Control of Electric Water Heaters

A homeowner can control the electric output of their electric furnances with the use of Energy Sentry demand controller. Please note: The following steps should be performed by a licensed electrician and is not recommended for homeowners.
Most residential furnaces have between 10 kW and 25 kW of resistance heat installed. Typically, the total wattage is divided into 5 kW elements, staged by timing relays controlled by a “sequencer.” The sequencer contains relays that turn on elements in stages. These relays allow some or all of the heat to come on, depending on the model or the controls. It is common to have 1st stage of 10kW; 2nd stage of 10kW and 3 stage (“emergency”) heat of 5kW.

The Energy Sentry demand controller can control the heat several ways depending on the number of elements and stages. Normally, those furnaces with 10kW or less are controlled via low voltage and those greater than 10kW can be controlled either in stages by disconnecting the low-voltage signals or by controlling each 5 kW element individually via line voltage.

Figure 1 relay installation on water heater figure 1
Figure 2 relay installation on water heater figure 2


  1. Turn water heater breaker OFF.
  2. Disconnect one wire from water heater breaker.
  3. Extend the wire just disconnected to the Energy Sentry relay, using #10 AWG solid copper wire and appropriate wirenuts as shown in the figure above.
  4. Add another #10 AWG wire from the now-empty breaker terminal to the other side of the Energy Sentry relay.
  5. After completing all other wiring on the controller, turn water heater breaker ON.


  • Beware of mislabeling. Make certain the 30A breaker is the water heater’s and not the dryer’s, the other 30A breaker common in most breaker panels.
  • If any loads connected to the Energy Sentry demand control system are wired with aluminum wiring, insure that connections between existing aluminum conductors and copper conductors of the Energy Sentry are properly done using a Copair tool, anti-oxident paste or anti-oxident impregnated wirenuts.

Quick Test

  1. Clip an Amprobe onto one of the water heater wires in the breaker panel. Turn the water heater breaker ON.
  2. Turn ON water heater by turning UP the temperature of the water heater or by running enough hot water from faucets until thermostat in water heater calls for heat.
  3. Check amperage going to the water heater. The amperage should be approximately 21 to 24 Amps when the water heater is on.
  4. Lower demand to limit on the demand controller to minimum kW settng. Turn ON oven and range until all loads are shed. This may take a few minutes. The water heater’s amperage should go to zero when the water heater is shed by the Energy Sentry.
  5. Turn OFF oven and range.
  6. Turn water heater’s thermostat temperature back to normal.
  7. Return the demand limit on the demand controller to the normal settng for the home.