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Note the horizontal location of the 15-degree check valve, and the orientation of the
gate/pivot. This valve is intended to relieve any vacuum forming in the condensate outlet of a
condensing steam coil, and to seal this port when steam pressure is again supplied to the
coil. It must not be installed in any other position, and should not be used in the supply line.
For coils used in tempering service, or to preheat outside air, install an immersion thermostat
in the condensate line ahead of the trap. This will shut down the supply fan and close the
outdoor damper whenever the condensate falls to a predetermined point, perhaps 120 F.
NOTE: Do NOT use an immersion thermostat to override a duct thermostat and open the
steam supply valve.
For vacuum return systems, the vacuum breaking check valve would be piped into the
condensate line between the trap and the gate valve instead of open to the atmosphere.
Figure illustrates the typical piping at the end of every
steam supply main. Omitting this causes many field
problems and failed coils.
NOTES:
1. A bypass is necessary around trap and valves when
continuous operation is necessary.
2. Bypass to be the same size as trap orifice but never
less than 1/2 inch.
COILS FREEZE-UP PROTECTION
WATER COILS — If a chilled water coil is applied with outside air, provisions must be made
to prevent coil freeze-up.
Install a coil freeze-up thermostat to shut down the system if any air temperature below 36 F
is encountered entering the water coil. Follow thermostat manufacturer’s instructions.
For outdoor-air application where intermittent chilled water coil operation is possible, one of
the following steps should be taken:
• Install an auxiliary blower heater in cabinet to maintain above-freezing temperature around
coil while unit is shut down.
• Drain coils and fill with an ethylene glycol solution suitable for the expected cold weather
operation. Shut down the system and drain coils.
STEAM COILS — When used for preheating outdoor air in pressure or vacuum systems, an
immersion thermostat to control outdoor-air damper and fan motor is recommended. This
control is actuated when steam supply fails or condensate temperature drops below an
established level, such as 120 to 150 F.
A vacuum breaker should also be used to equalize coil pressure with the atmosphere when
steam supply throttles close. Steam should not be modulated when outdoor air is below 40 F.
On low-pressure and vacuum steam-heating systems, the thermostat may be replaced by a
condensate drain with a thermal element. This element opens and drains the coil when
condensate temperature drops below 165 F. Note that condensate drains are limited to 5
psig pressure.
