Forced vent air intake

During a recent inspection I encountered the PVC air intake of a forced vent gas furnace which was only stubbed up two foot from the furnace. The exhaust was piped to the exterior, the intake was pulling inside air from the basement. Is this acceptable? Why would it be installed using this method?

this would depend on what’s going on in the basement.

my 90+ FA furnace takes air from my 1/2 basement ( 900+ sq ft ) and works well.
no extra 2ft of pvc

it seems odd that they would “stub” the intake up 2 ft.
i’m not so sure that was necessary.

if this furnace is taking air from a decent size basement, i see no issue.

CAUTION Statement from 90+ High E Furnace Manu Install Booklet.

Combustion air must not be taken from inside structure, because inside air is frequently contaminated by halogens, which include flourides, chlorides, bromides, and iodides. These elements are found in aerosols, detergents, bleaches, cleaning solvents, salts, air fresheners, adhesives, paint, and other household products. Locate combustion air inlet as far as possible from swimming pool and swimming pool pump house. Excessive exposure to contaminated combustion air will result in safety and performance related problems.


All combustion-air and vent pipes must be airtight and watertight. Pipes must also terminate exactly as shown in Fig 30,31,32,33, or 34. Failure to follow this warning could result in property damage, personal injury, or death.

In addition to above, Altitude, Size of Pipe (dia) for Btuh (also both I and E should be same size), Number of Elbows in run need to be considered.

If recall is correct, the length of the I run should equal length of the E run + or -5% (Static pressure issues ?). MANU INSTALL SPECS TO BE FOLLOWED.

Additional LINK

In my area, more often than not, H/E furnaces draw combustion air from the basement. My house was that way when I moved in and I found that it made the household air very dry in the winter. I extended the stub to the outside and it made a big improvement.

If the furnace is located within the living space of the House, failure to install combustion air vent to the outdoors will result in on condition they are being drawn around window and door openings to replace the air use during combustion. This is an efficiency lost that basically shoot yourself in the foot when having a high efficiency furnace.
If there is not a garage door to allow fresh air to enter easily into the basement, furnaces within unfinished basements cause negative atmospheric pressure and increases radon concentrations in the space.
Either way, the manufacturer design equipment to event to the outdoors. It’s only a few dollars to make the installation complete.

“it seems odd that they would “stub” the intake up 2 ft.
I’m not so sure that was necessary.”

It’s intended to reduce the possibility of debris falling into the intake.