STIEBEL ELTRON CNS Series, CKT Series, CK TREND Series, CK Series Installation Manual

THERMOSTAT WIR ING AND INSTALL ATION GUIDE

CNS, CK, CKT, CK TREND, ALL MODELS

1. Thermostat Wiring Options

There are a number of different options available if you
wish to control your Stiebel Eltron CNS/CK heater with
a room thermostat. There is a limited variety of line
voltage thermostats available to which you can directly
wire your heater. You can also use a transformer/relay
unit so that you can utilize the much wider variety of
low voltage thermostats that are available, including
some of the WiFi capable ones such as Venstar and
Nest. There are also wireless kits available, which
eliminate the need to physically wire the thermostat to
the heater.

When using any type of external thermostat to control
the CK/CNS line of heaters, set the internal thermostat
of the heater to its highest setting, as the cycling of
the heater will now be controlled by the external
thermostat. Do not, however, disable the CK/CNS
heater’s internal thermostat, as even when set to its
highest setting, it must still cycle off occasionally to
prevent the external parts of the heater from reaching
hazardous temperatures.

the room reaches the setpoint, turning the heater off.
A line voltage thermostat should not be used when
controlling more than one heater with the same
thermostat.

When wiring this type of thermostat, the line voltage
thermostat is connected to the circuit breaker on the
load panel (breaker box), and the CK/CNS heater is
connected to the line volt thermostat. If using a digital
thermostat, always use a 4-wire model, because the

4-Wire Line Voltage

Thermostat 120 V

15 A Minimum

15 A
Single Pole
Circuit Breaker

L1

120 VAC

L2

1
2

CK/CKT/CNS

120 VAC Units

Fig. 2: 120 V heater with line voltage thermostat.

1.1 Line Voltage Thermostats

A line voltage thermostat simply takes the high voltage
signal that is used to power the heater, closes a switch
when the room temperature falls below the setpoint,
which turns the heater on, and opens a switch when

4-Wire Line Voltage

Thermostat 240 V

15 A Minimum

15 A
Double Pole
Circuit Breaker

L1

240 VAC

L2

Fig. 1: 240 V heater with line voltage thermostat.

1
2

CK/CKT/CNS

240 VAC Units

internal thermostat of the CK/CNS heaters will turn off
occasionally, even at their highest setting. See figures
1 and 2 for wiring diagrams illustrating the manner in
which to hook up both the 120 V and 240 V heaters to a
line voltage thermostat.

1.2 Low Voltage Thermostats

A low voltage thermostat is the industry standard for
most types of heat sources other than electric resistance
heaters. These devices are typically connected to the
control board of a boiler or furnace, and they operate
on a 24 VAC signal. To utilize this type of thermostat,
it is necessary to use a transformer/relay device such
as the RC840T. A low voltage thermostat can be safely
used to control more than one heater, provided that it
is wired correctly. As is the case with the line voltage
thermostat installation, the internal thermostat of the
heater should be set to its maximum temperature when

17 West St., W. Hatfield, MA 01088 | 800.582.8423 | 413.247.3380 | fax [phone] | [e-mail] | www.stiebel-eltron-usa.com

wiring the unit in this manner.

To install one or more CK/CNS heaters to a low voltage
thermostat, each heater must have its own transformer/
relay device. This device will be connected at a junction
box near the heater and wired both to the heater and to
the load panel. Please see figures 3, 4, and 5 for wiring
diagrams illustrating the manner in which to hook up a

R
W
C

Low Voltage

R

L

N

CW

RC840T-120

1
2

CK/CKT/CNS

120 VAC Units

Thermostat

15 A
Single Pole
Circuit Breaker

120 VAC

R
W
C

Low Voltage

R

Thermostat

15 A
Double Pole
Circuit Breaker

L1

CW

RC840T-240

1
2

CK/CKT/CNS

240 VAC Units

240 VAC

L2

Fig. 4: 240 V heater with low voltage thermostat.

single 120 V heater, a single 240 V heater, and multiple
heaters, to a low voltage thermostat. Be aware that, as
with most transformer/relay devices, there are both 120
V and 240 V versions of the RC840T, so be sure to select
the appropriate device for your application.

Fig. 3: 120 V heater with low voltage thermostat.

Low Voltage

Thermostat

R

CW

1
2

15 A
Single Pole
Circuit Breaker

RC840T-120

CK/CKT/CNS

120 VAC Units

L

120 VAC

N

Fig. 5: Mulple heaters, both 120 V and 240 V, connected to single low voltage thermostat.

R
W
C

15 A
Double Pole
Circuit Breaker

L1

240 VAC

L2

R

CW

RC840T-240

1
2

CK/CKT/CNS

240 VAC Units

17 West St., W. Hatfield, MA 01088 | 800.582.8423 | 413.247.3380 | fax [phone] | [e-mail] | www.stiebel-eltron-usa.com