FORCED AIR HEATING:
DO NOT MOUNT A HOT WATER COIL ON THE RETURN SIDE OF THE FURNACE. Warm air will be flowing over the blower motor and may not provide sufficient
motor cooling. Doing so will void the furnace warrantee and the UL listing of the furnace.
DO NOT MOUNT A HOT WATER COIL IN SYSTEMS SERVED BY A HIGH EFFICIENCY CONDENSING FURNACE. Doing so will void the furnace warrantee and the UL
listing of the furnace and create the potential for flue damage and a building fire.
Size a coil that increases the air-side pressure drop by only 0 .25” to 0 .33” WC. Increase blower RPM to offset this increased static pressure and maintain CFM. Select a coil that will provide a supply air temperature of 110°F or slightly greater. Code limit is 140°F. Pipe all coils in a counter flow pattern. The “normal” range of water tempe rature drop through a coil is 8°F to 20°F. Mount hot water coils (flat and A-type) on the discharge side of the furnace. In almost all cases the coil will be physically larger than the existing supply air plenum. The plenum size will have to be increased. Sheet metal work must be designed and fabricated in accordance with SMACNA guidelines. If the furnace is more than 12 years old, consider installing a new unitized fan coil unit that provides a motorized fan, filter, hot water heating coil, DX cooling coil and controls all within one insulated sheet metal unit. Such units are manufactured to replace an existing residential furnace and reasonably match the existing furnace’s overall dimensions . When selecting a unit, make sure to apply a correction factor (if necessary) for the hot water coil output at the entering water temperature (EWT) expected in the new system versus the EWT at the manufacturer’s rated output (see water temperature table in the Hot Water Baseboard Heating section of this manual).
For more information on unitized fan coil units visit: http://www.firstco.com/ http://www.magicaire.com
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Hydronic System Design Manual ©DECTRA CORPORATION - March 2013
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