​Storage-Tank Water Heaters

 

The dominant form of domestic water heating for most of the 20th century.

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Early forms: range boilers and stove water-backs (19th and early 20th centuries) stored limited amounts of water heated by a cooking stove. These were transitional systems rather than dedicated water heaters.

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Automatic storage tanks: Edwin Ruud’s 1889–1890 design introduced the automatic storage-tank gas water heater with thermostatic control. This became the template for the modern tank-type heater: a vessel of water maintained at a set temperature by a controlled burner or electric element.

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Key later advances

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• Glass lining of steel tanks (A.O. Smith patent 1936, commercial production from 1939) — the single most important material improvement for corrosion resistance and affordability.

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• Sacrificial anode rods for additional corrosion protection.

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• Progressive improvements in insulation (fiberglass, then polyurethane foam in the late 1970s).

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• Dual-element electric designs and better thermostats.

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• Temperature-and-pressure (T&P) relief valves are standard safety features.

 

Storage tanks remain widely used because of their simplicity, lower upfront cost, and ability to meet simultaneous demand. Their primary limitations are standby heat loss and finite hot-water capacity.

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→ Full article: History of the Storage-Tank Water Heater (coming soon)

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Tankless / On-Demand (Instantaneous) Systems

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The oldest continuous technological lineage in modern water heating.

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Origins: Benjamin Waddy Maughan’s 1868 Geyser was the first practical gas instantaneous domestic heater. It heated water as it flowed through pipes over a gas burner, but lacked a flue and automatic controls.

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Ruud’s contribution: In 1897–1898, Edwin Ruud patented an improved automatic instantaneous heater using a copper coil and a thermostatic gas valve. This added the safety and control features missing from Maughan’s design.

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Electric instantaneous: Dr. Theodor Stiebel’s work in the 1920s produced early commercial electric tankless units in Germany.

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Modern development: After mid-20th-century refinement in Europe and especially Japan, high-efficiency gas tankless systems (including condensing models) entered the broader U.S. market in the 1990s–2000s.

 

Digital controls, better scale resistance, recirculation options, and modulating burners addressed earlier performance complaints.

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Advantages: Unlimited hot water (within flow-rate limits), lower standby losses, longer potential service life, and smaller footprint.

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Trade-offs: Higher upfront cost, more complex installation (gas line sizing, venting, water quality), and sensitivity to hard water without proper treatment.

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→ Full article: History of Tankless and On-Demand Water Heaters (coming soon)

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Solar Thermal Water Heating

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One of the earliest renewable approaches to domestic hot water.

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Key milestones

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• 1767: de Saussure’s solar hot box (conceptual foundation).

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• 1891: Clarence Kemp’s Climax — first commercial solar water heater (batch tanks under glass).

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• 1909–1910: William J. Bailey’s Day and Night system — separation of outdoor collector from indoor storage tank, enabling continuous supply.

 

Solar water heating enjoyed strong adoption in California and later Florida until inexpensive natural gas largely displaced it. Interest revived with the energy crises of the 1970s and has continued with improved collectors (flat-plate and evacuated-tube), better freeze protection, and active circulation systems.

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Solar thermal remains a viable technology, particularly in high-insolation regions, often used today as a pre-heat system paired with a conventional or heat-pump backup.

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→ Full article: History of Solar Water Heating (coming soon)

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Heat-Pump Water Heaters and Hybrids

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A fundamentally different approach: moving heat rather than generating it directly.

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Heat-pump water heaters extract heat from the surrounding air (or sometimes other sources) and transfer it to the water. Early conceptual and experimental work dates to the mid-20th century, with limited commercial attempts in the 1950s–1980s.

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Widespread residential adoption began after ENERGY STAR certification started in 2009 and was accelerated by federal efficiency standards that effectively required heat-pump technology for larger electric storage units.

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Modern hybrid models combine a heat pump with electric resistance elements for backup capacity in cold conditions or high-demand periods. Coefficients of performance typically range from 2 to 4, producing substantial energy savings compared with conventional electric resistance heaters.

