Thread: screen on furnace intake

20 year old Trane condensing furnace.
I'm suspicious of the screen on the intake.

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Originally Posted by John Arnold

20 year old Trane condensing furnace.
I'm suspicious of the screen on the intake.

JA: It all depends on the net-free area required by Trane.

Thats to keep the kids from dropping things into the burner chamber!

Recommend screen removal followed by extending intake to outside air source.

I have seen this on older furnaces too. I recommend extending the intake to the outside also. I think these were early versions of the high efficiency furnaces.

The manufacturer will have specific requirements when it comes to the intake and the exhaust flue. If the room has plenty of make-up air for the amount of BTU's then the current intake is just fine.

You really should check the BTU's vs. the room size and then what the manufacturer requires before you start making recommendations.

Originally Posted by Scott Patterson

The manufacturer will have specific requirements when it comes to the intake and the exhaust flue. If the room has plenty of make-up air for the amount of BTU's then the current intake is just fine.

You really should check the BTU's vs. the room size and then what the manufacturer requires before you start making recommendations.

I certainly agree with checking the OEM's requirement's prior to making any recommendations about changing the intake air routing. But putting that aside, regardless of available inside make-up air, wouldn't the furnace be more efficient (and maybe even achieve its efficiency rating for the first time since installation) by using outside air instead of inside air for combustion?

regardless of available inside make-up air, wouldn't the furnace be more efficient (and maybe even achieve its efficiency rating for the first time since installation) by using outside air instead of inside air for combustion?

Where is the unit located? In an attic? Same air! What difference would it make if it was outside or indoor air as long as it had the volume required.

Originally Posted by Mike Schulz

Where is the unit located?...

Unfinished basement.

In confined spaces, this arrangement can depressurize the CAZ and cause backdrafting and poor combustion in water heaters in the same space.
bob

Look...the bottom line here is that the HVAC guy was too cheap to extend the 2nd pipe to the outside where it should be!

They install it just like that here in attics, crawl space and unfinished basements. If it was in a smaller room I can see what Bob has pointed out. But in big open space what's the problem.

Look...the bottom line here is that the HVAC guy was too cheap to extend the 2nd pipe to the outside where it should be!

Show us where the instructions says that for unfinished basements. If they plan on finishing the basement running a section of pvc is no big deal but to write up something for nothing is.

Originally Posted by Mike Schulz

Where is the unit located? In an attic? Same air! What difference would it make if it was outside or indoor air as long as it had the volume required.

If it's indoor air that you're using for combustion, you're wasting money you've already spent to condition (put BTU's into) it, so that's a less efficient use of your money (and the furnace). If it's outside air that you're using for combustion, it's free, and therefore a more efficient use of your money (and the furnace).

Originally Posted by Michael Chambers

Regardless of available inside make-up air, wouldn't the furnace be more efficient (and maybe even achieve its efficiency rating for the first time since installation) by using outside air instead of inside air for combustion?

Cold air is harder on the combustion process than warm air is and less efficient.

Many industrial facilities preheat their combustion air to make the appliances operate more efficiently.

If you can use interior air safely it's the best bet.

Michael, I think you hit the nail on the head!
David, How is cold air harder on equipment? This makes no sense to me. Cold air contains less moisture than warm air which is a byproduct of combustion anyway.

This might answer your cold and hot air. Hot & Cold Running Oxygen | Ask Metafilter

Also, some manufacturers recommend or require an elbow on that intake to prevent obstruction:

Originally Posted by Robert Dalga

How is cold air harder on equipment? This makes no sense to me. Cold air contains less moisture than warm air which is a byproduct of combustion anyway.

It has nothing to do with moisture which is only relative anyway.
The moisture in the combustion process doesn't come from the air it comes from the chemical reaction in the fuel during the combustion process.

It revolves around the cold air being denser than the warmer air.