Hot water reset is an energy-saving automatic control

Automatic control

Automatic control is the application of concepts derived from the research area of modern control theory. Automatic control is also a technology for application of control strategies. The implementing requires prior of analyzing and modeling of the subject to be controlled...

 algorithm for hot water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 boiler

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

s that are typically fired with fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 oil or natural gas

Natural gas

Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

. A hot water reset control loop measures the outside air temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

; this information is used to estimate demand or heating load as the outdoor temperature varies. The supply hot water temperature is modulated up and down range in an inverse linear

Linear

In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

 ratio to outside air temperature. The typical range for conventional boilers is to vary the supply water temperature from 140 to 180°F as the outside temperature varies from 65 to 0°F.

Implementation

The control system can be made to modulate the supply water temperature in two different ways:

1. By acting as an operating control on the boiler burners, either modulated on/off, high/low fire, or fully modulating fire, depending on the burner construction. When modulating the actual boiler temperature lower, water temperature needs to have a low limit and be maintained above the flue gas
   Flue gas
   Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
   
    condensation
   Condensation
   Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
   
    temperature for non-condensing type boilers, typically above 140°F. Condensing type boilers can be made to operate at temperatures
   Operating temperature
   An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
   
    below the flue gas condensation limit and raise stated efficiencies
   Thermal efficiency
   In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...
   
   from the 85% to 95% range.
2. By acting as an operating control on a three-way powered mixing valve or proportional injection pump system that modulates the supply distribution hot water temperature. The mixing valve or mixing pump system recirculates the return water temperature and adds proportionally supply hot water from the boiler for tempering to achieve the desired supply water temperature.