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Install Manual

Rinnai’s recirculation technology is called Circ-Logic. Circ-Logic allows consumers to set recirculation patterns that coincide with their hot water usage patterns. Hot water is available when needed, without circulating it during times of inactivity. The GTK03U kit, manufactured by Grundfos, allows the user to enjoy Circ-Logic with VC, KB, and SENSEI series tankless water heaters on a dedicated return line with the use of an external pump.

For all models, Circ-Logic operates in either Comfort or Economy Mode (the modes are selected by dip switches/parameter settings inside the water heater):

• Comfort mode—Cycles the pump more frequently, ensuring the loop temperature remains higher (but also uses more energy).
• Economy mode—Cycles the pump less often, using less energy to maintain the circulation loop temperature.


The GTK03U Kit contains the following:
• Grundfos 15-55 pump and internal check valve
• 6 ft. (2 m) BX conduit pre-wired to pump (In-line 4 amp glass fuse included)
• Union Set
• Rinnai Circ-Logic Manual


A licensed professional must install the GTK03U kit.
The installer should have the following skills:
• Connecting gas lines, water lines, valves, and electricity.
• Knowledge of applicable national, state, and local codes.


The pump supplied with your GTK03U kit is meant for indoor installations only. It is designed to circulate water from 36°F to 150°F up to a maximum pressure of 145 PSI.

If you are installing a GTK03U Kit on one of the following tankless water heater models, follow the instructions in Section 2 in the install Manual:




If you are installing a GTK03U Kit on one of the following models, follow the instructions in Section 3 in the install Manual:

RL75i/e (VC models)
RL94i/e (VC models)
RU98i/RUC98i and RU98e (KB models)
RU80i/RUC80i and RU80e (KB models)

Housing Material Stainless Steel with Check Valve
Flow Range 20 GPM
Head Range 17 Ft (5 m)
Max Liquid Temperature 150° F (65° C)
Maximum System Pressure 145 PSI
Input Voltage 115 V
Frequency 60 Hz
Max Amperage 0.75 A
Connection Size 3/4” Flange
Weight (Pump/Flange Kit Combined ) 8.6 lbs. (4 kg)
Pipe Length
Equivalent pipe length using Grundfos GTK03 pump at
120°F (49° C) = Up to 200 Ft (61 m) (3/4” piping).
Equivalent pipe length includes supply piping, return piping and
fittings. The installer/contractor/engineer must calculate the total equivalent
pipe length of the piping system

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