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.

Measuring the flow rate through a circuit has always been more of a challenge compared to measuring temperatures. Very few circuits are equipped with permanently installed flow meters.

Single-stage non-modulating heat sources in zoned distribution systems often face short cycling, particularly with low thermal mass. When few zones are active, they struggle to dissipate generated heat quickly, causing temperature build-up and potential shutdown of the heat pump based on internal limits, with varied restart behavior depending on controls.

Can you speculate why heat delivery in zone 3 was insufficient before the helper pump? How could the piping system be improved, considering the wasted length of PEX-AL-PEX tubing from the manifolds to the floor panels in figure 2?

Last month, we reviewed the basics injection mixing. It’s a method for controlling water temperatures by regulating the flow of heated water “injected” into a circulating distribution system, while simultaneously removing an equal flow of cooler water from that distribution system. Any method that controls the rate of injection flow also controls the rate of heat transfer into the distribution system.

For boilers operating on natural gas or propane, flue gas condensation begins at inlet water temperatures below about 130 ºF. Boilers operating on low sulfur #2 fuel oil have lower dewpoint temperatures in the range of 110 ºF.