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The control action is the switching on/off of the boiler, but the controlled variable should be the building temperature, but is not because this is open-loop control of the boiler, which does not give closed-loop control of the temperature. In closed loop control, the control action from the controller is dependent on the process output.
OpenTherm (OT) is a standard communications protocol used in central heating systems for the communication between central heating appliances and a thermostatic controllers. [1] As a standard, OpenTherm is independent of any single manufacturer. A controller from one manufacturer can in principle be used to control a boiler from another ...
The control action is the switching on or off of the boiler. The process variable is the building temperature. This controller operates the heating system for a constant time regardless of the temperature of the building. In a closed-loop control system, the control action from the controller is dependent on the desired and actual process variable.
Hot water reset, also called outdoor reset (ODR), is an energy-saving automatic control algorithm for heating 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 definition of a closed loop control system according to the British Standards Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero."
The boiler will also fire (regardless of thermostat state) when the boiler water temperature goes below a range around the low limit, ensuring that the boiler water temperature remains above a certain point. The low limit is intended for tankless domestic hot water; it ensures that boiler water is always warm enough to heat the domestic hot water.
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