Fan in a Can?

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Old 12-18-07, 04:09 PM
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Fan in a Can?

I am in the process of finishing my basement. I have a utility room with a hot water heater and a forced hot air furnace (120 K BTU). My town's code requires a fresh air supply for the utility room.

My utility room does not have an "exterior" wall. I mean, three of the walls are interior basement walls and the fourth is a shared wall with the garage. That means that any fresh air duct will go into the garage, then duct will run ~15 toward a foundation wall.

I understand from other sources that I may need a "fan in a can" that will come on when the furnace blower comes on.

Does anyone have experience with this? What size fan and duct would I need for a 120,000 BTU furnace? Where would I buy the fan? Do I need an HVAC profession to connect it to my furnace?

Thanks.
 
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Old 12-18-07, 10:36 PM
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Id ask code there just what they want. Where do you get the air now??? Lot of code's just call for air from in the home .Thats why I say ask there. If gas units are in a inclosed small room . They need 1 sq" per 1000 Btu Like a 100sq" grill for a 100,000 btu furnace. Dont foget to add the btu of the water heater. So I say again ASK there
 
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Old 12-19-07, 04:54 AM
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Like Ed said, you better check the code requirements on this.
Here the combustion air supply to a room can not be powered.

If it is not permitted in your area you would have to size it large enough to allow for the distance.
 
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Old 12-19-07, 09:08 AM
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Originally Posted by GregH View Post
Like Ed said, you better check the code requirements on this.
Here the combustion air supply to a room can not be powered.

If it is not permitted in your area you would have to size it large enough to allow for the distance.
Some time ago when I spoke to an assistant inspector in my town, he said I should check with an HVAC pro. They would know what size I need.

Yesterday, I checked with an HVAC contractor in my area. He mentioned the the Fan in the Can and 4" pipe, given the distance between my small utility room and the outside. But, he quoted me $1500 to do the job. I thought that was high. I figure the fan is ~$300. The 20' of 4" duct and elbows is
 
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Old 12-19-07, 09:48 AM
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Some time ago when I spoke to an assistant inspector in my town, he said I should check with an HVAC pro. They would know what size I need.
Id say thats BULL. How can he be a inspector If he dont know what you need there.
 
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Old 12-19-07, 10:47 AM
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Unless your AHJ has some prohibition against doing so you can pull the combustion air from the basement if it's large enough.

You an do the basic calculations here:

http://www.houseofcraig.net/combustion_air_calc.html

IMO if you can afford it the best option is these situations are Cat IV direct vent appliances: there is usually some way to vent these and bring in combustion air through a sidewall, you can place them in a tightly close utility area to reduce noise, and you are guaranteed to avoid back-drafting and combustion air issues.
 
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Old 12-19-07, 12:12 PM
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Originally Posted by Michael Thomas View Post
Unless your AHJ has some prohibition against doing so you can pull the combustion air from the basement if it's large enough.

You an do the basic calculations here:

http://www.houseofcraig.net/combustion_air_calc.html

IMO if you can afford it the best option is these situations are Cat IV direct vent appliances: there is usually some way to vent these and bring in combustion air through a sidewall, you can place them in a tightly close utility area to reduce noise, and you are guaranteed to avoid back-drafting and combustion air issues.
I can't pull air from the basement, that much I know for sure.

My furnace may be a direct vent. Does that mean I have a double-walled exhaust vent--one opening for exhaust and and an outer wall for fresh air coming in? If so, I will contact the town to see if my current situation is adequate.
 
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Old 12-19-07, 02:02 PM
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Originally Posted by GaetanoL View Post
My furnace may be a direct vent. Does that mean I have a double-walled exhaust vent--one opening for exhaust and and an outer wall for fresh air coming in? If so, I will contact the town to see if my current situation is adequate.
If your furnace is recent direct vent unit, the vent pipe will be plastic, unusually white Schedule 40 PVC. Often (around here anyway) installers will attempt to save money by not connecting the combustion air intake to the exterior and pulling the combustion air from within the house. Some manufacturers allow this, some don't.

