Furnace installed below stairs without return air duct

I came across a strange set-up today. The furnace is located in the basement below the stairs. The underside of the stair are exposed so I did write up that they need to have drywall installed. My question is regarding the return air. There was a through the wall grill installed to combine the under stair space with the rest of the basement, and the ceiling is not finished. The main floor is on floor trusses. They appear to be using the underfloor space where the trusses are to return air from the main floor. Return grills are installed at two locations on the floor of the main level without any ductwork. This seams to me to be less than ideal and a fire hazard as the furnace will be pulling air through the trusses if it is operating during a fire. 1. Are these assumptions correct? 2. How do I explain that to the client without scaring the heck out of them?

This sounds like simply an amateur install, without ductwork. The home will never be comfortable.
You can jam a furnace in wherever you want but a balanced duct system makes all the difference.

As far as a fire hazard, no. Unless there’s a clearance issue we can’t see from here.

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Once upon a time it was comon practice to use foil coated cardboard stapled to floor joists for return air. Cheap alternative to ductwork. The open space between the joists was the return duct.

What Scott said^^^^^^^^^^.

I agree. Thanks Scott.

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Panning the joists from the return air grilles back to the return air trunk line is how its done today, in most areas.

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It becomes a fire issue with the equipment itself being under the exposed stairs.


Not in my area, panning joists and using it as return air is cheap installation in cheap builds.

How about a picture on how return air ducting is installed in your area?

I can’t see any return air ducting at all. The floor trusses run perpendicular to how the return duct would need to be installed. I think the whole under-floor area is acting as a return will just a couple of holes cut in the floor.

The return air isn’t properly installed or at all.
Have a heating contractor repair.

The furnace has been running but not able to properly heat/cool the home. Again, lousy install.

Using metal or flexible duct work:

Okay, simply a difference of opinion on what you perceive as cheap.

No difference in quality of system. In fact, if the installer isn’t careful with flex duct and there’s too long a run, too many bends, etc., the resistance and pressure drop is too high.

It’s not a mere opinion or installation issue, it’s a design and long-term performance issue. It’s extremely difficult, therefore nearly impossible, in practice to seal a panned joist cavity. It’s a lot easier to seal and keep sealed a dedicated duct work. Cost savings in labor and material is THE reason 99% of the time why panning is used in lieu of a dedicated ductwork. Per before, it’s not done here, I’ve only seen it a few times in person, definitely not common here.

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Forced air HVAC systems. Furnace require a fully closed and sealed supply and return ducts.
Closed returns minimize “suction” from the rest of the house. Issues. Induction of soil gases and radon. Increase the air leakage. Higher home heating bills. Poor comfort.

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It is just that, your opinion dude.

As we all have, I have inspected some large high end homes, ranging to 1500 SF ranch style. For me here in Northern Michigan all with panned joists.
In addition, all air balance qualified.
Not “cheap” homes just different from what you are used to.

Both, properly installed, style systems will give the end buyer a comfortable home.

How about a picture of high-end home with panned joists used as duct work? I’ll be surprised. I’ll wait :slight_smile:

This one has actual metal panning. Most use Thermopan.

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Simon, Scott knows his stuff around HVAC. He’s been in it for years…okay, plus he’s a friend. :rofl:

My house has Thermopam and there is 1 degree difference from one corner of the house to another…of course, I have foam walls and an R-60 attic. there is a couple of degrees difference in the basement but I have less insulation down there. I love my heat and AC bills

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