Avoiding negative pressure – GreenBuildingAdvisor

I am in Zone 5a, CT.  My 1953 ranch house has no VAC (Heating with new Navien LP tankless water heater and rads).  The house is very humid almost year round, in spite of all efforts to insulate and remediate. Crawlspace is covered in thick plastic and has a Santa Fe dehumidifier running 24/7/365.  

I would like to connect the aforementioned dehumidifier to a couple of supply lines (bedrooms) and returns (hallway and living room), as detailed as an alternate installation by Santa Fe. 

I am concerned about creating negative pressure due to the LP tankless water heater.  Should I be?  How can I manage this, if so?