My combustion air intake is ran directly into the furnace air return. I have read this is an old practice and its better to have it ran down near the floor just open to the room. It seems like running it directly to the return dumps cold air into your furnace and hot humid air when your ac is running. Running it open to the room would only draw the air in due to pressure differences and would allow it to mix with the inside air before being taken in by the furnace. I also like to run my fan 100% of the time when the ac is on to circulate the air since downstairs where my thermostat is at stays way cooler than upstairs. This would be constantly sucking in hot humid air from outside.

Should I change it?

How big is the room you are pulling from?

Its open to the whole basement right now but when finished it will be 6x12.5x 13ft ceiling. The room has a furnace and power vented hot water heater. The furnace has a pvc vent, assume power vented also?? They both run horizontal to the side of the house.

Your combustion air intake should NEVER be plumbed into the return ducting. If it's a 90+% furnace, it should be plumbed from the outside to use non treated air to support the combustion process as that could care less if the air is 70F or 0F. It also keeps a negative pressure situation from happening if the house is "airtight." Technically, [in Mi at least] NOTHING is supposed to be piped into or through ductwork.

Basement air that is not treated [heated] is still better then using warmed air that is being returned and keeps the return and supply balanced.

When using the A/C, there isn't any combustion going on on.

6 x 12.5 does not support a lot of BTUs unless the house is very old and drafty.

This is basically what I have now except i dont see a register on the output side of the furnace like in the picture.




This is what im wondering might be a better setup

Does your furnace have a combustion air intake  and a combustion air exhaust or just the exhaust? Can a combustion air intake be plumbed to the furnace, IE, does it have a sealed combustion box and a 2-3 inch inlet? NO combustion intake air should be drawn directly from the return ducting.

What I was thinking with the ac is, since its running from an outside vent to the air return, its like having a return outside sucking in hot humid air and recirculating it with the cool air from in the house.

Combustion air and treated [going thru the blower housing] air are two different things, with A/C there isn't any "combustion" air.

Heres a picture. The black line is what im talking about. Its insulated and runs outside.the round line on the top right is the downstairs return and the angled duct on the far top right is the upstairs return. You can see they all come together, go down to where the filter is and into the furnace. The pvc line is the combustion vent. It has a blower inside the furnace




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I'm sorry but that looks way janky.

From your diagram, that is a fresh air intake, not a combustion air intake.  Your combustion air is coming from the free space surrounding the furnace.  My suggestion would be to run another PVC pipe from the combustion air intake on the furnace to the outside.

OP, does your house have super  tight energy efficient  construction?  IE-  SIPS panels or field applied spray foam?

Are there any open fireplaces in the house?  Do they have a sufficient air supply?

I'm just wondering if the HVAC  installer tried to cover his or her butt to provided make up air for the whole house.   Your second hand sketch looks like an appropriate solution but that's assuming there are no fireplace/s or high CFM range hoods etc..  And, that doors are undercut to help the "whole" house maintain  a pressure balance.  Getting combustion air for the furnace from the exterior  is always recommended as mentioned above.

You're right, you shouldn't be pulling outdoor air inside the building envelope during the cooling season unless your house is so tight that a mechanical engineer etc thought differently for some reason, IE- air quality,  that's not evident so far.