Hot Water Reset
Hot water reset is a way of controlling your boiler setpoint based on some variable, typically outdoor air temperature. This is especially useful for condensing boilers, but can also be applicable to other types of boilers as well. For the purpose of this article, we will focus on condensing boilers utilizing an outdoor air reset. The below picture shows one potential curve that could be applied, but these values can be tuned to fit your particular building.
As you’ll see, the y-axis is inlet temperature and not leaving temperature. Your boiler setpoint is equal to leaving temperature, but leaving temperature will generally result in a certain returning temperature under normal circumstances. The reason we focus on returning (inlet) temperature is because we are concerned with the water that enters the boiler and is heated. The red line shows the inlet water temperature we want to control the boiler to at each outdoor air temperature. When it is really cold, the boiler will need to operate at maximum load. This will have the water being heated as high at it can go (typically around 180° F) and return at around 150 °F. When it starts to warm up outside, instead of heating the water to 180 °F, lets try heating the water in order to achieve a return temperature that fits on the curve. Continue this trend until the boiler is operating at minimum load.
The main point is to run the boiler at the minimal level that will keep your building up to temperature. The reason for this can be illustrated in the picture above. As the inlet water temperature gets colder, the boiler operates more efficiently. When the water temperature crosses below the blue line (130 °F) in the first picture, the efficiency grows exponentially. At water temperatures below 130 °F, condensing boilers can recover energy from the exhaust air. The exhaust air contains water, as a by product of combustion, that can condense under cooler conditions. This condensed water will drip over a heat exchanger that preheats the water, thus reducing the necessary boiler input.