Can main water be turned off with tankless gas water heater?
 

We are leaving for several months and want to turn off the main water valve to house. But we have a gas tankless on demand hot water heater. Do we need to leave the main water valve on so as not to damage the gas tankless on demand water heater?
Thank you.

You just turn off the water heater then your water shutoff.
We do this any time we go out of town.

I would suggest contacting the manufacturer of the tankless water heater to see what is the proper procedure to follow, I am betting it is different for gas and electric water heaters.

It's highly unlikely to be a problem leaving the tankless water heater on, but it's a safe precaution to shut it off if you shut off the water.

The tankless needs to see flow to turn on, so it likely won't fire when your water is off. But if anything happens, someone opens a faucet or something, or air seeps in, there could be something confusing the heater and it fires up without flow. That could cause damage. While very unlikely, it can't hurt to just turn it off.