Adding a dedicated air intake to my furnace
 

I've been wanting to do this for a while now and I think now is as good a time as any to at least start looking into it a bit more. I also haven't seen anyone else do it and I'm wondering why. In any case, my furnace pulls the air for the burner from my basement. While my basement isn't conditioned it does put negative pressure on the house, pulling in cold outside air, since its exhausting air when the furnace is on. Newer furnaces have a dedicated intake so that this does not happen. I would like to modify my own furnace to have its own dedicated air intake.

First off, is there any reason to avoid this? I'm way out of warranty. Would tampering with the furnace cause insurance issues? I'm guessing yes, but I think I'm still willing to give it a shot.

http://ecorenovator.org/forum/attach...1&d=1384799701

Here is the setup in the basement. Shown here is my old tank water heater. It exhausts into my chimney which is cut off in the attic. I was thinking of connecting the old water heater exhaust duct up to the furnace. It is a 4" corrugated flue pipe and would go up and pull warmer air from the attic (so a very slight efficiency boost there too). Obviously, all the holes in the furnace sheet metal would need to be sealed up. Some aluminum tape would probably make short work of that. The door (which is off the furnace in the picture), would also need a gasket added. Overall though, not a lot of work...

I wonder what kind of efficiency boost I would see out of this? Any ideas? Other Ideas? Comments? Suggestions?

I suppose one concern would be that the existing exhaust fan wouldn't be pulling as much air through a long corrugated intake pipe. I could hook up a vacuum gauge to see what the change is to see how big the effect is.

My first concern would be making sure you maintain approximately the same intake mass flow rate. You should put a balancing damper at the furnace air inlet. When the system is balanced the damper will neither draw nor exhaust air from the basement. It will also bypass the flue if it fails to provide combustion air.

A fan might be required on the old flue pipe to induce down draft of what you hope will be warmer air. I strongly suggest putting the fan at the top, not the bottom of the flue. Air should be pushed, not pulled, for best fan efficiency.

You will never see the difference in your gas bill. Condensing furnaces don't pull much air from the conditioned space like the old natural draft furnaces did. A cheap fart fan exhausts twice as much air as a typical condensing furnace does.

Beware of positive pressure!

In winter, any positive pressure inside the house will push warm, moist air into your envelope. This moisture will condense, soaking insulation or whatever occupies the warm/cold boundary. If the supplied moisture freezes, it can cause severe damage to your home in places that are hard to detect. If it doesn't have a way to dissipate, it will remain and grow mold in the cooling season.

This is the major reason most all gas heaters have an exhaust but no real intake. Unless your house is sealed up like a submarine, it doesn't take much pressure to move enough water vapor to cause problems. The manufacturers have learned the hard way not to expose themselves to this liability.

Virtually all condensing furnaces today have a dedicated intake pipe. No liability issues since the intake/exhaust of the furnace is sealed off from the house.

Ok, sorry. I wasn't sure what kind of furnace Daox has. My bad.