RitchieYellow Jacket 78060 Complete Test Kit, 0-35" W.C

what iam want to do is check how much pressure is coming in to my tankless hot water heater it is not working properly will this tester work

How many inches water column is this rated at?

Can this be used for checking if vacuum pressure is good on a furnace / pressure switch ? If not what do I need to get?

What size threads is the brass connector ?