Hydronic Floor Radiant Heating

Heat transfer by radiant heating works through the flow of heat from the warmer material to the cooler, at a rate proportionate to the existing temperature difference. Radiant heating is the principle used in heat sources such as electric space heaters, sun lamps, fireplaces, open campfires, and so on. A hydronic radiant floor heating system uses either a heat pump, boiler or water heater to heat water, and some type of circulator which pumps the heated water through a series of tubes located underneath the surface of the floor.

There are two types of installations of these systems. In a “wet installation”, the tubing can be installed embedded in the concrete slab of a foundation or installed over a subfloor and then covered with a thin concrete slab. In a “dry installation”, the tubing is not installed inside or underneath concrete, which allows for retrofitting an existing floor. Pre-manufactured wood panels can be fitted to the subfloor, which contain routed grooves into which the hydronic tubing is laid; the finished floor surface attaches to the top of the wood panel.

System Components

Typically, a gas fired boiler or oil fired boiler are used as heat sources in a radiant hydronic floor heat system. The temperature range used in radiant floor heating systems, 85 to 140 degrees F, is considerable lower than other types of hydronic heating temperatures, which run from 130 to 160 F. That means that floor heating boilers run cooler and last a lot longer, saving you money. In areas where the cost of electricity are more economical, electric fired boilers are preferable, due to their smaller size which allows installation in more compact areas, not to mention the lower heating costs.

The coils of tubing through which the heated water runs are made of either copper, synthetic rubber or PEX plastic. A supply line carries heated water from the boiler to a distribution manifold and from there to the heating loops beneath the floor. A return line carries water from the loops to the manifold and back to the boiler to be reheated.
Copper tubing should be of soft tempered Type L, due to it’s workability, bendability and shapability. The diameter of tubing varies, depending on flow rate and friction loss of the system. Copper tube that is sold in coils needs to be straightened in a decoiling jig or device before bending to the desired pattern. The tubes in underfloor radiant heat systems typically have multiple U shaped bends in them which can be prone to leakage due to improper bending. For this reason, cross-linked polyethylene (PEX) tubing is becoming more widely used in radiant heating.

PEX tubing is made from high density plastic manufactured with a cross-linking process. It is used in residential and commercial plumbing, but for radiant heating applications such as interior underfloor heating, and driveway, sidewalk and patio de-icing, it is treated with a coating which acts as an oxygen barrier so that air will not penetrate the tubing and contaminate the water inside, causing corrosion of metallic fittings or valves in the system. The flexibility of PEX allows for easy installation and bending, and it is quite resistant to leaks caused by crimping or kinkage.

Manifolds and Valves

Hydronic radiant heating manifolds connect all the tubing lines in the system to single supply or return line. There is at least one supply manifold and one return manifold in each system. The supply manifold, when equipped with zone valves, can control the flow of heated water so as to distribute it to different zones of the floor, possibly located in separate rooms of the house.

Manifolds are available that include temperature gauges to measure the water in the tubing, built-in circulator pumps, water sensors for supply and return flow, air vents for purging air from the system, drain valves, and isolation valves to allowing servicing of the system.

Manifolds are available which electronically monitor and regulate the flow rate and temperature of the water in individual loops of the heating tubing. This ensures the efficient distribution of heat and gets rid of any hot or cold spots on the floor to maximize comfort. Troubleshooting problems is easier with this type of manifold as well, since they give quick and accurate feedback on what is happening in the system.

A wide range of valves and flow control components are used in hydronic floor radiant heat systems. There are isolator valves for isolating components like overflow tanks and circulation pumps to allow replacement or maintenance, pressure regulating valves, air vents, high temperature check gate valves called aquastats, backflow preventers, and zone valves with actuated motors, among others.


A circulator pump is used to force the water to circulate through the radiant floor heating system. As mentioned previously, some are built-in to the manifold, others are stand alone units. Variable speed pumps are sometimes used, in order to provide a wider range of temperatures, but most often the pump is of a single speed type. In certain zoned systems, a zone thermostat can control the operation of the circulating pump in order to balance and control temperatures between zones; in other zone systems, the temperature is controlled by motorized zone valves.

Sizing of hydronic heat circulator pumps depends on the required rate of flow that the water circulates, and the overall pressure drop of the system. In turn, the required rate of flow depends on the heating load and the temperature drop of the system. It is calculated by taking the total heating load, divided by a figure which represents temperature drop of the system, usually 20 degrees F, multiplied by minutes per hour, multiplied by weight in pounds of a gallon of water (temperature drop times 60 times 8); the resulting figure is the rate of flow expressed in gallons per minute.

All closed hydronic heating system must include an expansion tank. The tank provides extra space to store any increased volume of water created by the expansion of the heated water. In this way, potentially damaging stress on the system is avoided.