Hot water reset, also called outdoor reset (ODR), is an energy-saving

automatic control Automation describes a wide range of technologies that reduce human intervention in processes, namely by predetermining decision criteria, subprocess relationships, and related actions, as well as embodying those predeterminations in machines ...

algorithm for

heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...

s that are typically fired with

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

oil or

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

. A hot water reset control loop measures the outside air

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

; 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 Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...

ratio to outside air temperature. The typical range for conventional boilers is to vary the supply water temperature from as the outside temperature varies from . Buildings cool down slowly when the outdoor temperature is moderate, and cool more rapidly when the outdoor temperature is colder. Hot water reset reduces the energy output by the heating system to approximately balance it against the energy lost as the building cools. This makes it easier to maintain the temperature within the desired range (as the radiant heating systems that boilers usually power respond slowly and are subject to overshoot) and can allow boilers to operate more efficiently.

Implementation

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

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

temperature for non-condensing boilers, typically above .

Condensing boiler Condensing boilers are water heaters typically used for heating systems that are fueled by gas or oil. When operated in the correct circumstances, a heating system can achieve high efficiency (greater than 90% on the higher heating value) by cond ...

s can be made to operate at temperatures below the flue gas condensation limit, typically below and raise stated efficiencies from 70-80% to around 90-95%. #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. Another method is to adjust boiler temperature from thermostat activity. This is indoor reset control. It is more responsive to real needs inside the building. {{DEFAULTSORT:Hot Water Reset Heating, ventilation, and air conditioning