Question about gas furnace placement / air intake

Since this is random ramblings, I’m hoping someone has tips for my personal home:

The air quality in my home is very bad and I think I finally understand the problem. When my furnace was installed, it wasn’t installed flat on the floor with the base covered. It’s sitting on two large cement bricks with the bottom of the furnace open and allowing air to be taken in.

And, that’s the problem. It’s taking in direct air that doesn’t pass through a filter. I can attempt to seal the bottom of the furnace, but then I’m concerned that the furnace may not have enough available air to “breath properly”.

I read that it needs adequate air for combustion otherwise it can even explode or produce carbon monoxide. And this furnace is in a pretty closed area.

So rather than seal it, I’m thinking of taping a filter down there to serve as the base.

My question is, does anyone understand air combustion and can you help me determine the best way to solve this problem? Should I seal the bottom base? Tape a filter onto it?

Here are 2 pics:

I’ve dealt with similar issues. You need an opening no less than 90% of the raw opening to the furnace intake. It looks like your furnace fan would labor with the existing intake opening(s).

Also, the source of intake air should NOT start inside the furnace closet. Otherwise, your furnace will suck exhaust fumes back through the flu and circulate carbon monoxide all through your house every time the equipment fires up.

You need to isolate the intake air so that it ONLY ‘inhales’ common air from the interior of the house NOT from the furnace enclosure.

I’m surprised you guys aren’t deathly ill by now…

Meantime, build an open-ended box for the furnace to sit on. Train air into the box from outside the closet. On the intake opening, I would install an intake vent cover that holds filters in place. This is an intake vent grill that opens to contain a filter; you insert filters in back of it. Any Home Depot or Lowes has this.

The configuration you currently have is absolutely a death waiting to happen.

Hope this helps.


There are two parts to a furnace, first combustion air which is adiquate to create a strong completely burned flame causing heat, and second is the re-circulation system which picks up air through the air return and moves it through the fan, the heat exchanger and redistributes it through the vents back to the house!

If what your showing is the combustion air intake it is not designed to have a restriction such as a filter which limits natural intake levels of combustion air!

Re-circulated house air should never intermix with combustion air inside the re-circulation area of the furnace like Javipa said, but your system should have 2 very clear and seperate systems! Combustion air once fired with the gas and super heated should all flow up the exaust stack and out of the building above roof level!

Combustion air can be supplied through a loovered door, a wall vent to the exterior, or through a double walled exaust stack system drawing combustion air through the outside big stack and venting through a smaller inside stack!

This is an item so important that you do not want to make mistakes, people die of carbon monoxide in their sleep, it is a huge killer of people every year and well worth spending $100 on a service call for a professional to evaluate and make sure it’s safe!

Also code requires a carbon monoxide detector in any home using propane or natural gas!


I missed the part about having a separate source of air for combustion. That is so true and absolutely a must.

I have a feeling this furnace has only one source of air; the house.

Why has nobody died yet?

Thanks guys for the detailed responses, I’ve been reading a lot on this and I think I have a better understanding now:

Combustion air:
I do have a separate intake for combustion air. It’s just a PVC pipe with an elbow and it takes in air directly from the area surrounding the furnace. (see pic)

Intake air:
The furnace does get its intake area from the interior of the house. But, the installers left the base of the furnace open. So it’s also been inhaling intake air directly from its base, and that’s the issue. The previous pics I attached have arrows that point to the base of the furnace, and the floor in that area usually has dirt so the air gets dirty. I taped a filter onto the opening. Now my concern is will the blower be able to intake enough air from the interior of the house without blowing out?

The exhaust is a PVC that goes right out the side of the house.

Here’s a better photo:

What do you think of this set up? Do you need a better photo?

Meanwhile, I do have carbon monoxide detectors installed and they’ve never gone off. Since writing this post, I’ve placed a carbon monoxide detector right on top of the furnace to be on the safe side and it hasn’t gone off.

I would consider putting the combustion air inlet somewhere so it gets outside air. Really the only reason for this is that it’s pulling heated air into the furnace and applying a negative pressure to the house, causing the windows and doors to leak cold air in.

As far as the base goes, I’d start by putting a piece of plywood in the bottom and seeing how it does. A furnace starving for return air is going to get too hot and trip the limit switch. I’d check the temp rise across the furnace, lay the plywood in there, and check again. If it’s not really any different you solved the problem.

If the bottom is open because the return air is inadequate Japiva’s advice about a piece of duct from elsewhere in the house is great. I’m just not 100% convinced it’s necessary.


Plywood is never a good idea near open flame!