Water Heater (CONT) - TM-3-4230-209-20-P0030

TM 3-4230-209-20&P

1-20

(11) In the holding relay circuit a fail-safe circuit forms an important

(10) If ignition has occurred, as soon as a fire is going the temperature

function. Shown here is the fail-safe circuit. It starts basically at relay K2-A1

in the exhaust flue will rise sharply. When the exhaust flue reaches 300F,

and stops at S4. An over temp condition at S1 or a low pressure at S3 will

the flame switch (S2) contacts will open. This will remove the circuit voltage

cause an immediate shutdown of the heater. If the exhaust temperature is less

through S4 contacts 2 and 3, the heaters of both TD-1 and TD-2 to ground.

than 300 F, S2 will close. This would allow voltage to be applied to TD1 and

When TD-1 and TD-2 cools down, TD-1 will open its bimetallic contacts. With

TD2 heaters. If this were to occur in approximately 120 seconds, the TD2

TD-1 contacts now open, they no longer form a bypass circuit from K2-A2 over

bimetallic contacts would open up and the circuit between pins 5 and 7 would

to A1 and the whole circuit acts as a holding relay. The heater should heat the

open. The fault would have-to be corrected before the circuit would hold

water.

again.