GARN - vacuum relief duct

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Ecky

New Member
Aug 9, 2008
63
Central Ontario, Canada
The last paragraph of page 8 of the GARN installation manual refers to a vacuum relief duct (shown on drawing A-1 on page 6). I can, for the life of me not figure out why this is required. Could someone please enlighten me? The manual states,

"Because the GARN WHS unit utilizes an induced draft combustion blower a vacuum relief duct is required. Thus a vertical 4"x14" sheet metal duct is installed along the north wall within the insulation. The outdoor portion consists of a screened weather hood positioned 36" above grade. The top of the duct is open-ended and terminates above the insulation that surrounds the GARN unit. The upper end may also be screened. No damper is required."

In the diagram, this vent is shown by the tank and does not come out in the portion of the building containing the blower. In my GARN barn, the ceiling is open between the GARN unit and the area in front of the unit. Does it matter where this duct is located? And again, what does this duct do anyway?

Paul
 
I don't have a Garn, but it reads like outside air supply to me. Attempt to make sure the ID fan always has a supply of air.
 
JimboM said:
I don't have a Garn, but it reads like outside air supply to me. Attempt to make sure the ID fan always has a supply of air.

That's what I thought except that it is not supplying air to where the blower is. That is what I don't get.
 
Ecky said:
The last paragraph of page 8 of the GARN installation manual refers to a vacuum relief duct (shown on drawing A-1 on page 6). I can, for the life of me not figure out why this is required. Could someone please enlighten me? The manual states,

"Because the GARN WHS unit utilizes an induced draft combustion blower a vacuum relief duct is required. Thus a vertical 4"x14" sheet metal duct is installed along the north wall within the insulation. The outdoor portion consists of a screened weather hood positioned 36" above grade. The top of the duct is open-ended and terminates above the insulation that surrounds the GARN unit. The upper end may also be screened. No damper is required."

In the diagram, this vent is shown by the tank and does not come out in the portion of the building containing the blower. In my GARN barn, the ceiling is open between the GARN unit and the area in front of the unit. Does it matter where this duct is located? And again, what does this duct do anyway?

Paul

The fresh air intake acts as a vacuum relief when the door is opened. By having the fresh air intake located at a lower point than the discharge, it acts as a trap and does not allow hot air to continually discharge out the ductwork from inside the building. I think the logic of locating the intake on the north wall is that the south wall of a Garn barn has wood storage, east wall has the chimney, and the south wall has the entry.The front wall of my installation does not go up to the ceiling and my f/a intake is located midway down the left side of the tank when facing the boiler door.

Hope this helps


Another Paul
 
Hi Paul. So, if I understand you correctly, I should actually have the top of my relief duct located below the ceiling in the area containing the front door access. After reading page 8 of the installation manual, I see the front wall for the GARN barn is "to be constructed to a height of 6'6" to allow access to the top of the GARN unit". This means that there would be a path for the air to reach the blower. In my situation this is not the case. Hence, I need to have the top of the relief duct located below the ceiling. This would make sense to me. I had not noticed the fact that the plans for the GARN barn had a "cathedral" ceiling. Does this make sense to you?
 
Ecky said:
Hi Paul. So, if I understand you correctly, I should actually have the top of my relief duct located below the ceiling in the area containing the front door access. After reading page 8 of the installation manual, I see the front wall for the GARN barn is "to be constructed to a height of 6'6" to allow access to the top of the GARN unit". This means that there would be a path for the air to reach the blower. In my situation this is not the case. Hence, I need to have the top of the relief duct located below the ceiling. This would make sense to me. I had not noticed the fact that the plans for the GARN barn had a "cathedral" ceiling. Does this make sense to you?



Paul


Yes, I think if you have the choice, I would keep it below the ceiling and lined up with your boiler door(close as possible).


Good Luck

Paul
 
You can get OAK's for most Pellet stoves and some wood stoves.

With a Pellet Stove you would not usually open the dor when it is operating, with a Garn you do.

I would have thought an air supply on a timer would suffice?
 
Hi David. What is/are OAK'S? As to an air supply on a timer. I surmise that would do the same thing. The advantage to the ductwork is, of course, the fact that it is passive.
 
Outdoor Air Kit

Basically the same as a Garn but 2inch air supply to the back of the stove.

Avoids using conditioned air and especially useful in tightly built buildings.

In this context I have always thought of it as slightly overkill, not sure I like a permanent open duct to the outside.
 
Thanks on the OAK. The GARN is located in an outbuilding. Based on my building skills - outdoor air should not be a problem. ;^) I see your point on a permanent open duct. I suspect that you would not lose too in this case since hot air rises and this is unconditioned space for the most part anyway. Thanks for your input David.
 
I was thinking more about it, and as I will find out rather than have the experience I was thinking it would also depend on the size of the building.

You open the door, you have the fan running to avoid smoke in the face, you are pulling air from inside the building. In a small space could be a big issue. You finish and open the door to leave so instant equalisation.

I want my space conditioned. So slightly different situation.

I like the comment on building quality, you will have a very good excuse!
 
Thanks David. Different situations for sure. It would have been helpful if the GARN manual actually explained the purpose of (and science behind) the vacuum relief duct so that educated adjustments could be made if need be. If the purpose of the duct is to prevent back smoking (as seems to be the case) then I need to know that so the top of the duct is in the same area as the firebox door. With a closed ceiling as I have - that would not have been the case. In the end, it may not matter because I'll likely have the door to the barn open will loading anyway. Thanks for your thoughtful input.
 
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