I just replaced my old natural gas furnace with a 95% AFUE version which is dual certified (direct vent or non-direct vent).  My old furnace was non-direct vent.  I'm trying to decide if running a combustion air intake pipe from the outside is worth the cost and effort.  I know that using outside combustion air is better for the heat exchanger (less corrosion), and that a direct vent will not create a negative pressure differential that exacerbates air infiltration, but does the use of direct venting also increase the overall energy efficiency of the furnace?  Also, why do the installation instructions specify that the direct vent air intake be placed a maximum of 24" from the exhaust?   It would be nearly impossible for me to meet that requirement as a retrofit install.

You really don't want to have CO and excess CO2 recycled back into the fresh air. The result is more CO and improper combustion. Are you sure you cannot get 24" and it could be more if that is easier.

Posted By MikeSolar on 09 Oct 2012 08:27 PM
You really don't want to have CO and excess CO2 recycled back into the fresh air. The result is more CO and improper combustion. Are you sure you cannot get 24" and it could be more if that is easier.

Mike, the installation instructions state that the maximum distance between the intake and exhaust should be 24" and a minimum of 3".  The only reason I can think of is that if the exhaust is blocked by snow, the intake would likely also be blocked and the furnace would shut down.  Another possibility is that the intake and exhaust need to be located in the same atmospheric (wind) conditions to maintain pressure balance.  I don't know.  I'm forced to separate the intake and exhaust by several feet (20 or more feet).  Is that going to be a problem?  The alternative is to go non-direct vent as before.  Any thoughts?

You might consider using a concentric vent pipe. Both exhaust and combustion air delivered in one penetration. Google it.

Is the manual available on line?

The BTUs of induced infiltration loss per cubic foot= 0.018*V*ΔT (Fahrenheit delta between conditioned space air temp and outside air temp), which can add up to a real (but hard to measure) number during the heating season, peeling a percent or three off the average net system efficiency. (The AFUE test does not account for those losses, nor does it account for jacket losses of the furnace or boiler, presuming that heat loss accrues to a conditioned space, which is not always the case.)

And having an open vent to supply combustion air increases infiltration losses even when the blower isn't actively pulling that air in, which can also add a few percent to the heat load. When you view the furnace and the house as a combined system, it's ALWAYS better for net efficiency to go with a direct-vent/sealed combustion appliances.

Posted By 3cityblue on 10 Oct 2012 10:26 AM
You might consider using a concentric vent pipe. Both exhaust and combustion air delivered in one penetration. Google it.

Thanks for the suggestion, 3cityblue.  It isn't a second penetration that concerns me, but, rather, the difficulty of routing another 3" PVC intake pipe alongside the existing 3" exhaust pipe which is buried in a finished ceiling. 

I'm wondering, does it matter if I route the intake pipe to another spot some distance from the exhaust outlet?  The installation manual states on pages 17-18 that the intake and exhaust vent openings must be within 2 ft of each other, but doesn't explain why that is important.  I'm inclined to ignore that restriction, but I'd like to understand the possible downside. 

Thank you, Dana, for explaining about the added heat loss through the exhaust vent of a non-direct vented installation.  That loss would be greatest for a vertical vent, I presume.  Is the heat loss for a horizontal vent also significant?

--Clark

I think the separation maximum rule is for air balance, so one pipe length does not greatly exceed the other. I would call and ask if you maintained the same length of piping, they could be more than 3 feet apart (assuming they both offset same distance from furnace). Its not like the pipes know where the other is located...

Posted By NFC on 10 Oct 2012 12:39 PM
I think the separation maximum rule is for air balance, so one pipe length does not greatly exceed the other. I would call and ask if you maintained the same length of piping, they could be more than 3 feet apart (assuming they both offset same distance from furnace). Its not like the pipes know where the other is located...

That's an interesting explanation.  I will attempt to contact the manufacturer.   I wonder if your theory makes sense, however, given that a non-direct vent installation is inherently unbalanced between intake (zero length) and exhaust.  Seems to me that a problematic imbalance would only arise if the intake pipe was longer (more restrictive) than the exhaust pipe.  On the other hand, the volume of the cooler intake air is considerably less than the heated exhaust gas mixture.  That would imply that the intake pipe could be longer or smaller in diameter than the exhaust pipe without impairing thorough combustion of the natural gas.   The manufacturer specifies the same 3" PVC pipe diameter for the fresh air intake pipe.  Maybe this recommendation reflects a rule of thumb which is known to always work in the absence of an engineered design.

Horizontal or vertical, if it's a hole in the building near the either bottom or top of the building it's a major contributor to the stack effect. The building height from the lowest air leak to the highest determines the amount of stack pressure (more or less), and the size of the leaks determine the flow.

With sealed combustion units it doesn't matter if it's side or top vented- it's not a hole in the pressure envelope of the building, isolated from the conditioned space air and thus has no effect on stack effect pressures or flows.

On p17 of the manual they state: "Both the combustion air intake and a vent/flue pipe terminations must be in the same atmospheric pressure zone." which means they should be on the same side of the building (or roof ridge), and at approximately the same elevation, but the 24" max separation spelled out in multiple drawings on pp17-19 is a mystery.

On the InterNACHI website, Inspector and PE Robert O'Connor states: "...higher efficiency furnaces can be very sensitive to any restriction in available combustion air when using a pipe for the intake. Some furnaces start shutting down from simple things like marginal intake pipe size not terminated under the same ambient conditions as the exhaust ... I have seen it happen."

Sounds like I would be risking a furnace shutdown if I don't follow the manufacturer's installation instructions concerning the placement of the intake/exhaust penetrations. Maybe if the intake pipe is oversized and/or kept short, the risk could be minimized.

BTW, I called Goodman. They won't provide any direct technical support to the homeowner. Must go through a licensed HVAC contractor.

The length of the intake piping is a bigger issue relative to flow restriction than proximity to the exhaust vent. I presume they must have a reason to specify that proximity, but that ain't it.

Minimizing the lengths is always a good idea, but there IS such thing as "too big" of an exhaust vent (less so for intake vent.)

I would take the air from outside however you can.
Sometimes installation instructions have to do with certain approvals. While they know it can't be closer than 3" they aren't going to test the product out to 50'. The atmospheric comment is the catchall for bigger spreads.

Also a consideration is the minimum clearences from windows and such permitted by direct vent appliances. If you spread the pipes 10' apart it no longer qualifies for "direct vent" rules as you are outside the manufacturers installation instructions.

As was noted you can't get much different atmospheric conditions than one pipe inside and one out side.
I think your furnace will like it more if it doesn't have to re-heat air you blew outta the house. Is there a chance that it will interfere with performance if you draw from "different atmospheric conditions"- a little but not likely.
If for some reason it does, terminate your new "combustion air duct" really close to the furnace inlet and close the door to the mechanical room.

Is this code? Nope.

I cannot see any document about this furnace but I have NEVER seen a manual that said to keep them so close together. They all seem to have the same picture showing minimum X radius from the fresh intake or X from the exhaust to a window or gas reg. It used to be, in the early fan assisted furnaces that a big imbalance would be a problem but the fans are much better now and we regularly take combustion air from the side wall and then put a vent in the old chimney to the roof if we could not get clearance from the intake, for example. Don't constrict the suction piping as it changes the max length of total venting and is not good for the fan anyway.