Hi,
I am looking for some advice on my first time hot water tankless installation to look professional.
Here below is the illustration showing what I am thinking.
There are 2 existing vents to outside of the house above ground that I would like to use. However they are little large @ 6in and one of them is really down below close to the ground.
- Is there any problem to use a 6" to 2" reducer PVC mounted to existing sheet metal?
- Any issue if I use the very bottom vent for air intake? Will the air flow through like this?
Can I put one inline duct fan in the air intake just to make sure air coming in and up to the Tankless?

For the water part, can I use PEX tube (after the existing shut off valve) for cold and hot water?
For gas, the main supply is 1.5" I think and the soon to be removed old water tank use 1/2". I understand the tankless manual says must e 3/4". Can I just connect the flex hose to a very short run (1feet) of existing 1/2" pipe? The reason is the gas valve is after the 1feet long 1/2" pipe.