I don't see why not.

EDIT: Now that I think about it, I DO see why not. The intake and exhaust are supposed to be at the same atmospheric pressure. A difference in elevation would mean a difference in pressure.

Check the installation manual for your furnace. I think I recall reading in a Trane manual years ago that there was a maximum horizontal and vertical distance allowable between the intake and exhaust pipes.

where will furnace be located?...

I have an oil-fired System 2000 furnace. It works fine connected to the previous furnace MUA ducting to the side of the house and the exhaust going to the pre-existing chimney. With oil (and I assume with gas), it is just necessary to properly set the draft. However, my furnace's exhaust is too hot for PVC. There is a version of my furnace with colder discharge, but it uses an air mixing blower and hence has an improved draft as a result.

Nonetheless, it is true that in various conditions such as high winds, there can be a significant pressure variation between a chimney top and the side of a house. Usually the pressure differential can be overcome with a blown burner rig such as all oil furnaces have.

In cases where there is no augmentation of the draft -- hot water heater, perhaps -- it would be best to take the MUA from the same local area as where the exhaust is discharged.

The installation manuals I have seen indicate a minimum distance between the combustion intake and exhaust vents. But I have never seen a maximum distance.

The manual I have for my Carrier furnace shows in take being installed in a ventilated attic, and the exhaust continuing upward through the roof deck. No maximum distance is indicated.

I'm thinking that the pressure in the attic will tend to average the pressures on the house faces, and the pressure at the chimney top may not be sufficiently different for the products recommending this approach to have any difficulty. The very fact that external make-up air is being used goes a long way toward achieving interior CO safety.

I appreciate the comments and the muddier water I'm in with this. While I may be putting my foot in my mouth here, I'm not too worried about voiding my warranty. I would be worried about comprising the furnace itself.

The furnace will be in the basement in a confined closet.

The manual states "For Direct Vent systems, all pipe penetrations through roof or sidewall must be installed so that the vent and combustion air intake pipes terminate in the same atmospheric pressure zone."

I take it that 16" above grade (more than required) on a north facing wall would be a different "atmospheric pressure zone" than the roof (about 10 ft higher)? Thoughts?

It is always possible to posit or observe a wind speed and direction for a given structure that will result in different pressures at two separated locations on the structure. How different vs. what the unstated actual requirement is would be hand-waving at this point. It is notably difficult to construct an outside absolute atmospheric pressure gauge that is, within the desired accuracy, unaffected by even low speed wind, so differential pressures between locations where the wind is different should be expected to be different.

For reference, 50 ft of altitude is worth 1.4 inches of water column, so compared with the back-draft safety specification of 0.03 inches implies a maximum altitude difference of about a foot, although in this case the lower altitude as an intake would be positive for draft, all else being equal, such as temperature. The point here being that variables such as intake vs. exhaust altitude affect the draft in ways that the manufacturer may not want.

If the furnace manufacturer would list a specific differential pressure value not to exceed, then the pressure differences between two candidate intake and exhaust points could be measured by not very expensive means (excluding the labor involved in moving tubing around and in taking measurements over a wide range of conditions). It is probably easier to just follow the instructions because where the pressure difference requirement is low, it is generally safe to assume that a roof port will see a different pressure than a wall port in windy conditions.

I'm not there and don't see the situation, but cutting another hole to run the exhaust through the wall next to the present intake doesn't seem like it should be too much of a challenge. This is why the ancient Roman god Janus gave humanity hole saws. :)

I'd just cap the old furnace's roof exhaust at both ends, and not worry about it until it was time to re-roof.

If you used a concentric vent kit you only need to make one penetration out the side of the house. It has to terminate high enough above the ground to be above anticipated snow.

Thanks for all the comments and help. I've learned a thing or two which is always helpful. The decision was not simply about taking the path of least resistance. There were some other factors like having one less pipe to have to box in for a soffit, one less hole to punch through into my rim joist and brick, etc. Nevertheless, I will be going with a concentric vent as the second least path of resistance.

Many thanks all!

Direct vent furnaces can have concentric vents which can be installed horizontally or vertically. I don't know if there is any limitation on the length of a vertical installation to replace your existing vent. this should be outlined in your installation manual. The condensing vent MUST be PVC - I'm guessing your old vent was double wall, metal, B-Vent?

Just FYI the in atmospheric pressure at ground zero and your roofline is negligable.

"Just FYI the in atmospheric pressure at ground zero and your roofline is negligable."

FWIW, with a 30 foot change in height, the difference in atmospheric pressure will be about 0.02 psi, or roughly 0.1 percent. I'm not an engineer, so I can't say how much that matters to your furnace. But I suspect that the manuals wouldn't specify that intake and exhaust should to exit the home together if it weren't significant for some reason.

Thanks, sktn77a, for the idea. The old vent was metal, looks to be close to 6" in diameter from the point where the old furnace and water heater converged going vertically through the roof. Obviously I would need to check the termination on the roof but I can't see what I couldn't run concentric pvc pipes inside the one metal pipe through the roof with a concentric cap. The vertical length from the furnace to termination would be less than 20ft. Problems with this?

"The vertical length from the furnace to termination would be less than 20ft. Problems with this?"

You'd need to check the installation instructions for your furnace - should be in there. The actual concentric portion of the PVC vent pipe is only about 3' long and then splits into a 2" fresh air supply pipe and a 2" flue gas vent pipe:

https://www.homedepot.com/p/3-in-PVC-Concentric-Vent-Kit-CVENT3/204514478

I was thinking of making a 20' length of concentric piping through this 6" metal chase of sorts. I realize they come in 3' lengths and was thinking I could find the same size pipes at 20' length to run the whole length. Oh well.