Combustable air

Would you recommend adding vents for the furnace doors. The back is open but the face of the furnace is right behind the doors.

I did from the amount of dirt at the opening at the return.

Also I recommended a proper sael at the chimney where a incinerator probably was.


From the picture it appears that there may be a lack of combustion air. There are several remedies, one being the installation of door vents.

I would simply state is as “combustion air appears to be inadequate” and let them decide how to fix it.

There should be other indicators for inadequate combustion air as well - evidence of flame rollout, excessive soot in the heat exchange, etc.

The position of the PVC looks like it my be the combustion air supply. . .

Hi Dave, Jeff,

It looks from the pictures (it could be a High efficiency Cat 4 condensing FAU). Hopefully this install is pulling combustion air from the outside via the PVC piping… IF this is the case then the “closet” install is fine.

But I always say: Check the MFG install instructions first then the AHJ modified is next… BY the way if this is a Cat 4 unit with indoor combustion air being used then you have another problem. First it is defeating the purpose of the FAU cat 4 concept and efficiency, second …See the return air duct on the left side of pic… A no no with in ten feet of the FAU … Should not be there period if you ask me…Stupid to have that behind a solid door in a closet install…

Who had that idea??..

Plus the chimney pic… Duct tape is not the correct "product " to use in this case… A metal cap would be nice to see and metal / foil tape for rated temps is recommended… Do you have a complete picture of the install with doors open??:smiley:

Here is the picture with the doors open.

I like the case of flourescents next to the cabinet waiting for me to explode when I open the doors and start to inspect. I luckily lunged for the box and maid a catch that would of made the ESPN top 10 plays of the day.


Yup…You’ve got yourself a high effieciency unit there.

It does not need additional venting.

pbolliger is Correct…

No return vents should be installed in the same room as the furnace. CO issues could develop. If that furnace starts to give off CO, it will be sucked into that vent (you can see on the return duct-left side) and sent directly into the living area.


One more thing…

Why is the A/C condensation pipe capped? And where’s the neutralizer box (aka-the pump)?

Is the A/C evaporator installed yet? I can’t see the plenum above the furnace.

Oops, I can barely see the installation holes for the A/C presssure lines. It don’t appear to have A/C yet.

Don’t like the return air vent, it should be outside the closet, not in the same room as the furnace.

The last picture you posted indicates this is a Janitrol 90% furnace. While it is piped to the out side for intake (combustion) and piped to the out side for flue gas exhaust it most likely is not a sealed combustion unit, that is where the burners are enclosed and not open to the room air.

Based on the amount of louvers on the furnace door my guess is that this is not a sealed combustion unit, they normally have solid doors.

If this was a sealed combustion unit the closet installation would be ok. Sealed combustion units that are directly piped to the out side normally have a 1-inch clearance to combustible in the front of the unit and do not require house air for operation so the doors are ok. The return air is permissible on a sealed combustion unit when no H20 is in the same room because the flue gases can not be pulled into the return

If this is a non sealed combustion 90% which is very common with this manufacture then the air intake pipe that feeds the burner is optional because the combustion box is open to room air and the pvc pipe just connects to the side of the furnace. The return air grill would not be allowed, the doors are to close and they would need to be louvered. The manufacture will print the front clearance for combustible material on the data plate (most of the time)

In a furnace closet where the back of the room is open to allow sufficient combustion air (on furnaces where the combustion air is taken from the room not from out side) many code jurisdictions will allow this install with out additional make up or combustion air being provided by piping or louvers, while it may not meet ICC it often is ok by local code. Unfortunately if the homeowner should close off the room as the one I was on this week, the furnace is starved for air and a dangerous situation is created. I would make note of it in you report and warn about closing off the room but I would not refer to code.

John’s got a point about the louvered furnace door. They’re usually solid with a small view glass. I was going by the piping on the sides of the unit.

What’s it look like behind that louvered door?

John if it is open combustion I have a problem with the return air duct. Do you? Like your post, good comments.


David - I don’t think the condensate drain is capped–I think that’s an elbow. I can barely see it but I think the drain is going to the rear of the unit.

If you mean water, I don’t follow that. Can you help me out a little more?

Great info John!

You have to love the NACHI BB… Someone somewhere will have the info you need.:wink:

Definitely not a cap. Look at the size of it. An elbow.

Yes I mean Hot water heater

With only the furnace in the room and if it’s sealed combustion the return air in the same room can not pull flue gasses from the furnace into the house air, however if the room contains a normal draft water heater then the return air grill suction can be so great to cause the water heater to back draft when the doors are closed on the room, pulling the flue gases into the house return air.

question: when is a fresh air intake needed? I don’t often see units that do not have fresh air intakes, but yesterday ran into one that uses only combustion air drawn in to the front of the unit from the basement.