Some local jurisdictions have specific venting requirements for tankless gas
units. You would be wise to check into those venting requirements before
purchasing your water heater. In my county, the building and codes venting
requirements for propane and natural gas tankless water heaters are so strict
as to almost eliminate their installation inside a home. Cases like that are
where the external units like Rinnai’s model 2020 and 2520 come in handy.
As with any appliance or piece of mechanical equipment, misuse or abuse can
cost you money. If you keep your water heater’s thermostat turned up too high,
requiring you to add cold water to your shower temperature, for instance, you
will waste energy. If you take advantage of your water heater’s ability to
provide unlimited hot water by regularly filling your hot tub or jetted tub or
taking extremely long showers, you may also experience increased utility bills.
Don’t worry about your dishwasher; they all have their own heating elements to
boost the incoming hot water temperature to that required for proper cleaning.
Some people confuse tankless water heaters with instantaneous hot water
heaters. Tankless water heaters do not provide ‘instant’ hot water. If your
water heater is at one end of your house and your bathroom is at the opposite
end, it may take a few seconds or longer for the hot water to arrive at your
faucet. A tankless water heater will not improve this kind of situation.