Can I Combine Two Direct Vent (PVC) Appliances?

Discussion in 'Water Heater Forum, Tanks' started by RockRiverShooter, May 19, 2014.

  1. RockRiverShooter

    RockRiverShooter New Member

    Jul 8, 2013
    Hello All --

    Quick question I cannot find an answer for:

    I recently installed a new High Efficiency (2-pipe - PVC) Direct Vent furnace. The unit is 96+ 100,000 BTU. The installation specifications indicated I was required to only use 2" PVC for the length/#'s of elbows of my run. As an upgrade, I increased the size to 4" PCV as soon as the vents exit the furnace.

    I would like to install a direct vent condensing (2-pipe PVC) tankless water heater next to the new furnace and combine the exhaust and intake of both units. The specifications of the tankless water heater indicate I would only require 2" PVC for the length of run. My plan is to splice a 4X4X2 Wye (in correct direction of airflow to outside) into the main 4" runs going to the outside of the house.

    Some manufacturers like Rinnai and Navien make references to combining the venting for multiple units. Here's a pic of two units combined. Not sure why it wouldn't work with a HE furnace and Tankless. With both units operating, I really don't think there would be any static over pressure issues with the 4" vents. Of course all runs would be slopped correctly to allow condensate to flow.

    My only concerns would be if there is any problem with only one unit running. Would it backfeed into the other? I assume the combustion chambers for both appliances are sealed and the exhaust/intake would find the path of least resistance and suck/blow from the outside.

    Appreciate any feedback.. The exterior of the home is brick and the protrusions from the first two PCV vents were very difficult. Really do not want to have 4 holes on the side of the house!!

    Thanks so much!!


  2. zl700

    zl700 DIY Senior Member

    Aug 26, 2009
    Western NY
    You may never combine category IV positive pressure commonly vented with plastic vents according to most manufacturers.
    This will be stated in the installation instructions. Doing so will potentially exhaust dangerous CO gases into the other non-running appliance.
    The Naviens' above may because they work together and have a cascade cable to operate the fans together along with check valves in the fans to maintain positive draft.

    Read the instructions, before someone dies.
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