Replacement Parts - A.O. Smith 650A, 740A Installation & Operation Manual

Commercial gas, glass-lined, tank-type low nox water heater

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Specification sheet (2 pages)

Table Of Contents

Table of Contents

Enlarged version

• Do not proceed to Step 5 unless 24V is detected at "24V"

terminal of the Honeywell Module.

5. As soon as the pilot gas valve is energized, check for the

burner pilot flame through the burner peek-hole. If the pilot

flame is established inside the burner, then proceed to

Step 6.

• Turn the burner switch to "OFF" position for at least one minute

to reset the heater.

• Check for 24V between "PV" terminal and "MV/PV" terminal

after 45 seconds prepurge cycle (this 24V may only last

15 seconds starting after 45S prepurge). If there's no 24V at

the "PV" terminal ever, and there's 24V at "24V" terminal, the

Honeywell flame control module needs to be replaced.

• If 24V is detected for a period of 15 seconds, check for the

changes of gas supply line pressure when the pilot gas valve

is energized. If no visible pressure change during the pilot

ignition period, check for loose connection of the pilot valve or

a defective pilot gas valve.

• Adjust the pilot gas regulator setting during the pilot ignition

period (15 seconds window). Do not set too much or too little

pilot gas. This adjustment should only be performed during

the 15 seconds ignition period. A clockwise or counter-

clockwise 0.5 to 1 turn should be able to correct the problem.

Refer to burner manual for detailed pilot gas setting procedure.

• If still no pilot flame inside the burner, check for the spark

ignitor wiring, ignitor tip location and visually check the spark

(burner pilot assembly needs to be taken out for the visual

check). Refer to the burner manual for detailed spark ignition

troubleshooting procedures.

6. Pilot flame established but not detected by the flame control

module. Use Micro Amp meter to measure flame signal during

the pilot ignition period. Refer to burner manual for testing

procedures.

• If pilot flame is confirmed visually through the burner peek

hole, and the flame signal is zero, the thermal-cutoff safety

switch (mounted nearby the flame rod, inside the burner mixing

chamber) may be broken or check for loose wire connections

or low gas switch (mounted inside control panel) may be

broken. If the thermal cutoff switch or low gas switch is broken

(by checking the switch continuity), the switches must be

replaced before the heater can operate again.

• If it's confirmed that the thermal cutoff switch is damaged

(open), call the A.O. Smith Technical Center to analyze the

root cause (too lean combustion) and proper gas/air ratio

adjustment. Flashback will cause the thermal cutoff switch

broken (open).

• If the flame signal is not zero but below 1µA, relocate flame

rod or replace flame rod will be required. Refer to burner

manual for detailed procedure of replacing flame rod.

MOTOR RUNS CONTINUOUSLY, BURNER LIGHTS ON

MOMENTARILY THEN LOCKS OUT OR EXCESSIVE

ON/OFF CYCLE

Under this condition, the burner will start the purge cycle again,

then pilot ignition and main flame cycle. If main flame cannot be

sustained:

1. Make sure main manual gas valve (right after the Robertshaw

gas valve) is fully open;

2. Check for 24V at the Robertshaw gas valve when the main

gas light is on. If there's no 24V at the valve, check loose wire

connection.

3. Check for gas supply line pressure. The gas line pressure

before the valve should be maintained at minimum

8" w.c. when main flame is "ON". If the gas line pressure

drops below 8" w.c. (2 kPa), the service regulator pressure

setting must be increased to keep 8" w.c. (2 kPa) minimum

gas pressure. After adjusting the gas regulator setting, turn

off the heater, make sure the gas line pressure does not

exceed 14" w.c. (3.5 kPa) if it does exceed 14" w.c. (3.5 kPa)

static pressure, the gas supply line size must be increased

accordingly to avoid gas line pressure exceeding 14" w.c.

(3.5 kPa)

4. Check the gas valve manifold pressure (refer to Table 10),

see ADJUSTMENT PROCEDURE to adjust the manifold

pressure.

5. Check for defective gas valve for failing to open or shut off.

6. Verify that the air supply is adequate. The air inlet screen or

blower wheel may be restricted.

7. Check the installation for proper ventilation, see AIR

REQUIREMENTS.

8. Verify the air switch setting is not too high.

9. Check the venting system for the correct sizing, excessive or

insufficient draft, and proper operation of the barometric draft

control, see VENTING.

NOISY BURNER OPERATION

1. Check for flue gas excess O

10). If combustion is too lean (flue gas O

7.5%), the combustion flame front may not be stable, which

will cause noisy operation. See ADJUSTMENT.

2. Check the venting system for the correct sizing, excess or

insufficient draft, and proper operation of the barometric draft

control, see VENTING.

3. If the burner operation is still noisy, then remove the burner,

examine the burner diffuser, blast-tube tip, anti-flashback

corrugated tube, center mixing, refer to the burner manual for

details.

GAS FAILS TO SHUT OFF

Check for defective gas valve or thermostat. If operation is

incorrect, replace.