Hot Air Outlet Size

[Wayjamus]

Hi all,

I'm new here and I asked this question on another forum but I figured I'd ask here, just in case there were other opinions.

I'm in the process of putting in a Traeger GBU-70 Pellet Furnace and
while I'm waiting for the cement pad to cure, I've got a question
about the hot air outlet. I bought it used and the prior owner
enlarged the outlet to 18" by 17" but the manual specifies the outlet
size as 12 by 12". Does the outlet size effect the efficiency in any
way? My thinking is it might decrease it a little by letting the air
pass through the tubes quicker and not allow as much heat transfer.
The air out of the register might be a little cooler possibly but I'm
not sure. Any thoughts on this? I may be totally wrong and that's ok
too. My current plenum on the oil furnace is large enough to allow for
17 by 17 so either way I'm going to have to change the size. I don't
want to do any more fabricating than I have to. I'm working with my
old wood furnace duct.

Velocity of the hot air was brought up and it seems like with 12 by 12" opening, the air would move to where it needs to be faster, less loss in the ducts. Any ideas on this?

Thanks,

Wayne

 

[Eric Johnson]

Welcome to the Boiler Room, Wayjamus.

Without knowing anything about it, I'd recommend going with the mfg. recommendations, as stated in the manual. Who knows why the previous owner went with a different size? The point is that the mfg. engineered the thing a certain way, and I don't think you can go wrong by following that recommendation. If you have to tweak it later, sheet metal beats the heck out of brick and mortar.

 

[Wayjamus]

Hi,

Thanks for the welcome!

That's kind of what I've been thinking. It seems there must be a reason for the size and I don't understand why it would be good to more than double the area of the opening. It seems like there must be some time allowed for the heat to transfer and opening it up like that would hurt more than help. The guy I bought it from is an HVAC tech though which is why I figured I'd ask.


Wayne

 

[deerefanatic]

Well, it depends. If the guy had to duct that heat very far, a smaller duct will have a higher static pressure and will require a bigger blower........ So that may have been his reason......

 

[Wayjamus]

It appears that the blower is original but then it might not be so obvious. He was very helpful and even fired it up for me in his garage. He replaced 2 of the motors prior but he didn't say anything about the blower. It makes sense about the static pressure though. I measured the exchanger tubes and they're about 15 X 17". It seems like the 12X12 would be too small? Seems like the sheet metal would be absorbing more heat from the tubes than it should. I want all the heat I can get out of it. : )