Hot Air Duct work

[Jeff May]

I added an addition onto my shop. The old section is being heated with an oil fired forced hot air furnace.
I need to run some type of ducting from the furnace to the addition.
It is about a 35' run.
My question is,
Has anyone ever used PVC pipe for hot air duct?
I was thinking of using 6" pvc from the furnace to the addition.
There is a partition between the addition and the old shop.
I was going to use the 6" for the main run and where it gets to the new section add a Y fitting and drop down to 4" and let it turn down and exit the 4" pvc through the ceiling of the addition.
Has anyone ever used pvc, or should I use something else?
I was thinking the pvc would be smooth, easy to work with and less expensive than regular duct work.
I'm open to ideas, thoughts and suggestions for this.
Many thanks,
Jeff

 

[pseudorealityx]

How are you returning the air?

 

[Jeff May]

The furnace is out of a mobile home. The return is in the front of the furnace, the hot air is blown out the bottom. Currently I have plenum on the bottom with several outlets that just blow into the shop. I should mention that the furnace is sitting about 24 inches off the floor on brackets against the wall.
The addition is attached with double doors between the shops.
I was thinking the return air could be via the part of the shop it sets in and just duct the hot air into the other shop area.
Maybe, maybe not... I'm not a hvac guy, that's why I'm here asking for help.
Thanks,
Jeff

 

[offroadsteve]

I think you'd be better off using the right stuff, which I would think could be as simple as a couple lenghts of flexable fiberglass insulated duct (Lowe's website has a 25' length of 6" duct for $27, 4" is $23 for 25'). Flex duct is super easy to run, just need some large size zip-ties and a roll of the aluminum backed tape to seal up the insulation. Unless you have a great source for 6" PVC fittings, I think you'd come out ahead with the flex.

Return air could be through a gap under the double-door seperating the two spaces, or a grill through the adjoining wall if you need more flow. But to be clear, you do need to establish a flow path for air from the remote room back to the furnace to be sure you get good circulation.

 

[Rockhead261]

The furnace is out of a mobile home. The return is in the front of the furnace, the hot air is blown out the bottom. Currently I have plenum on the bottom with several outlets that just blow into the shop.

Is this a wall furnace? If so, it's not designed to be ducted - the blower will just cavitate.

Why not use round duct/stovepipe? It's cheap and easy to work with.

 

[Jeff May]

Offroadsteve,
I looked at the regular heat duct.
Yea, you're right.
I didn't think it was that affordable.
So, my next question.
What size should I use from the furnace to where I will "Y" off to feed the ceiling difusers?
Should I drop the size down after the "Y" ?
Thanks everyone for the help.
Jeff

 

[Higgins]

Q. Is the furnace large enough to handle the addition of the new space????

AL

 

[Jeff May]

Don't know.
The only thing I know about furnaces???
They make heat...
I will check for the BTU rating and get the total square footage of my shops.
I will also try to figure out how to post pics and get a few of the furnace on here.

 

[offroadsteve]

Rockhead brought up a good point about the furnace not handling ductwork well... you'd have to pull a manual on the unit and see if it can handle the back pressure of ductwork. Furnace sizing is also a consideration as Higgins pointed out, but it doesn't sound like you're in the mode to replace right now. the only problem with an undersize unit is that you won't be able to get to your desired temperature when it gets really cold outside.

If that all works out and you can duct it, a 6" feeding two 4" ducts is fine. (6" round is 28 sq in, 4" round is 12.5 sq in). you can put the "Y" whereever its convenient.

 

[OccupantRJ]

Round duct is much cheaper at a supply house than at the big box stores, an example being a fitting that is $7 at the big box can be had for 25% of that in the supply house. Noland is one such company. If you don't like wrapping insulation on round duct, get an appropriate size flex duct, remove the inner sleeve, then slide the insulation over your round duct, as the smooth metal duct will give better air flow than the flex. Seal all joints and screws with tape or mastic to prevent air loss.

 

[Jeff May]

OK guys,
The furnace is a 62,000 BTU oil fired forced hot air out of a mobile home.
The return is basically at the top front of the unit, it blows the air down through and around the heat chamber and exits out the bottom.
When it was removed from the trailer it was blowing the hot air into the ducts running under the trailer and coming up in each room/section in the trailer.
All I did was sit it up on brackets about 20" or so, built a plenum and attached it to the bottom of the unit, cut two 6" holes in the front to allow the hot air to blow into the shop area.
I will try to get pics of my setup later tonight on here.
I'm off work tomorrow, I will be checking out our local supply houses for the duct work.
Again,
Thanks everyone,
Jeff

 

[Jeff May]

OK, Here are some pics of my furnace.

 

[OccupantRJ]

Maybe this will help.

http://www.google.com/search?q=hvac+duct+sizing+chart&ie=UTF-8&oe=UTF-8&hl=en&client=safari