Water – How to determine tankless water heater requirements

Without natural gas or propane, your only source of heat is electricity. Electric tankless water heaters would probably require a new circuit to be run. Here's a table of typical power and wire requirements:

http://www.rheem.com/documents/rte-13-tankless-electric-spec-sheet

The smallest unit they recommend for a shower is the RTE13 which takes 13 KW or 54A@240V. This requires #6 wire. If your incoming temp is 40F and desired out temp is 110F, you need 70F of temp rise and the RTE13 will provide 1.27 GPM.

Many people think they want tankless because the mfg tell them tankless is more efficient. There is some truth to this for gas fired water heaters but in the case of electric water heaters, the difference in efficiency is very small.

It's mostly the gas pipe. A tankless water heater is going to need a 1" or even 1.5" pipe to feed it. Your current water heater probably uses a 1/2" pipe. The new pipe probably needs to come off the main trunk, and replacing it requires a precise gas volume calculation and gas plumbing work that is very skilled labor and not cheap. If your plumber can't do this work himself, he's probably subbing it out and you're eating the cost.

Secondarily, there's also the re-plumbing of the water lines to feed the new heater. The inlet and outlet ports are probably in different places compared to where they are on your tank water heater. By contrast, if you replace it with another tank water heater, the water lines are already in the right place, and all you need to do is connect the tank with flex lines.

Finally, the exhaust may be different, too. Particularly if you go with a high-efficiency model, it can't be exhausted through the current pipe that's likely going through the roof.