Redesigning furnace plenum

[FarmerSid]

I installed a Newmac add on wood furnace a few years ago beside an existing oil furnace. The plenum on top of the oil furnace is box shaped with the old holes covered up and new ducting added to it to connect it to the wood furnace. I'm wondering if I had a new plenum built that would be curved in hopes of improving air flow.

What do you all think of the plan?

 

[CrappieKeith]

What you've got is fine...I'd be looking at increasing your blower's cfms 1st...on another note your installation is not to code.
You need 18" of clearance between the top of the plenum and your floor joists unless it has been tested to be within 6".
Your runs also need 6" of clearance for the 1st 6 feet.It does not look like your ducting meets these clearances.

 

[FarmerSid]

Thanks for the feedback! I know the runs were installed at 8" below the joists for the first 8 ft but not sure why the plenum is at 8" as well. I will have to ask my installer why things were installed this way. He mentioned on more that one occasion during the installation that he did things the way he did for code clearance reasons. I'm not sure what the cfm is of my current furnace fan but it is the one that came with the furnace. The filters in the oil furnace really reduce the amount of air flow out the vents in each room. You can really notice the difference with them in.

On another note, there is two cold air returns approx. 10 ft apart from each other up stairs. One down the hall and one in the kitchen which is right above the furnace. That return is right above the water softener and the one in the hall is about 10 ft farther away. Both returns are approx. 12"X24" and in the floor. The only other cold air return is in the furnace room beside the water softener. You can see a bit of the round ducting of it going down to the floor. I have a rag over it so it does not suck in any dirt, dust etc from around the furnace area.

 

[CrappieKeith]

I do not believe that having a cold air return in the room that the furnace is installed in is a good thing or proper. It can take air or create a mini vaacume in the basement robbing combustion air from the burn process.

 

[freemind]

The biggest killer to air flow is restrictions. If you want airflow, you must not put any 90 degree elbows in the system. Nothing more than a 45 degree. The straighter the run to the plenum the better.

You will want a 45 degree elbow inside you plenum pointed toward your register runs inside the plenum.

Trust me, I just put one in last year.

If you have a standard fan, like on the back of the add on woodstoves, you may also have a CFM problem if your stove is very far away from the plenum.

I put a squirrel cage motor out of a forced air furnace in my system to boost airflow.

 

[FarmerSid]

I agree about the cold air return in the furnace room. There is no return in any part of the finished part of the basement. Anybody have anymore insight?