COIL SELECTION
Check the nameplate on existing heating system for BTU/HR output, blower CFM and allowable external static pressure. Measure the external static pressure with a clean filter in position.
Choose coil based on desired BTU/HR output and LOWEST entering water temperature (usually 125°F to 130°F EWT). Choose circulating pump based on required water flow and total system pressure drop, Determine if EXISTING furnace blower is adequate. If NOT adequate, and furnace is in GOOD condition, replace blower assembly or blower motor and pulleys to yield proper flows. If NOT adequate and furnace is in POOR condition, replace the furnace with a new furnace of proper blower capacity. Or replace the furnace with a package fan-coil unit. If the furnace is adequate and in good condition, install the coil. DO NOT install a coil in a system that utilizes a high efficiency or condensing furnace. Call your local GARN® dealer for coil selection and pricing.
HIGH LIMIT SWITCH (DUCT STAT)
If the HWS temperature to the coil is greater than 140°F an optional high limit switch (sometimes called a duct stat), must be installed on the downstream side of the coil. The duct stat provides overheating protection for the space being heated. If the temperature of air discharged from the coil exceeds 140°F, the switch stops the fluid flow through the coil.
BLOWER SPEED AND CFM ADJUSTMENT
It is very important that the proper air volume is supplied to the heated space, across the furnace’s heat exchanger, and across the coil. These air volumes are to be determined by design specifications. A draft gauge reading of pressure drop across the furnace is taken before the coil is installed. This yields the initial system air volume. After the coil is installed, a pressure drop across the coil should be taken to indicate the new system air volume. This new system air volume must be adjusted to supply: 1. T he minimum air volume across the furnace’s heat exchanger as specified in the manufacturer’s engineering data. 2. The proper air volume across the coil to yield the required output 3. The proper air volume to heat the space. A minimum of three 1/4” air test holes must be drilled. One in the ductwork on both sides of the furnace and one on both sides of the coil. Refer to following diagram. Connect draft gauge across the blower. The zero end of the draft gauge scale connects to the air entering side. Insert the hoses so about 1/4’ extends inside th e plenum. Seal around holes.
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Hydronic System Design Manual ©DECTRA CORPORATION - March 2013
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