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Mod/Con equivalant vent length

aemeeich Member Posts: 16
When figuring out the equivalent vent length of a venting system, do you add up both the intake and exhaust pipes, or do you just add up 1 way?

For example, say the exhaust pipe has a total of 20ft of straight pipe, and (4) 90° elbows. 

The intake pipe also has a total of 20ft of straight pipe and (4) 90° elbows.

When looking at the manufacturer's max vent length - in particular the Lochinvar Knight WBN-081 which says for 2" vent a max equivalent of 40ft - would the above example count as 40ft, or 80ft?

Thanks for your help,



  • Ironman
    Ironman Member Posts: 7,399
    It Applies...

    To one pipe, not both added together. The "vent" pipe is the exhaust, not the intake. Since both pipes should terminate in the same pressure zone for direct venting, it is only necessary to calculate the "vent" pipe.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jack
    Jack Member Posts: 1,047
    The way

    We do it with the Rinnai's when the two pipe option is used is to figure each pipe individually for fitting equiv, pipe length and then X .5. Doing each X.5 accounts for any discrepancies in the pipe/fittings used
  • aemeeich
    aemeeich Member Posts: 16

    Thanks for your guidance.  Sorry about the delay in responding - I wasn't able to log in the past couple of days. 
  • Steve_210
    Steve_210 Member Posts: 646
    split venting

    some manufactures do allow split venting (not in the same pressure zone)
  • bob eck
    bob eck Member Posts: 930
    length of vent pipe

    Take a look at using Triangle Tube Prestige Solo or Prestige Excellence 95% AFUE condensing gas boilers. With 4" PVC you can go to 100' (must add 90 ells and 45 ells to determine total lengths of the exhaust pipe only.) With there PS250 it only goes 60' These boilers do NOT have to have the vent pipe and fresh air inlet in the same pressure zone. Great boilers. 
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