On Noritz Tankless why can first vertical run of vent be 3' or less?

[billhayton]

Installing Noritz 931 and read the first vertical run cannot exceed 3'..if you add a horizontal section and then go vertical again that ok.

Any reason for this...does too much vertical venting cause suffocation of unit?

[GrumpyPlumber]

Most likely for condensate, high efficiency gas units leave a lot of water behind in the exhaust.

[srdenny]

Put in a vertical condensate tee and all will be well.

[billhayton]

Makes sense the condensate accumulation is not something noritz or anyone else wants running back to unit.

thanks for the posts