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[Gunpowder]

Plenum is 13x20. Would prefer a 13x21 actual dimensional exchanger.  Looks like my choices are 12x18 (approx 13x20 dimensional frame) or maybe 12x21 (box size not published). 

Problem is the outlets are on the end of the long end. I can forget the back. No one will ever be able to fit around the side to do the cutting, let alone the plumbing.
As shown in the pic, the front side has the exhaust pipe right in front of it 2" away. I might be able to get the outlets to match one one each side of the vent but there is no room for the cross pipe to fit as the clearance between the vent and the plenum is only 2" at most.

What should I do?

[mlappin]

Would placing the HX in the return duct be a better option?

[GUSWHIT]

Your span from the furnace to the rest of the duct line doesn't look like it would be too hard to change out.  I would have a HVAC guy come over and take measurements and get 1 built around an exchanger and then slide the prefabbed unit back in place.  He could put tapers on the inlet and outlet sides to accommodate the slightly larger exchanger you want.

[shepherd boy]

Looks like you got a lot of area to play with. Can you ell your vent forward or to either side? Should be able to get a 20x20 in with a little sheet metal work.Wonder how many BtU and cfm you need? Hate to put a heat exchanger in return on a oil or gas furnace.

[Gunpowder]

Could probably add a second adjustable elbow section at the top and rotate both away from the plenum. That would add more space if I installed the exchanger 1/2 way up the plenum from the furnace. Go with either 12x18 or 12x21 exchanger. Benefit is I can do this myself vs paying for HVAC guy to make me a plenum to fit my application


The reason I was thinking 12x21 was that I reviewed a video where they cut an H slit and then wrapped the top and bottom over the lip of the exchanger to get a good seal and help support the exchanger without brackets inside. I can't get to the back so would stillneed a bracket on the sides or back to support the back.

https://www.youtube.com/watch?v=nuXPjFxg5bU

[Gunpowder]

Benefits of a 20x20 in a newly fabed plenum vs a smaller exchanger in my current plenum?

[shepherd boy]

The bigger heat exchanger will get you some more btu but the main thing is it will keep static pressure down, meaning better air flow. How many cfm are you pushing now? As a rule of thumb, we want 80 sq. in. of coil for every ton or 400 cfm. But you can squeak by with 70 at times.

[Gunpowder]

No CFM listed. It is a Bard IH60D36B Furnace with LP conversion. 58,000 BTU. Installed 94/95 time frame.

The proposed 12x18 (12x20 box) exchanger is BTU 81,400 - 90K BTU depending on manufacturer
The Proposed 12x21 (box) exchanger ranges  90K - 100,800 BTU depending on manufacturer

[shepherd boy]

That doesn't sound like a big furnace. I think a 12x20 will work. Look at the fan HP. Should find a 1/4 or 1/3 HP. I think you have 1200 cfm.Could squeak by with a 12x18. You should have a 3 speed blower, make sure it is on high speed.

[Gunpowder]

Can't get to the motor side view to examine the motor but according to the internet, the IH60D36B uses a 1/3 direct 3 speed motor.  Low-735 CFM, Medium-950 CFM,  High-1175 CFM @ .20 inches H20.

The wiring goes into the old style metal wiring box. I read in the online manual that it is set at the factor at low speed. Not sure if the speed is changed via rewiring in the old style metal connection box

[shepherd boy]

You should have a blue, red,white and black wire coming from the motor.The black is high speed. see if that is connected. Or you could have three prongs on blower for high,med and low speed. White is normally common on 115 volt. You got what you need to work with.