The diagram below is from the Rinnai instruction manual for R50LSi, R75LSi, R94LSi, R98LSi(-ASME) tankless units. Link for manual. Were looking at page 33:
Why is there a maximum clearance between exhaust and air intake? It mentions something about a pressure zone. Can anyone explain this to me? I’ve never heard of the concept.
That is a great question! I have read somewhere in the past that basically the reason is to keep the intake and exhaust pipe runs approximately the same length. If the intake length is let’s say, double the length of the exhaust, it may result in the intake being insufficient due to difficulty drawing air (friction) through the longer pipe.
That said, I am wondering if there is more to it than that. I come across non-direct vented high efficiency furnaces periodically where the intake is terminated right at the furnace (indoors). Can that be an issue since the exhaust is obviously outside? 
Hopefully one of the great HVAC gurus on the board will chime in.
The Rinnai RU199i, RU199e, RU180i, RU180e, RU160i, RU160e, RU130i, RU130e manual says nothing about a maximum clearance between a two pipe venting set up.
Well, since you refuse to join Internachi and continue to squeeze education out of the Members for free, here is the source for you to research and come to your own conclusions…
Examining the maximum vent lengths for the two may give us a hint.
The appliance that specifies the max termination distance (first pic) has a shorter max pipe length and some other settings with dip switches to consider.
The appliance that does not specify the max termination distance (2nd pic) has a longer max distance and appears to be not as complicated having to change dip switches or anything.
Is it possible that a fan design or hardware change opened up more options for terminations, specifically differences in the pipe lengths?
2nd picture
What is the question?
The OP is gone.
What Jeffery attached is the beginning of the answer. 
ASHRAE wrote the Bible on this stuff and is what I was taught.
There is a bunch more that must be considered. You’re looking at Mfg Service Manuals which have already calculated their equipment design and performance. Fan performance has a lot to do with everything there.
Do you have something specific you don’t understand?
His questions were under the graphic in the first post.
He blocked me 
I suspect it’s to reinforce the pressure zone concept.
If they’re too far apart, and especially on difference faces of the building, then
wind could result in more pressure at the exhaust than at the intake. Too much of that and the appliance backdrafts.
What I really want out of these tankless units is a dip switch to set the MAXIMUM BTU. That way they can be installed in lesser gas pressure situations without risk of a bad flame. Good flame requires gas pressure, intake vent, exhaust vent, pipe size and environmental temperature to all line up.
A concentric pipe exhaust sort of sidesteps all this.
Ryan, depending on the unit’s design, the reason why the intake and exhaust should be in the same pressure zone is to avoid over or under pressurizing either the intake or the exhaust, causing the unit to fire improperly or shutting down completely due to an error code during wind loading (high wind days). If you have a 30mph wind blowing on, say exhaust but not the intake, the pressure inside gets imbalanced. For example, take an exhaust blower inside the unit, if the controller is monitoring its speed… imbalance of pressure can slow down the fan, causing the main board to throw an error code. With both intake and exhaust in the same pressure zone, high wind blowing at both balances things out inside, preventing this from happening. How they overcome this issue on units that don’t call for same pressure zone for intake and exhaust, I’m not sure of. I heard from techs that even those units that don’t call for it, those units can still go out of whack on certain windy days, the reason why it’s always best to install them in the same pressure zone to avoid issues on windy days.
Thanks, his first post is not showing up here for some reason.
My question was if you had a question I could help you with?
Not in particular. I just have the same curiosity as the OP. I guess the one question that comes to mind is if the exhaust and intake must be in the same pressure zone, what are the implications with a non-direct vent installation? Obviously an intake located in the basement near the furnace is not in the same pressure zone as the exhaust which will obviously be outside, sometimes way up at the roof.
That would be correct. 
I think these measurements listed have to do with the same pressure zone issue.
However, the “Max Distance” measurement is not clear as to the purpose.
I think the point is, if the two terminations exceed this distance, the length of pipe differential would become greater. It would also require both to be located in the same pressure zone.
They do not discuss this further. There is no design standard that requires keeping them together. Too close, has had historical problems from snowballs blocking air flow. Discharge gas has water vapor, and we sure as hell don’t water in the intake side.
This system design and operation addresses many things which create ppotential problematic conditions. It’s a finicky design.
May just be a typo.
BTW: Seeing this as a HI, without the system not responding to normal operational controls, should not be addressed in a HI Report.
