tom is using a fan to exhaust air the reason to use a Barometric damper on the Inlet ?

Yes and Yes...........

as fyi, my potential HVAC supplier suggests that I provide a fan on the inlet. He also suggested avoiding Barometric relief, however, i've searched on this topic here and read page 122 time and time again and my conclusion is that it is critical. Reading Tom's thread about where he is planning to put the fan (ie in the exhaust) has made me stop and think...yet again...

OK - there are a ton of ways to make it from A to B - and plenty of reasons at one particular point in time to do it that way......... Some fans push better than they pull - for example - those Broan Fans I provided info on earlier- - the don't want to see duct before the inlet........... I usually want my fans to be connected to the exhaust side of the equation - that is then taking the fan noise and sending it away from the critical space......... which is what I do with stand alone specialty fans......... When tying into an HVAC air handler unit - you don't need a fan if your tie in is close enough to the body of the fan coil - ( that's what you see in Figure 7.3 - page 123) The only reason to have a fan is if you can't force the air handler to do the job -i.e.: long duct runs - or you can tie in at the unit - so if you need a fan in that case then use one. I always try to avoid that because it is just one more motor to break down over time - one more piece of gear costing you energy to run - one more maintainece expenditure to keep up - one more capital expenditure that you eventually have to replace. I don't know what his plans are, perhaps you should ask him why he doesn't want to use a BD - I find them perfect for adjusting to the pressure level I want the space to be. How exactly does he propose to do that? LIsten it's easy for one person to say A and another to say B - and what you are supposed to do (sitting in the middle) is ask the reason why......... in each case they should be able to articulate clearly what their design reasoning is - no math - no smoke and mirrors - just simple "it works thusly"........ clear enough for a non mechanic to be able to understand, If they can't do that and satisfy you - then their whole thought process is bull***t.......... If they can do that and satisfy you - and I can do that and satisfy you differently - then that doesn't mean that my design is good and theirs is bad - it just means they are 2 different means of arriving at the same point............ So ask him to explain to you - and tell him to use graphics - exactly how his system is going to work......... why does he require the fan =- what does he use in lieu of the BD to establlish and maintain positive pressure within the space......... etc., etc., etc. When he's done let me take a look at it........... Rod I

First off - you need to brush up on your math skills - a 4" round is a reduction of 44.45% of the volume (28.2743 s.i.) of a 6" round. who are "they? Rod

"They" would be the manufacturer, suncourt. I'd like a second opinion, do you think 6" ducting all the way, without reducing to 4", using the DB206 fan would be adequate for 1375 cubic feet of space? Assuming the ducts are done air tight with very minimal use of flex duct (1 foot each) and a duct length of no more than 10 feet each. Would I be better off using the 8" fan with 6" ducts? Or something totally different. Let me know if i'm not providing enough information. Thanks for the link to the barometric damper! p.s. I saw it as a 33% reduction because reducing 2 inches of 6 is 1/3 of 6.

TomM wrote:

p.s. I saw it as a 33% reduction because reducing 2 inches of 6 is 1/3 of 6.

But it doesn't take 3 - 3" round ducts to equal one 6" - it takes 4........ Picture easier - 6x6 square duct =- 36 s.i.. 1/3 of the area = 36/3 = 12 s.i. 1/3 of the area = 2 x 6 leaving not 4x4 but 4x6 as a remainder. See how easy that was? Rod

rod gervais wrote:

TomM wrote:

p.s. I saw it as a 33% reduction because reducing 2 inches of 6 is 1/3 of 6.

But it doesn't take 3 - 3" round ducts to equal one 6" - it takes 4........ Picture easier - 6x6 square duct =- 36 s.i.. 1/3 of the area = 36/3 = 12 s.i. 1/3 of the area = 2 x 6 leaving not 4x4 but 4x6 as a remainder. See how easy that was? Rod

Extremely easy... sorry I wasn't disagreeing with the 44%. I realized I was wrong because the outer circumference is much larger for the outer 2 inches than the inner 4 inches. duh, Thanks for correcting me :roll: Do you have a way of calculating how appropriate this fan would be based on the factors I described above? Based on the specs of the fan, the size of my room and the max. length of ducting? What would give me the recommended amount of fresh air?

Tom - the volume of a room is meaningless without knowing the square footage (which is what you base max occupancy on). Please provide info.......... Rod

The square footage is 185 feet.

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