there are two issues often over looked with tankless water heater installations,
1. make up air for combustion, depending on the btu size of the tankless water heater it can be very large, and typicly its open to outdoors 24/7 year around
2. the correct size of the gas line, if you can get a copy of the installation manual and see how much work it will be to run that gas line over to where you need to install the water heater.
these are the two most common things we see and are asked to repair at our company, the make up air issue can be life threating, the gas line will just cause you service issues and mad wife issues when she doesnt have hot water.
in the early eighties we replaced lots of tankless water heaters and installed conventional ones, in the past parts were very hard to get, of course a lot has changed since then, but we are still taking a "wait and see" approch to tankless water heaters.
if there is something i can do to assist, with this or any other house hold heating and a/c problems just let me know
sincerely
duane