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→ Full article: History of Heat-Pump Water Heaters (coming soon)

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Materials and Construction

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Materials science has been as important as heating methods.

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• Early tanks: Galvanized steel, copper, or pure steel (prone to rapid corrosion).

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• Glass lining (1936 onward): Fused glass coating on steel — the industry standard for residential storage tanks.

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• Anode rods: Magnesium or aluminum sacrificial anodes to protect the tank steel.

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• Foam insulation: Major reduction in standby losses beginning in the late 1970s.

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• Stainless steel and advanced alloys: Used in many modern tankless heat exchangers and some premium tanks for superior corrosion resistance, especially in hard-water areas.

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• Heat exchangers: Copper coils in early instantaneous designs; stainless steel in most current high-efficiency tankless units.

 

Material choices directly affect longevity, maintenance requirements, and performance in different water-quality conditions.

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→ Full article: Materials and Construction History (coming soon)

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Cross-Cutting Themes

 

Several forces have repeatedly shaped technological change:

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• Safety (thermostats, relief valves, proper venting)

• Energy cost and efficiency regulations

• Water quality (especially hardness and its effect on scale and corrosion)

• Manufacturing scale and cost reduction

• Changing expectations of convenience and unlimited supply

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The gas-control valve (the actuator on a tank)

 

This is the part that made the automatic storage heater possible.

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1783. Jean Simon Bonnemain in Paris patented a “fire governor” that used differential expansion of iron and copper bars to regulate a calorifier fire. It is an early thermal actuator on a fuel supply, not yet a household water heater.

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1889–1891. Edwin Ruud, working in Pittsburgh for the Fuel Gas and Manufacturing Company (Westinghouse orbit), put a temperature-sensitive valve on the gas to a storage tank. U.S. Patent 443,797 (filed June 21, 1890; issued Dec. 30, 1890) is the foundational automatic storage heater: expansion rods in the water move a lever that opens or closes the gas valve and a burner plate as tank temperature changes. U.S. Patent 460,513 (Sept. 29, 1891) refined the regulator so small draws of water would not slam the burner the way a large draw did. That is the ancestor of every tank-type gas control.

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Ruud kept working on the same problem after he founded Ruud Manufacturing in 1897:

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• 610,281 (1898) — automatic instantaneous heater with thermostat-controlled gas valves

• 853,738 (1907) — storage heater with a rod-and-tube thermostat driving a snap-open / snap-shut gas valve

• 875,217 (1907) — “thermostatic-valve-operating mechanism”

• 1,257,932 (1918) — water valve for instantaneous heaters (flow, not just temperature, actuating the machine)

 

1920s–1930s. The shop form that lasted for decades is the rod-and-tube thermostat screwed into the tank: a copper or brass tube (expands) around an Invar rod (almost does not). Relative movement is only a few thousandths of an inch over a 15–20 °F band, so later designers added levers or snap disks to get a usable valve stroke. John H. Grayson’s water-heater thermostat patents (1,699,468, 1929; later snap-action work into the 1930s) sit in that line. Grayson Heat Control became the Grayson Controls division of Robertshaw. The familiar Unitrol combination control — thermostat, gas cock, pressure regulator, and pilot safety in one body — is that lineage productized.

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Safety add-ons, mid-century. Standing-pilot valves added a thermocouple or thermopile so that a dead pilot dropped an electromagnet and shut the gas. Energy cut-off (ECO) high-limits, thermal cut-offs, and later flammable-vapor ignition resistance (FVIR, ~2003–2005) were piled into the same housing. After 1979, heating combination gas valves had to be dual-seated.

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Late 20th century to now. Mechanical Unitrols gave way to sealed electronic combo valves — Honeywell WV8840-type, White-Rodgers IntelliVent, Robertshaw 3000 Series — powered by a thermopile, with an LED flash code instead of a rebuildable mechanism. The valve is no longer serviced; it is swapped as a unit. That concentration of function is why a failed “gas valve” today is expensive.

