Hot water reset

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Hot water reset is an energy-saving automatic control algorithm for hot water boilers that are typically fired with fuel oil or natural gas. A hot water reset control loop measures the outside air temperature; 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 ratio to outside air temperature. The typical range for conventional boilers is to vary the supply water temperature from 60 .. 82 ⁰C as the outside temperature varies from +18 .. -18 ⁰C.


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 condensation temperature for non-condensing type boilers, typically above 60 ⁰C. Condensing type boilers can be made to operate at temperatures below the flue gas condensation limit and raise stated efficiencies from the 85% - 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. Another method is to adjust boiler temperature from thermostat activity. This indoor reset control. It is more responsive to real needs inside the building.