If you have two 2" or larger white plastic pipes connecting to the furnace, it is likely pulling it's combustion air from the exterior.

Post a link to a pictures of the furnace and the vent pipe(s) at the exterior, and we can tell you more.
 
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Old 12-19-07, 07:37 PM
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Originally Posted by Michael Thomas View Post
...If you have two 2" or larger white plastic pipes connecting to the furnace, it is likely pulling it's combustion air from the exterior.

Post a link to a pictures of the furnace and the vent pipe(s) at the exterior, and we can tell you more.
Here's a link to a photo of the 2 white pipes as they leave my garage. Each is 3".

http://gaetanol.myphotoalbum.com/vie...1&id=image0001

It's too dark to take the photos of the other side, but I can do that over the weekend if it is necessary (it's dark out when I leave for work in the morning).

I can tell you that the other side has a PVC vent cap in the shape of a funnel out of which drips water when the furnace is running.
 

Last edited by GaetanoL; 12-19-07 at 07:38 PM. Reason: added pipe diameter
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Old 12-23-07, 07:22 PM
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That’s appears to be a “concentric vent termination”:

http://johnsonsupply.com/york2001/PU...T/N2_4u799.pdf

and assuming that those pipes are both continuous back to your furnace, and properly installed, then you are pulling your combustion air for the exterior of the building.

Disclaimer: I’m looking a picture, not your actual installation, and only a qualified HVAC technician looking at your actual installation can tell for certain if it is correct.
 
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Old 12-24-07, 06:06 AM
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Yes, that's how the vent terminates.

The info you've given will be a great starting point for when I talk to the inspector or an HVAC pro. Thanks.
 
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Old 01-09-08, 08:25 PM
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Originally Posted by Ed Imeduc View Post
... If gas units are in a inclosed small room . They need 1 sq" per 1000 Btu Like a 100sq" grill for a 100,000 btu furnace...
I have confirmed that my furnace has it's own powered fresh air supply.

I spoke to the town and the inspector said that since my furnace has it's own fresh air supply, I only have to supply air for the hot water heater.

Does this requirement of 1 sq inch per 1000 BTU relate to the size of the opening or the size of the pipe? If my geometry is correct (Area of circle = pi X radius squared), then a 38K BTU furnace would need a 7" pipe (3.14 X (3.5 squared)>38).

Or, can you use a smaller pipe as long as you have the appropriate size vent opening?
 
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Old 01-10-08, 08:03 PM
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Gaetano:
you need fresh air supply if your heater is located in what is considered a "confined space"...which in a nutshell means that there may not be enough air in this mechanical room for good and safe combustion.
How does one know if the space is (or is not) confined?
The rule is as follows: Confined space means that the appliance room has less than 50 cubic feet of volume per 1,000 Btuh of all appliances located in the space. Since your furnace is direct vent, it needs not be counted.

Having said that, since you're finishing your basement, the space will automatically be considered confined b/c you'll frame and sheetrock the walls and infiltration of fresh air will no longer be.

So...what should be the size of the opening?
Well, from your first posting, it appears to me that you'll be running horizontal ducts from your basement, through the shared garage wall, to the outdoors (please correct me if I'm wrong here). If so, you need "2" horizontal ducts, each one of them having a MINIMUM free area opening of 1 square-inch per 2,000 Btuh of total combined input capacity (in your case only the water heater). Let's say for example's sake that your water heater has a 40,000Btuh input...that means that each one of the openings shall be 20 square inches (20x2,000=40,000)...BUT...BUT...BUT... you surely won't have just the bare ducts coming off your exterior wall, you'll likely add a louver/damper, etc. to each duct. Since the louvers will reduce the original opening area, double up its size. You're then looking at a minimum opening area of 40 square inches per duct...and from the formula of area for a circle: Diameter = square root of [4xA/3.14] = sq-rt of [4x40/3.14] = 7.17 inches
go to the next size up.

