Radiant & Hydronics

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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. 

The upward pricing trend that carried into the new year is continuing through February and into March, with a steady stream of plumbing, hydronic, HVAC and PVF manufacturers announcing increases. While many adjustments are landing in the low- to mid-single-digit range, several manufacturers are implementing category-specific or “various” increases, with select product lines moving higher.

AHR Expo 2026 made it clear that hybrid systems are no longer an emerging idea; they are the answer to the ongoing balancing act of meeting decarbonization goals while remaining realistic about the grid's capacity for electrification. Manufacturers emphasized systems that combine electric technologies, gas-fired equipment, and advanced controls.

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.

A case study of a ThermaCEILing radiant system in a Connecticut home.

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.