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T3EH 072/150, 25−35 kW Y Voltage − Sequence of Operation
First Stage Heat Call
1. TSA − W1 heat demand energizes the economizer
relay K43 and heat relay K9. K43−1 closes energizing
blower relay K3
TPA − Unit will operate in heat mode" from the indoor
thermostat.
2. K3−1 closes energizing indoor blower B3. K9−1
closes energizing contactor K15. Second stage
heat relay K19 is energized. (Unit is ready for se-
cond stage heat but ONLY if there is a W2 call)
3. K15−1 closes and assuming primary limit S15 and secon-
dary limit S20 are closed, heating elements HE1 and HE2
are energized.
Second Stage Heat Call TSA / TPA
4. TSA − W2 calls for second stage heat. K19−1 closes ener-
gizing one side of contactor K16 which energizes one
side of relay K32. K32−1 closes energizing K16. K16−1
closes energizing HE3 and HE4.
TPA − W2 heat demand energizes K9 and K19. K9−1 and
K19−1 closes energizing K15, K16 and K32. K15−1 and
K16−1 closes energizing HE3 and HE4.
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