The opening size varies from 1 square-inch per 1,000 Btuh to 1 square inch per 4,000 Btuh depending on (a) the type of fuel [gas v. oil], and where are you getting the air from [the attic? and adjacent room? horizontal ducts?, etc.]

I=B=R has a superb manual for all things hydronic, it is called Guide-2000. I strongly recommend to anyone who is a serious hydronic dude and wants to get deep into the know hows in this area of HVAC.

Notice the location of the ducts on the drawing. One has to be within 12 inches off the ceiling, and the other within 12 inches off the floor, ELSE natural air circulation won't happen, the air in the room will become stagnant and your heater won't get all the air it needs for safe combustion.
 
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Old 01-11-08, 05:25 AM
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Thank you for a very thorough reply.

Originally Posted by pflor View Post
So...what should be the size of the opening?
Well, from your first posting, it appears to me that you'll be running horizontal ducts from your basement, through the shared garage wall, to the outdoors (please correct me if I'm wrong here).
My original thought was to go through the garage to the outside. I have since considered the possibility of getting to the outside air through a pantry, which is adjacent to the furnace room. If I go through the garage, the duct will have a 90 degree bend (otherwise it will have a very long run to the outside). If I go through the pantry, it is a straight shoot.

Either way, the problem with this "high-low" set-up is that if I go horizontally 12 inches above the ground, the low vent will be well below grade. If I got the air from directly from the garage, I could do "high-low." But, I'm not sure if that would affect the fire rating of the furnace room.

Originally Posted by pflor View Post

The opening size varies from 1 square-inch per 1,000 Btuh to 1 square inch per 4,000 Btuh depending on (a) the type of fuel [gas v. oil], and where are you getting the air from [the attic? and adjacent room? horizontal ducts?, etc.]...
My water heater is gas. When would the requirements call for 1 sq inch per 4000 BTU?
 
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Old 01-11-08, 07:26 AM
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Sorry...I thought the basement and garage were at same level (a walk-out bsmt of sorts but w/o the walk-out door ).

The fan-in-a-can won't work for you here...I'm assuming you have a tank-type DHW heater with standing pilot and atmospheric draft.

IF you have an exterior wall in this mechanical-room-to-be, and a hole were to be cut that would allow direct admission of outdoor fresh air, then this is what you need:

Keep in mind that the values given are for free area opening sizes. Since you'll likely add a louver, the actual opening will have to be larger...how much larger? that depends on the efficiency of the louver: wooden louvers - 50% [final opening size doubles-up]...plastic metal - 70% [increase opening size by 30%]

IF final admission of the air into this room needs ducting, then the above table is not the correct one to use.
 
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Old 01-11-08, 07:39 AM
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Thanks again.

Originally Posted by pflor View Post
Sorry...I thought the basement and garage were at same level (a walk-out bsmt of sorts but w/o the walk-out door ).
No problem. I don't I was very clear in my explanation.

Originally Posted by pflor View Post
...IF you have an exterior wall in this mechanical-room-to-be...
...IF final admission of the air into this room needs ducting, then the above table is not the correct one to use.
I would have to run a duct to get to the outside air. Three of the walls are interior walls (including the one I mentioned that is shared with the pantry), and the 4th wall is shared with the garage.

So, if you could provide the table (or sq inch to BTU ratio) for a set-up where a duct must bring in the fresh air, that would be hugely helpful. I will keep in mind the info you provided regarding the louver efficiency.
 
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Old 01-11-08, 07:57 AM
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Read from the bottom of pg-7, through the upper part of pg-9.
Table & diagrams shown are pretty standard, apply not only to the maker of this equipment but to any gas-fired appliance in general.
http://icpindexing.mqgroup.com/docum...4101260302.pdf
 
 

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