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The same family of controls had a dark chapter: Robertshaw Unitrol 110/200 and A-1 LP valves (1950s production), and some Honeywell V5130 LP water-heater controls were the subject of CPSC recalls and death/injury investigations when the pilot-safety function failed, and unburned gas reached the burner. Those cases are why modern valves are sealed, coded, and not field-rebuilt.

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The mixing-valve actuator (outlet temperature)

 

This is the other device people call an actuator valve. It does not fire the heater. It blends stored hot water with cold, so the house sees a safe setpoint while the tank can sit hot enough to discourage Legionella.

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1911. Frederick Leonard is credited with the first practical plumbing thermostatic mixing valve after a barbershop scald. Early Leonard valves used a bimetal coil — two metals bonded together that wind or unwind with temperature and move a slide or rotary valve. That coil technology dominated mixing valves for about sixty years. Leonard Valve Company still exists on that line.

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1934. Sergius Vernet invented the wax (paraffin) thermostatic element for automotive cooling. Wax melts and expands with a large force in a small capsule; a piston becomes the actuator. Plumbing was late to it. First sanitary/heating uses are usually dated to Sweden and Switzerland around 1970–1971. After that, the wax cartridge became the standard “thermal actuator” inside residential and commercial mixing valves (Watts, Honeywell, Caleffi, and others). Manufacturers still advertise an “advanced thermal actuator,” meaning that a wax motor.

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Standards that fixed the device in code:

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• ASSE 1016 (1973) — individual shower/tub-shower compensating valves (pressure, thermostatic, or both)

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• ASSE 1017 (1976) — temperature-actuated mixing valves for the distribution system (the master mixer at or near the heater)

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• Later ASSE 1069, 1070, 1071, 1082, 1084, 1085 split point-of-use, fixture, and water-heater-integral controls.

 

1939. Symmons pioneered the pressure-balancing shower valve. That is an actuator of a different kind: a piston or diaphragm that senses inlet-pressure change, not temperature. It stops thermal shock from a toilet flush; it does not, by itself, guarantee a safe mixed temperature. Later valves combined P and T functions.

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2000s onward. Digital mixing valves replace the wax motor with sensors and a motorized or solenoid-driven valve. Recirculation balancing valves such as ThermOmegaTech’s Circuit Solver (2011) use a self-actuating thermal element at the end of each branch.

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Tankless/instantaneous valves

 

Maughan’s 1868 Geyser had almost no automatic control. Ruud’s 1898 instantaneous heater added thermostat-controlled gas valves; his 1918 patent is specifically a water valve for instantaneous heaters. Modern tankless units split the job:

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• a flow sensor or flow switch (the water “actuator”) tells the board that a tap has opened

• a modulating gas valve (often a proportional solenoid or stepper-driven valve) matches burner rate to flow and setpoint

 

That pair is the current form of Ruud’s original idea: water movement and water temperature both actuate fuel. Scale on the heat exchanger is why those valves and sensors fail early in hard-water country.

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How the pieces fit on a modern job

 

Era What “actuates” What it moves

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1889–1930s Expansion rod/tube in the tank, Gas cock to the burner

1930s–1990s Rod-and-tube + snap disk + thermocouple Combination Unitrol-style gas valve

1911–1970 Bimetal coil Mixing-valve shuttle

1970s–present Wax cartridge Mixing/tempering/balancing valve

1990s–present Thermopile + electronics Sealed electronic gas control

 

Tankless era Flow sensor + board, Modulating gas and sometimes water valves

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The short version: Ruud invented the water-heater actuator in the gas-valve sense in 1889–1890. Leonard invented it in the mixing-valve sense in 1911. Vernet’s wax motor, borrowed from cars in the 1970s, is what most people now mean when they point at the little thermal element inside a tempering valve and call it an actuator.