Hi,  I have a bit of radon coming though the basement.  Measured between 4-8pci (averaging at 6).  I’d like to start with the least invasive method of mitigation, meaning sealing the sump and other openings in the slab (cracks and a cutout for a shower (that will be sealed with a sump cover blank).  This means no ventilation (I’ll ventilate if this isn’t enough).

My question is, do I need ‘make-up-air’ for the sump, (as it’s pumping water out, so will be depressurizing the sump pit and associated drain tile)?

My gut says no, since the volume of air is high enough, and air will still get in via whatever means.

A second question, can both a tankless gas water heater and gas furnace be supplied via a single duct from the outside? (cross sectional area would be the same (two 3 inch to a 4 or 5 inch, or something.)  I do plan on replacing with energy/green options eventually, but didn’t get much choice with where I’m starting.