First things first, this is not an endorsement for or against tankless water heaters. It is only information pertaining to them for your consideration if you are contemplating the purchase and installation of a tankless water heater.
In addition, the only “green” consideration in this discussion is the color of money since that is relative to everyone. There are probably not any two individuals that have the same definition of “environmentally green”, so we really cannot effectively discuss that.
Tank Type. Tank type water heaters have a tank for storing heated water. When a demand for hot water is created, i.e., taking a shower, using the dishwasher or clothes washer, etc., hot water flows out of the tank to supply the demand. As hot water flows out of the tank, replacement cold water flows in. When the cold water entering the tank reduces the temperature to a set level, the burner comes on to heat the water in the tank. In addition, if the water in the tank sits long enough, it will naturally cool, if the cooling reduces the temperature to a set level, the burner comes on to reheat the water being stored.
Tankless. Tankless water heaters do not have a tank for storage of hot water. When a demand for hot water is created, water flows into the water heater, triggering the heating elements or burner. As the water is heated by the burner, it flows to where the demand is located. When the demand is turn off, the heating system also turns off until the next demand is created.
Tankless water heater manufacturers state that tankless units use from 30 to 50 percent less energy than tank type units. National studies have shown that heating water typically accounts for about 30 percent of the average home’s energy budget. This energy savings of 30 to 50 percent applies to only 30 percent of the bill, not your total energy bill. Again, national studies say this translates to approximately $100.00 in annual savings for the average household.
Installation of a tankless water heater in an existing house is considerably more complicated than replacing an existing tank type water heater with a new tank type. Most tankless water heaters require a different type of vent piping, i.e., double wall vents and some even require stainless steel vents. Tankless water heaters also have a high demand heating unit, which could require a larger gas supply be installed. Water piping will more than likely need to be reconfigured. Electronic ignition elements also required electricity, so you may need to have electrical work performed to complete the installation. The installation of a tankless water heater is not recommended for the normal home owner unless you have professional skills.
Tankless water heaters required considerably less room than traditional tank type water heaters.
Tankless water heaters do not run out of hot water, they continue to produce hot water as long as there is a demand. The water temperature may reduce some if the demand is extremely high for a prolonged time, but hot water will continue to be produced.
Tankless water heaters do not start producing hot water unless a minimum flow rate is in demand. If you need a small trickle of hot water for some reason, you may have to increase the amount of flow in order to activate the heating cycle.
Tankless water heaters may require more maintenance that conventional water heaters. Calcium build up from hard water decreases efficiency, restricts water flow and damages tankless units much more than in tank type units. Annual cleaning is recommended for tank type heaters, but may be very necessary to tankless units.
Natural gas tankless water heaters require electricity for their electronic components, so if there is a power outage, they will not produce hot water.
There are many very good references for water heaters, both tankless and tank types, on the internet. If you are considering moving to a tankless water heater, it may be of great benefit for you to spend some time in research to assist in making the correct decision for your specific needs.