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AOS WPC - Tech Training 71 of 72 Ashland City, TN © 2007
Servicing should only be performed by a Qualified Service Agent
VF BOILER SERVICE MANUAL
GLOSSARY
ΔT:Symbol used for “Delta T.” Engineering term to convey “temperature rise” or “difference”
through a boiler or heat exchanger. IE: If the inlet water temperature to a boiler is 120°F and
the outlet water temperature leaving the boiler is 140°F; the temperature rise through the
boiler is 20°F, which is commonly expressed as “20°
ΔT.”
Blocked Flue Switch - Blocked Exhaust Switch: Two terms describing the same device.
This is a normally closed air pressure switch that opens on a rise in pressure. Used to detect
a blocked exhaust vent. Connects to J5 socket on the MCB, see page 28.
EJB - External Junction Box: Abbreviation used in wiring/connection diagrams. This is a
wiring box mounted outside of boiler. Connections for 120 VAC supply power, external
thermostat/enable/disable control, and remote Tank/Loop temperature probe are made here.
Hall Effect Sensor: A small sensor mounted in the end cap of the blower motor that
converts blower rotation motion into electrical data that is interpreted by the MCB as actual
rpm data.
High Blower Prover Switch: This is a normally open air pressure switch that closes on a
fall in pressure. It is used to detect that the blower is running at the proper higher speed
used during the pre/post purge operational states. Connects to J17 socket on the MCB, see
pages 24 and 28.
Low Blower Prover Switch: This is a normally open air pressure switch that closes on a
fall in pressure. It is used to detect that the blower is running during the heating cycle.
Connects to J17 socket on MCB, see pages 24 and 28.
Low Gas Pressure Switch: This is a normally open pressure switch that closes on a rise in
pressure used to detect supply gas pressure is maintained above minimum requirement.
This switch is standard on VF boilers. Connects to J5 socket on MCB, see page 28. The
MCB board’s master dip switch SW1 (page 30) must also be configured when a low gas
pressure switch is used to activate and monitor this circuit.
Low Water Cut Off:This is an electronic continuity based control used to prove water level
in the boiler. This is an optional component usually used to satisfy local code requirements.
Connects to J4 socket on MCB, see page 28. The MCB board’s master dip switch SW1 (see
page 30) must also be configured to activate and monitor this circuit.
MCB - Modulation Control Board :This is the main control circuit board for the EMC 5000.
It controls all centralized functions including temperature control, diagnostics, automatic high
limit etc. It sends information to/from UIB and implements the External Network Interface.
See pages 25 and 26.
PDB - Power Distribution Board:Distributes 120VAC and 24VAC power and supplies
fusing to the MCB circuit board and the pump. Contains a test circuit for determining if line
power is properly applied to the system. See page 35.
TSB - Touch Sensor Board:Circuit board containing touch switch pads (user input buttons)
which is a portion of the UIM assembly.
UIB - User Interface Board:Part of the UIM assembly. Communicates with the MCB.
Controls the operation of the LCD. Receives inputs from the touch switches. Activates the
LED's according to commands sent from the MCB.