69-1219—1 G.R. Rev. 12-00 www.honeywell.com
VS8421 VENTFREE MILLIVOLT GAS VALVE
Printed in U.S.A. on recycled
paper containing at least 10%
post-consumer paper fibers.
Home and Building Control Home and Building Control
Honeywell Honeywell Limited-Honeywell Limitée
1985 Douglas Drive North 35 Dynamic Drive
Golden Valley, MN 55422 Scarborough, Ontario
M1V 4Z9
Fig. 7. Identification of VS8421
terminals for troubleshooting.
Procedure:
1. Test A (Coil Resistance): Have thermostat contacts
open and Pilotstat knob turned to OFF. If coil
resistance exceeds 3.6 ohms, replace the valve.
2. Test B (Thermopile): Have thermostat contacts
open and the pilot lit with Pilotstat knob turned to
PILOT. If the reading is less than 460 mV, replace
the thermopile.
3. Test C (Thermocouple): Have thermostat contacts
open and pilot lit with Pilotstat knob turned to
PILOT. Reading should be a minimum of 12 mV in
a new ODS pilot burner assembly. If the reading is
less than 12 mV, replace the ODS pilot burner
assembly. The power unit will hold in down to 3
mV. If the output of the thermocouple is less than 3
mV, replace the ODS pilot burner assembly.
4. Test D (Operator Pull-In): have thermostat contacts
open and pilot lit with Pilotstat knob turned to
PILOT. Close the thermostat contacts. Operator
will pull in with a minimum of 155 mV. Operator
makes audible sound during pull-in. If pull-in does
not occur, replace the valve.
5. Test E (Resistance System): System resistance
from the remote switch or thermostat and lead
wires should not exceed 1.7 ohms. If resistance is
above 1.7 ohms, reduce the resistance.
Table 2. VS8421 Troubleshooting Table.
M17559
WIRING TERMINALS
OFF
PILOT
ON
TPTH
TH
TP
Test Test Title
Connect Multimeter
Between Terminals Set Thermostat Meter Reading Should Be:
A Coil Resistance TP and TH Open Maximum of 3.6 ohms
B Thermopile TP and TPTH Open Minimum of 460 mV.
C Thermocouple Tip (and Ground) Open Minimum of 12 mV.
D Operator Pull-In TH and TP Open Minimum of 155 mV.
E Resistance System — Closed Maximum of 1.7 ohms.
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