I have a lot of experience with tankless water heaters, both gas and electric. I put them in about half my rehabs.
Pros - takes up less space, more efficient, unlimited hot water
Cons - generally more expensive up front, requires high amperage or a large gas line
Key when buying is to size it right; you’ll see a table on the spec sheet showing how much it will heat water at various flow rates. Take the max concurrent usage of hot water in the house and add that together to figure out what flow rate you need to size for. For example, 2 showers at 2.5gpm each, and 2 faucets at 1.5gpm each would put a house at a peak concurrency of 8gpm. Lookup ground water temp in your area during the coldest time of year, for example 50 degrees. For this example you’d want to find a water heater that is rated to heat 8gpm up by 70 degrees (to get 120 degree water)
You’ll also need to make sure you have either enough electrical amps available (these water heaters can take 60-80 amps) or if gas that you have enough gas volume, often 1/2 in pipe required for the larger units.
A standard gas tank water heater gas line will not have the capacity, and you’ll have to re pipe back to where it’s big enough.
If you know all these things upfront and purchase correctly they are really really great units. I’ve had good experience with both electric and gas Rheem models. An RTE-13 in a studio and an RTG-64DVLN in a 4bed/2bath single family house.
