Hot water from a boiler or hot water heater is the medium for heat transfer in a hydronic system. Radiators are the most common source of heat transfer, but radiant loops are gaining popularity. Installation and materials costs have made aluminum finned copper tubing the most effective style. Once the heat loss of a room has been determined, a measured length of baseboard (low output or high output) or the correct size free standing cast radiator can be installed. Baseboard, whether copper or cast iron, is run in a continuous loop from room to room, but the larger cast standing radiators work better as a supply and return loop or as a monoflo loop. Using a fitting called a monoflo T, One pipe can feed the entire loop of standing radiators. Unique but simple, the monoflo T uses an orifice placed to one side of the t arm to restrict the flow of water through it. Down side of the flow it forces water into the arm. Upside of the flow and suction is produced. Water is forced in and out of the radiator without valves or a separate pump.

Using the concept of warm feet as an important comfort factor, radiant heating is becoming more popular, but continues to be more expensive to purchase and install. It can be stapled to the underside of the sub-floor, embedded in a concrete slab, or strung between the ceiling joist to radiate down from the ceiling for double the pleasure or when refitting a room with an existing slab floor. When embedded in a concrete slab, response time is slow and gradual, so the water must circulate constantly and the thermostat set at the desired temperature and left alone.