How Radiant Heating Works — Phillips Hydronics

How it works

From the boiler to your floor — one quiet, sealed loop of warm fluid

Every hydronic system has four subsystems: a heat source (boiler or heat pump), a distribution system (piping, circulator, valves), heat emitters (radiant floor tubing, baseboards, fan-coils, or panel radiators), and controls (thermostats, sensors, mixing logic). In a radiant floor system, warm fluid — usually water, often blended with antifreeze (propylene glycol) — circulates through a sealed, closed-loop circuit: the boiler heats it, a mixing valve tempers it, a circulator moves it, a manifold splits it across the floor, and PEX tubing in the slab warms the whole room from the floor up. Step through each part below.

Warm supply fluid Cooled return fluid Heat radiating into the room

Additional essential components. A complete installation includes several more safety, control, and maintenance components beyond the core loop shown above:

Pressure-relief valve — a code-required safety device (typically rated at 30 psi) mounted on or near the boiler; opens to prevent dangerous overpressure if controls fail.
Air separator — creates a region of reduced pressure that causes dissolved gases (oxygen, nitrogen) to form bubbles, which are collected and vented automatically. Placed where fluid temperature is highest, on the supply side of the heat source.
Flow-check valve — prevents unwanted thermosiphoning (gravity-driven flow caused by hot water rising) when the circulator is off, so heat doesn’t migrate into rooms uninvited.
Make-up water assembly — a pressure-reducing valve (feed water valve), backflow preventer, and shutoff valves that maintain system pressure and allow safe refilling from the domestic water supply.
Purging valve — allows air to be flushed out when the system is first filled with fluid.
Room thermostat & temperature-limiting controller — the thermostat signals when heat is needed; the temperature-limiting controller keeps the boiler within its safe operating range.

Every component is sized to your space: heat-loss calculations, water temperature, loop lengths and flow rates, circulator head, and tank sizing are all engineered by your Phillips Hydronics installer. The exact piping configuration varies by home. It’s precise work — and exactly why it’s done by a professional.

Piping Configurations

Multiple ways to distribute the heat

The diagram above shows a home-run layout with a manifold — the most common for radiant floors. But hydronic systems can be piped in several configurations, each suited to different building layouts, heat emitter types, and zoning needs. Here are the most common options:

Series Loop

The simplest configuration. A single piping loop connects heat emitters one after another — fluid flows through each in sequence. Low cost and easy to install, but every emitter must be on the same loop, and downstream emitters receive progressively cooler fluid. Best for small, simple zones with finned-tube baseboard.

One-Pipe (Diverter Tee)

A main loop runs through the building with diverter tees at each heat emitter that route a portion of the flow through the emitter while the rest bypasses it. Allows individual emitters to be valved off for zone control without affecting the main flow. Common in older and retrofit systems.

Two-Pipe (Direct & Reverse Return)

Separate supply and return mains run in parallel. Each heat emitter connects between them, receiving fluid at the same temperature. Direct return sends the return back the shortest path; reverse return balances piping lengths so every emitter sees nearly equal flow resistance. Ideal for larger buildings with many emitters.

Home-Run (Manifold)

Each heat emitter (or radiant floor zone) gets its own dedicated supply and return tube running back to a central manifold. Every loop can be individually balanced and controlled, making it the go-to choice for radiant floor heating and multi-zone comfort. This is the configuration shown in the diagram above.

Primary / Secondary

A primary loop circulates fluid continuously through (or near) the heat source; secondary loops branch off through closely-spaced tees, each with its own circulator. This provides hydraulic separation — the flow in one secondary circuit doesn’t affect the others. Essential for complex, multi-load systems combining radiant floors, domestic hot water, snow melt, and more.

Hybrid Systems

Many real-world installations combine piping strategies — for example, a primary/secondary layout where one secondary circuit feeds a manifold for radiant floors while another supplies panel radiators in a two-pipe arrangement. Phillips Hydronics designs each system around the specific needs of your home.

Which piping layout is right for your home? That depends on the type and number of heat emitters, the number of zones, the heat source, and the building layout. Your Phillips Hydronics installer evaluates all of these factors and engineers the best configuration for your space.

How hydronic radiant heating works