The first tankless water heater dates back to somewhere around the Roman days, but not quite what you might expect or think of and there most certainly was not any plumbing to speak of.
Another type of water heater developed in Europe predated the storage model. In London, England, in 1868, a painter named Benjamin Waddy Maughan invented the first instantaneous domestic water heater that did not use solid fuel. Named the geyser after an Icelandic gushing hot spring, Maughan’s invention made cold water at the top flow through pipes that were heated by hot gases from a burner at the bottom. Hot water then flowed into a sink or tub. The invention was somewhat dangerous as there was no flue to remove heated gases from the bathroom. A water heater is still sometimes called a geyser in the UK.
Maughn’s invention influenced the work of a Norwegian mechanical engineer named Edwin Ruud. The first automatic, storage tank type gas water heater was invented around 1889 by Ruud after he immigrated to Pittsburgh, Pennsylvania.
Another company that made great strides in producing the tankless water heater was formally known as Rinnai & Co., the company was co-founded by Kanekichi Hayashi and Hidejiro Naito. The origin of the business started in November 1918, stemming from Hidejiro’s inspiration from the blue flames emitted from an imported oil-burning imagawayaki cooking stove. Using the particular stove as a model, Hidejiro subsequently developed a petroleum-fuelled stove.
In 1920, Hidejiro left his job at Nagoya Gas – currently Toho Gas Co., Ltd. – and established Rinnai & Co. together with Kanekichi, a childhood friend who lived in the same dormitory. The company name was coined from characters in the two founder’s last names – “Rin” (it’s another way of reading “Hayashi“) and “Nai” (from “Naito“).
Tankless water heaters are and have been more predominant in Europe and Asia, not really getting a market share in the US for some time to come. It’s also worth noting that the Asia Pacific water heater market exceeded six billion in 2019, and the annual installation is expected to exceed 56 million units by 2026. Rapid industrialization across the developing countries coupled with growing urbanization owing to increasing standard of living and growing income levels is anticipated to augment the product demand. Ongoing requirement of hot water across single & multifamily apartments coupled with shifting consumer trend toward the usage of smart water heating systems will boost the industry growth.
The thermodynamics and economics of tankless water heaters only makes sense and considerably surprising to me that the US took so long to catch on, regardless of whether there was/is an abundant supply of fuels.
Water typically enters residences in the US at about 50 °F (10 °C), depending on latitude and season. Hot water temperatures of 122 °F (50 °C ) are usual for dish washing, laundry and showering, which requires that the heater raise the water temperature about 72 °F (40 °C) if the hot water is mixed with cold water at the point of use. The Uniform Plumbing Code reference shower flow rate is 2.5 US gal (9.5L) per minute. Sink and dishwasher usages range from 1-3 US gal (4-11L) per minute.
Natural gas is often measured by volume or heat content. Common units of measurement by volume are cubic meter or cubic feet at standard conditions or by heat content in kilowatt hours, British thermal units (BTU) or therm, which is equal to 100,000 BTU. A BTU is the energy required to raise one pound of water by one degree Fahrenheit. A US gallon of water weighs 8.3 pounds (3.8 kg). To raise 60 US gal (230L) of water from 50 °F (10 °C) to 122 °F (50 °C) at 90 % efficiency requires 60 × 8.3 × (122 − 50) × 1.11 = 39,840 BTU. A 46 kW (157,000 BTU/h) heater, as might exist in a tankless heater, would take about 15 minutes to do this. At $1 per therm, the cost of the gas would be about 40 cents. In comparison, a typical 60 US gal (230 L) tank electric water heater has a 4.5 kW (15,000 BTU/h) heating element, which at 100 % efficient results in a heating time of about 2.34 hours. At $0.16/kWh the electricity would cost $1.68.
No matter which way water is heated, there are still safety concerns to bear in mind. The risk of scalding from excessively hot water greater than 131 °F (55 °C), and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them. Both risks are potentially life threatening and are balanced by setting the water heater’s thermostat to 131 °F (55 °C). The European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease recommend that hot water should be stored at 140 °F (60 °C) and distributed so that a temperature of at least 122 °F (50 °C) and preferably 131 °F (55 °C) is achieved within one minute at points of use. Tank temperatures above 140 °F (60 °C) may produce limescale deposits, which could later harbor bacteria, in the water tank. Amazing to think that anything could survive in water that hot, it is all part of our wonderful world.
Having a water heater that is too small will result in it not being able to produce enough hot water for your household demand. For example, you might not be able to run more than one shower at a time without someone getting an arctic blast of cold water.
On the other hand, a water heater that is too large will end up being more costly than necessary, since you will be paying more upfront for the larger size. To work properly, a tankless water heater needs to be just the right size to meet your home’s hot water demand and preferably located at the point of use.
The good news for consumers is that the tankless water heater has evolved considerably, bringing the price point and use to a very reasonable price. For example, the Panasonic Bathroom Shower Heater DH-3JL2P will set you back about $150 US (7,226p) while the Imarflex Shower Heater ISH-3500BP will set you back about $242 US (11,658p). Both are a good choice for use in the shower and there are many other smaller tankless water heaters that can be used at the kitchen sink or next to the washing machine.
One advantage and disadvantage to having more than one tankless water heater is that you are only using what you need at any one point in time at any one place in the house as well as if one stops working, the other two are more than likely to still be working, but the disadvantage comes from more upkeep and cost of repairs. None-the-less, having tankless water heaters in your house is a smart investment in the long run.