Articles by John Siegenthaler, P.E.

Figure 1 shows a hydronic system that’s intended to supply four panel radiators, each with their own thermostatic radiator valve and an indirect water heater from a gas-fired sectional cast-iron boiler. The system is designed using primary/secondary piping. The primary circulator operates whenever the space heating load or the indirect water heater call for heat. 

In hydronic systems, the circuit usually passes through the boiler, which adds heat. When using a block heater and the boiler is off, it dissipates heat from the circuit. This heat loss must be considered when measuring the circuit's true "net" heating output.

Figure 1 shows the piping layout for an actual installation. It uses two boilers, fired in stages, to supply three areas of a building. One of those areas is sub-divided into three zones. The other two areas are handled by individual zone circulators. The "primary circulator" operates whenever any load circuit is on. All circulators are fixed speed. There's a partially closed ball valve at the end of the "primary loop."

Modern hydronic systems benefit from wider temperature drops, energy scavenging, and careful integration with domestic hot water.