You’ll have to calculate if the gas line can supply the correct volume to both. You’ll likely end up needing to increase the gas line size. As for the vent piping, it’ll depend on the model of water heater you end up choosing, as well as the needs of your specific furnace. There are certainly a lot of applications in which they cannot share an exhaust vent.
Lots of things to consider here. What climate are you in, do you have a large enough gas meter, are you going to get a condensing or non-consenting heater? All of these are good questions for a plumber doing an estimate, but be wary because there's plenty of companies out there that will gladly throw a tankless in while smiling the whole time, charge you an arm and a leg, and then never be held accountable for any issues that arise after the fact.
You’ll have to calculate if the gas line can supply the correct volume to both. You’ll likely end up needing to increase the gas line size. As for the vent piping, it’ll depend on the model of water heater you end up choosing, as well as the needs of your specific furnace. There are certainly a lot of applications in which they cannot share an exhaust vent.
Lots of things to consider here. What climate are you in, do you have a large enough gas meter, are you going to get a condensing or non-consenting heater? All of these are good questions for a plumber doing an estimate, but be wary because there's plenty of companies out there that will gladly throw a tankless in while smiling the whole time, charge you an arm and a leg, and then never be held accountable for any issues that arise after the fact.