Teco US TR20 - Manual (Page 6)

Unscrew and remove ring nut over Heater port.
Remove plug and o-ring from ring nut.
Place o-ring over the top of the Heater and push down
approximately 3/4 inch to 1 inch onto the Heater.
Insert Heater into heat exchanger until o-ring seats into
outlet lip. If you encounter resistance while inserting the
Heater, gently turn the Heater and apply very slight force.
Step 3
Installing The Heater