Hot water source

Most existing greenhouse boilers are the non-condensing type f a boiler system is heating the greenhouse, it can probably think be modified to deliver the 100oF-120oF water needed for root zone heat. The existing boiler capacity should be adequate as the heat is just being redirected from fin or pipe radiation to the root zone area.

If the greenhouse has a condensing boiler, one in which the boiler can operate safely with a return water temperature of less than 140oF, then the high limit switch is set to the highest temperature water that is desired in the root zone piping. If a boiler is to be used for both high temperature (180oF) and root zone heat, then a bypass loop and mixing valve are necessary to achieve the low temperature water.

Most existing greenhouse boilers are the non-condensing type and require the above bypass loop and mixing valve to keep the water temperature in the boiler above 140oF. A three-way valve is commonly used to mix the return water from the root zone with the hot water in the boiler to achieve the desired root zone water temperature.

For providing root zone heat to a hoop house or small bench area, a domestic hot water heater does an excellent job. These heaters fired by gas, oil or electricity are available in capacities of 30,000 to 40,000 Btu per hour and can heat up to 2,000 square feet of bench area. Multiple heaters or larger commercial water heaters and instantaneous water heaters have also been used with good success.