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Honeywell VS8421 Installation Instructions Manual Download Page 8

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.

69-1219-1.fm  Page 8  Thursday, August 18, 2011  7:32 AM

Summary of Contents for VS8421

Page 1: ...icate C2030022 European Community CE Certificate Applied for INSTALLATION When Installing this Product 1 Read these instructions carefully Failure to follow them could damage the product or cause a hazardous condition 2 Check the ratings given in the instructions and on the product to make sure the product is suitable for your application 3 Installer must be a trained experienced service technicia...

Page 2: ...lies Tubing installation must comply with approved standards and practices 1 Use new properly reamed pipe free from chips When tubing is used assure the ends are square deburred and clean All tubing bends must be smooth and without deformation 2 Run pipe or tubing to the valve If tubing is used obtain a tube to pipe coupling to connect the tubing to the valve 3 Install sediment trap in the supply ...

Page 3: ...s valve M12670 HI LO REGULATOR WIRING TERMINALS INLET PRESSURE TAP OUTLET PRESSURE TAP GAS CONTROL KNOB OFF PILOT ON PRESSURE REGULATOR ADJUSTMENT UNDER CAP SCREW M12673 WIRING TERMINALS INLET PRESSURE TAP OUTLET PRESSURE TAP GAS CONTROL KNOB OFF PILOT ON PIEZO SPARKER APPLY WRENCH FROM BOTTOM OF GAS CONTROL TO SHADED AREA M12679 69 1219 1 fm Page 3 Thursday August 18 2011 7 32 AM ...

Page 4: ...he pilot burner The piezo creates a spark when the plunger is depressed The connecting wires of the piezo include a terminal to connect to the electrode of the pilot burner The piezo is replaceable Refer to the installation instructions for the piezo envelope assembly part no 396079 form 69 1206 Fig 6 Millivolt system wiring diagram with ODS pilot burner Pilot Gas Lighting Procedure Lighting Stand...

Page 5: ...rate using the meter clocking method or measure the pressure by attaching a plastic tube with a 1 4 in shell ID to the manometer and connecting the manometer to the outlet pressure tap on the gas valve See Fig 4 3 If necessary adjust the pressure regulator to match the appliance rating See Table 1 for factory set nominal outlet pressure and adjustment range a Remove the pressure regulator adjustme...

Page 6: ...pliance being checked NOTE Other appliances must remain off with the pilots extinguished or the consumption must be deducted from the meter reading 2 Convert the flow rate to Btuh as described in the Gas Controls Handbook form 70 2602 3 Compare to the Btuh input rating on the appliance nameplate Checking Gas Input With Manometer NOTE Both the inlet and outlet pressure taps have a captive screw 1 B...

Page 7: ...UTION Equipment Damage Can burn out heat anticipator in thermostat Do not apply a jumper across or short the valve coil terminals even temporarily If Main Burner Does Not Come On With Call For Heat 1 Confirm that the gas control knob is in the ON position 2 Adjust the thermostat several degrees above the room temperature 3 Use a dc voltmeter to measure the voltage across the THTP and TP terminals ...

Page 8: ... 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...

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