Direct vs non-direct venting for tankless

[iperiod]

I'm looking at replacing my old 50 gallon water heater (propane) with a Navien tankless unit, and I have a question about venting. The manual for the NPE-210 S2 says that it can either be set up with direct venting or non-direct venting. Direct venting requires two pipes to the outside of the house - one for intake, one for exhaust - while non-direct venting has just one pipe (exhaust) going outside, with air intake coming straight into the top of the unit from whatever room it's in - but the room has to meet certain size requirements. I'd much prefer to have only one vent pipe, but I'm wondering if there are any advantages to direct venting versus non-direct? I'm also wondering, since the room where the tankless heater will be installed does not have a door that closes it off from the rest of the house, can I assume that the unit can pull air from the entire house? Thanks for your help!

 

[Reach4]

With a single pipe, exhaust gets blown out. That creates a vacuum, and that sucks outside air in thru cracks. Hot sucked in in the summer, and cold in the winter.

 

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[Fitter30]

You buy a condensing heater for the efficiency why not use all of it. Be sure to do all the maintenance it takes and look at yearly cleaning on you tube.