Hot Water in an Instant: Whole- or partial-house tankless water heaters

By Mark C. Ode | Mar 15, 2023
Tankless water heater against a yellow background. SHUTTERSTOCK / NSIT

Tankless water heaters were invented in 1868 by Benjamin Waddy Maughan, an English painter who patented the first “gas-powered” heater. He invented what he called the “Gas Geyser” instantaneous water heater. It did not include a vent for carbon monoxide, however, making it dangerous for daily use.




Tankless water heaters were invented in 1868 by Benjamin Waddy Maughan, an English painter who patented the first “gas-powered” heater. He invented what he called the “Gas Geyser” instantaneous water heater. It did not include a vent for carbon monoxide, however, making it dangerous for daily use.

The first commercially available storage-tank water heater was invented in 1889. It was built with thick copper coils and was very energy-efficient, but extremely expensive, and only the very wealthy could afford it. In the 1890s, there were many different water heater designs, including electric, solar and those fueled by gasoline, wood, coal or natural gas.

The rise of the water heater

Modern tankless water heaters were first introduced in 1970 and became popular in Europe as space-saving devices. In many European homes, each sink, shower and bathtub had its own “point-of-use” instantaneous water heater, which saved space and only heated when water was required at the specific area.

The heating system in my home when I lived in Hanover, Mass., was a combination boiler supplied by heating oil with a 500-gallon tank in the garage. Heat exchangers were located in each room. The combination boiler was also tied into the domestic water system and supplied hot water to the bathrooms and kitchen. That was my first introduction to a semi-instantaneous hot water system, since the bathtub, shower and various sinks were located fairly close the boiler room.

When I moved to Cary, N.C., the home I purchased had a storage-tank water heater in the attic. After a water tank failure in my neighbor’s attic flooded his two-story home and caused major water damage, I decided to replace my attic water heater with an instantaneous tankless one.

Location matters

Researching tankless water heaters, I discovered that these units were often located outside the home or in the crawl space below. Locating the instantaneous water heater outside was possible with adequate insulation and external fixed electric heating for the water pipes on the cold and hot water piping.

I also noticed in my research that there were “water heaters” and “hot water heaters.” I had not thought much about it up to that point, but I discovered that a hot water heater was supplied with hot water and actually boosted the temperature. The extra-hot water faucet is usually located at the kitchen sink for making hot tea and coffee without having to heat it in a kettle or microwave. The extra-high temperature water was usually tied into the dishwasher. I was only interested in water heaters where the water supply to the heater was cold water. 

My tankless water heater was destined to be installed in the attic as a replacement for the tank-type water heater. Determining the gallons of water per minute requires knowing the number of points of hot water supply and the water flow required for simultaneous operation of the unit, such as two to five showers, the dishwasher and the washing machine operating at one time (as a worst-case scenario). This can be a challenge. The manufacturers of these instantaneous water heaters provide tables online to help determine average gallons per minute, depending on the type of water load. These systems can save around 34% of the former energy cost of a tank system for whole-house hot water and up to 50% for point-of-use.

Installation concerns

Depending on the kilowatt load of the water heater and the voltage available, the electrical circuit can be anywhere from 29A at 120V for a 3.5-kW unit to 150A at 240V for large homes with a 36-kW unit. Individual branch circuit conductors for the instantaneous water heaters are required by 422.4(A)(1) to be sized at not less than the marked rating of the appliance. These instantaneous water heaters will not operate for three hours or more, so the conductors are not required to be sized at 125%, whereas a fixed storage-type water heater is required by 422.13 to have the conductors sized at 125%.

Branch circuit overcurrent protection is located in 422.11(F)(3)(2) and states that water heaters with resistance-type immersion electric heating elements are permitted in circuits not exceeding 120A and protected by an overcurrent device not greater than 150A. Some of these units, such as the 36-kW unit, may have subdivided and staged resistance elements with up to four 40A overcurrent protective devices to the unit. If an existing tank-type water heater is being replaced, make sure there is enough load capacity at the service and spare space in the panelboard.

Header Image: shutterstock / Nsit

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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