Last month we discussed a scenario in which a new pellet boiler would be added to an existing hydronic system with an oil-fired boiler. The piping for this system is shown in Figure 1.
The control logic for this system was based on a pellet boiler that operated under its own internal controller. That controller monitors two temperature sensors within the thermal storage tank. When the temperature at the upper sensor (S1) drops to some lower setpoint (such as 150° F), the pellet boiler fires. It remains in operation until the temperature at the lower sensor (S2) has climbed to some high temperature limit (such as 180°). This stacks the tank full of hot water before the boiler is turned off.