In checking the Rinnai heater guidelines the total equivalency cannot exceed 41 feet. This is not straight run of the pipe but includes all the elbows and the termination. 45 degree elbows = 3 feet and 90 degree elbows = 6 feet. Termination = 3 feet. These are special pipes that are both exhaust and intake. Find the local dealer for your water heater and have him go over the installation specifications and then decide what kind of water heater you want.

Well are you sure you have all the necessary piping already for a tankless water heater? Mostly an appropriate gas line. Remeber these water heaters are up at 200,000 btu’s, efficient yes but you can’t just slap one anywhere. You could buy and o.d. model. Mounted ”out door” or move the piping to the water heater. Depending on where you live you can direct vent a tankless water heater to the exterior. The flue material is quite expensive and proprietary to the brand of heater you buy. If you vent to the exterior you need to be 4ft from any operable window. Remember as well these babys get hot so limit your horizontal runs as much as possible, if you touch your head or neck to the flue your gonna get burned. Best case pick a wall that has good area (no windows within four feet, or doors) mount the heater to some plywood. One flue 90 a short transition piece and the correct manufacturers termination piece. Done.

By the by tankless water heaters require a pressure relief valve. This must go to the exterior as well, just another reason to pick a good spot. Good luck.

The installation guide for whichever brand you are considering will have these guidelines.Check the websites and they can be found there.
Regular tank water heaters are up to 95% efficient these days.

you’re gonna find that a tankless heater should be mounted to the wall and it may be easier to move the gas line than the vent. however, , w/o asking a furnace/heater expert, i’d say you should be fine w/o moving anything.