Thread: Can this be explained? Wooden box installed in firnace

I came across this yesterday in a townhouse that's about 20yrs old, but the system only looked about 5 years old. Basically this wooden box was constructed as part of the return air portion of the system (pardon my lack of knowing the proper terminology). You can see the filter resting on the top of the box, which was cut out at that section so air flow was in fact happening.

Can anyone explain why this would have been done? I wrote it up as being "unusual" and recommending an HVAC technician come check it out. "Unusual." Probably an understatement.

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Pardon the typo in the title of the thread

Most likely the newer system was a different size than the old unit, and they decided to build a platform to facilitate the connections.

Is the inside of the box lined with duct board or sheet metal, and sealed?

That is weird. Most sheet metal guys would have made a plenum of... well... um... sheet metal. Funny how that works, huh?

I would probably make a comment that it looks like an amateur installation.

I did not remove the filter to see into the box.

Often its easier to raise the bridge than lower the river. New FAU unit and building a box to elevate it was cheaper than redoing the upper plenum.
That's my guess.

You guys think this would affect the efficiency of the system? I was not getting optimum cooling at the time of the inspection, which I'm wondering if it can be somewhat attributed to this box.

I did not remove the filter to see into the box

Any reason why? It looks very accessible. Curiosity would have compelled me to look into that area.

Call it lack of experience. In hindsight I obviously should have.

The word efficiency is probally not the thing you are looking for as this wooden box may not be proper and should be written up as not a professional installation. The efficiency of the furnace should not be changed as long as the box does not restrict the proper amount of air flow.
I believe this is what you are asking?

Yes, but I have a feeling it still might affect the efficiency somewhat since it's not a proper installation. But I will leave it to any of the HVAC guys out there who may want to set me straight.

Not for nothing I have been in the HVAC trade for over 26 years.

LOL, i figured that might be the case. Good deal, thanks for the input.

Somebody was either too lazy or didn't have the skills required to do the install right.

The pleated filter is going to kill the efficiency of that equipment more than the wooden box as ugly as it might be.

I saw the same thing Saturday only the filter was vertical. The return grill on the other side of the wall was missing and I could see the filter from that side but the filter was not accessible from the furnace side. Because the plywood sections were resting directly on concrete it was damaged by moisture and the inside of the box was home to some dark fungus type growth. Considering the wood is in contact with the ground/slab I would say that alone was reason to call it out for what I described above.

In your situation it looks like that section is raised so I don't know what problems that install could cause. Lifting up the filter would have been a great idea.

It almost looks like you have been in my house.
Seriously, as others have stated, the box was to compensate for a difference in dimensions of the original equipment and the new equipment. Ideally, you would like to have the box at least covered on the inside by something that provides some insulation. My own home has the return air simply formed by the interior wall with dry wall acting as the return air ducting. I havent seen any real issues with this type of application, but always thought it might be a llittle more efficient if the interior of the plenum had some insulation added to it (like the hard shiny stuff). Havent seen any evidence of moisture problems or mold in my existing set up.

"Re: Can this be explained? Wooden box installed in firnace"

"Pardon the typo in the title of the thread"

Is that I typo or a pun?

Eric