Troubleshooting Ignition System - A.O. Smith VB-500, VB-750 Instruction Manual

Series 200/201 gas-fired copper boilers for hydronic heating and hot water supply

Hide thumbs Also See for VB-500, VB-750:

User manual (24 pages)

Service handbook (74 pages)

Service parts (8 pages)

Table Of Contents

Table of Contents

FAULT DISPLAYED

"Display Fail"

Communications with UIM interrupted

Check communication cable to UIM. Try moving to other Internal Communications connector on MCB. Also check the

connectors where the cable is plugged in for best connecting wires.

"Comm. Fail Stg.*"

Communications with FCB interrupted

Same as above. Also try swapping FCB and UIM communications cables.

"Low AC Voltage"

Line voltage less than 90 vac

Check incoming power line for loss of voltage. May also be caused by a power line brown-out (momentary loss of voltage)

"Low 24VAC"

Voltage from transformer less than 18 vac

Check transformer output. Should be over 24vac. May be caused by excessive current drain or a faulty transformer

"Brown Out Reset"

Indicates a brown out reset occurred

Caused by a momentary dip in voltage on the MCB +5vdc power bus. Contact factory. (Not considered a serious problem

if it only occurs on very rare occasions.)

"Timeout Reset"

Indicates a watchdog reset occurred

Caused when the micro executes the software in an improper way. Contact factory. (Not considered a serious problem if it

only occurs on very rare occasions.)

"FCB Comm Timeout"

FCB did not receive command from MCB

Caused when communications between the MCB and the FCB are interrupted. May occur when a surge of power on a

nearby device (i.e. arc welder) creates an EMI burst. Not a serious problem if it occurs rarely.

"No Flow"

Water is not flowing

Caused by the flow switch being open when it should be closed. Check water lines, pump, flow switch contacts, wiring.

"Blower Prov Stg*

Blower pressure is too low it they should have been closed.

Check blower, switch contact, wiring, and for air leaks.

"Blocked Flue"

Flue is Blocked

Caused by the blocked flue switch contacts being open when they should have been closed. Check for flue blockage, switch

contacts, wiring.

"Blocked Inlet"

Air Inlet is blocked

Check inlet terminal, venturi screen and burner tube for debris. If inlet is clear check blocked inlet flue switch for continuity.

Inspect hose to switch to ensure it is connected and not cracked or spilt.

"High Limit"

Outlet temp. exceeded high limit setpoint

Caused when the temperature of the outlet probe exceeds the high limit setpoint. This is an internal "software" limit switch and

not an actual device. Check that the over temperature is not being caused by improper setup or operation of the boiler.

"MRHL"

Outlet temp is too high

Caused when the MRHL (a thermostat) contacts in the Outlet probe are open when they should be closed. Fault may be due

to high outlet temperature, bad switch contacts in the probe, or disconnected wiring. Note: The MRHL can be checked by

disconnecting the outlet probe from the MCB and reading the resistance across the two active pins on the connector at the

end of the probe cable. The value should be approximately 0 ohms when the temperature of the probe is less than 220°F.

"Low Water"

Water level is too low

Caused by the Low Water Cut Out device not closing its feedback switch contacts when it should. Check water line, switch

contacts, wiring, LWCO.

"Low Gas"

Gas pressure is too low

Caused by the Low Gas switch being open when it should be closed. Check gas line, switch contacts, wiring.

"Powered Accessory"

Power vent not running

Caused by the Powered Vent device not closing its feedback switch contacts when it should. Could also be caused by a

problem with the relay output on the MCB. Check output to powered vent, switch contacts, wiring.

"High Gas Stg*"

Gas pressure too high

Caused by the High Gas switch being open when it should be closed. Check gas line, switch contacts, wiring.

"Inlet Probe"

Inlet probe shorted or open

Caused when the thermistor in the probe or the wiring to the probe is shorted or disconnected. Check the probe. Note: The

thermistor and wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two

active pins on the connector at the end of the probe cable. The value should be approximately 10K ohms (value will change

slightly with changes in temperature).

"Outlet Probe:"

Outlet probe shorted or open

Caused when the thermistor in the probe or the wiring is shorted or disconnected. Check the probe. Note: the thermistor and

wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two active pins on

the connector at the end of the probe cable. The value should be approximately 10K ohms (value will change slightly with

changes in temperature).

"Tank Probe"

Tank (Remote) probe shorted or open

Caused when the thermistor in the probe or the wiring is shorted or disconnected. Check the probe. Note: the thermistor and

wiring can be checked by disconnecting the probe from the MCB and reading the resistance across the two active pins on

the connector at the end of the probe cable. The value should be approximately 10K ohms| (value will change slightly with

changes in temperature).

"Igniter Stg*"

Igniter current is too low

Caused by a low current draw problem with the igniter. This may occur if the igniter is old, damaged, or disconnected and

no longer draws the proper level of current. This condition will affect the ability of the igniterto get hot enough to fire the gas

properly. Check the igniter and its associated wiring.

"Igniter Pwr Stg*"

Improper power applied to the igniter circuit

Caused by improper line power being applied to the igniter circuitry. Check line connections. Paying particular attention to

the earth ground connection. Also check that line voltage does not exceed 132 VAC RMS.

"Igniter Hdwr Stg*"

Hardware problems with igniter circuit