Furnace Flue

Can someone tell me what the purpose of the flue on the left side of the gas furnace is for? It’s the one going from the floor and into to supply plenum. Could it be for combustion air? This room was 6X12X8 and didn’t have a source of combustion air. Thanks in advance for the answer.


It looks like it is going to the air plenum. It could be a fresh air supply, although I can’t tell if that is the return air or the condition air side (fresh air would be on the return air side). It could also be a duct that was added for heated air, although I would expect it to be insulated.
It does not look like a combustion air duct.

It’s on the conditioned side of the air.

That is an upflow furnace return air on the bottom A-coil on top. Can not tell from the pic but it appears that the 3 inch round duct is just resting on the floor I guess it did not penetrate would that be a correct assumption.

I would not have a clue why someone would dump supply air into that space so close to the hot water heater. Did you run the blower to see how much air was being discharged if any. All that would do is bring air from the other section of the home thru the return and discharge into that room. They perhaps were thinking combustion air but to my way of thinking that is not the way to provide it.

Thanks Charley. I should have described the other things that were going on in this house. There were serious electrical issues and issues in other areas. It was a two year old production home gone wrong.

In addition to the strange flue configuration, the condenser/compressor fan would turn on for a fraction of a second while the furnace was operating. Also, the furnace burners would turn off before the thermostat indicated the desired temperature was reached. I think that might be combustion air issue. Also, the house was very cold when I got there, but the thermostat was set to 65 degrees and in heat mode. There wasn’t a return vent on the first floor.

It was very strange indeed.

Anybody got any ideas? Sounds like a botched installation and/or bad circuits boards.

What is directly under that space? I am assuming this is the basement, but you never know with some houses. Could this be an attempt to half a$$ a sub-slab heating system?

The first floor is underneath this room. It’s a two story slab home. The supply plenum goes into the attic and the ducts feed down from the there. It’s rare to see the water heater/furnace located on the second floor in the same room here. I’ve only seen this type of configuration here once before in several years. It was almost exactly like what I saw today. Must be the same rouge builder!

My guess is they forgot to run ducting to an area of the house that was supposed to be heated and cooled and bastard fit in a duct.

LOL…want to find out where it goes and have some fun…drop a golf ball down there and see where it comes out. Just kidding…don’t do that for real!

Sounds like you already explained it (Botched) you never stated where the duct terminated in the room with the furnace or below into the first level. Makes a big difference.

Let me see if I can explain a little on controls they basically fall into two categories either are a operating control or a safety control. Operating controls do just that they operate the equipment and when they do not work they remain in the position that they were in when they failed either normally open or normally closed and the burner would not fire. A safety control can be a normally open or normally closed type switch or sensor and has the capability of resetting its self unless it has a lockout feature which is most usually incorporated within the circuit board or a red push type button on the sensor. If it is incorporated within the circuit board you can trick this board and reset it by simply un-plugging the furnace or turning the breaker off and back on.

If you are observing the flame on a furnace and it goes off before reaching the set point of the Stat and then re-fires within a couple of minutes you can almost be sure that a safety switch is the guilty party.

With this being stated it does not really matter to the HI why it is not operating as intended, just that it is not. A burner that is cycling on a safety is very easy for the Hi to miss he turns the stat up and goes about his business. I have inspected many many homes with the burner short cycling and the owner was never aware of this. Most only open their eyes and ears when the furnace does not operate at all.

Skip, I am a 12YR HVAC Tech and what I can tell you is “IF” the 3" duct along side the heater is in deed feeding combustable air to the unit, that is aginst code. Carbonmonoxside can enter the conditioned air steam. As far as the unit short cycling… Yes it could be a high limit switch opening on heat rise. 1st check filter condition. Bad air flow across your heatexchanger and cause this problem. 2nd check your fan opration. Make sure the fan in turning and turning in the right rotation. 3rd make sure your A-frame or N-frame coil is clean.( This is the Evaporator coil on the unit). these are a few simple things to look for.

It looks like a supply air duct to a register below to me.