Interesting find: HVAC

Did a condo inspection, today. Newer building (2007) multiple unit condo. The unit was, basicly, a studio with a bedroom area partitioned off.

The unit had a utility room with a cat 4 high efficiency furnace. It was a sealed combustion unit and was properly vented, with both combustion air and combustion gas flues to the exterior.


It had no return ductwork. The utility closet (furnace only in there, common water heater) had a solid door and an 18 x 36" return register over the door. The return intake on the furnace had the filter in place, but no ductwork attached.

I checked with my local HVAC expert, the VP of operations for the local gas utility, and after asking some questions and thinking about it, he sair it was OK. Essentially, they were using the utility closet as a return plenum. Since the furnace was a sealed combustion unit and properly vented, there was no need for return ductwork.

It was a cheap build, but it was technically OK.


What the heck would I know. I live in Central Florida. What’s a furnace anyway. lol

This is very common with mobile/manufactured homes.

Before I research a reason let me ask you a question Will.
If you find a gas pipe run through the middle of a furnace vent or return do you call it out ?

I think you know where I am going with this.

I call it out. Gas leaks are a different issue, but I get your point.

Since the entire utility closet is a plenum, the gas piping for the furnace is, essentially, in the ductwork.

so when (assuming you have) you find a situation with gas pipe in the vent how do you word it in your report ?

National Fuel Gas Code…looking right now and cat 4 furnace install doc.

I ran into this a few months ago, first time, in a single family two story home. An electric unit supplying a 1,200 sq. ft. upper level, in (attic) closet with no return ductwork. It seemed very inefficient to me.

I wrote this.
No return air supply ducts in individual rooms. The possible result: room-to-room pressure imbalances that lead to uneven room temperatures, comfort complaints, higher energy costs, and even moisture problems in walls and ceilings*


Return Air
The return air ductwork may be connected to any
or all of the following: left side return, right side
return, or bottom return. Tables 1 and 2 show
the airfl ow data for each furnace model. Where
maximum airfl ow is 1800 CFM or more two
openings must be used.
…so far.

Furnaces installed in a confi ned space which
supply circulating air to areas outside of the
space must draw return air from outside the
space and must have return air ducts tightly
sealed to the furnace.

Good catch, Bob.

Will further research.

Last thought before the experts (Charlie :)) come in.

If there is a carbon monoxide issue how would drawing the invisible and poisonous gas inside the return effect your life/safety detectors.

“do not like what you described so trying to find foder here”
Now back to my report…damn it is busy.

Agree. Thanks for that last quote Bob.

Ha ha made that one up but it has me itchy…later.

I was referring to this:

		  			*Furnaces installed in a **confined space** which

supply circulating air to areas outside of the
space must draw return air from outside the
space and must have return air ducts tightly
sealed to the furnace.* 11](

Using doc you posted and keying in return found this on page 7…

Return Air
In applications where the supply ducts carry heated air to
areas outside the space in which the furnace is installed, the
return air must be delivered to the furnace by duct(s) sealed
to the furnace casing, running full size and without interruption
between the outside space and the one in which the furnace
is installed.


Nicely done Bob :slight_smile:

You see how I am when focused …lol.
Adult ADD is a bit…c…

Actually Chris found it,Will posted it and now it is in the search.

if the furnace is in the closet with a solid door, being closed all the time, this is considered confined area. most furnaces manufacturers require a min limit for confined area, and whether or not to replace the solid door with a louvered closet door for air ventilation and return air.

True, but that does not apply to a sealed combustion cat 4 furnace, which takes its combustion air from the exterior (if installed properly). The issue is not negative pressure but that the room is being used as a return plenum.

Let’s try not to reinterpret (misinterpret) the code. The code talks about gas and electrical utilities going through “ductwork”.

The design you are discussing is a free air return. It is no different than using a ceiling Plenum in a commercial building.

This type of free return system is usually incorporated into very small square foot applications.

Christopher, there is no requirement for return ducts to be installed in every room. Yes you are correct that there are pressure imbalances that lead to weatherization and comfort conditions, but still it is not a requirement, (nor is it desirable in some cases).

So it is and the same area.

I’m getting dizzy! :vomit: