Although this article is about a specific application, the wiring diagrams below can be used for any Heat Pump system with 2 stages of AUX (Auxiliary / Emergency) Heat.
When you’re installing a zoning system on a heat pump, wiring can feel a little more complex than with a standard furnace and air conditioner. That’s especially true with modern high-efficiency systems like the MrCool HyperHeat Ducted Heat Pump, which uses advanced inverter technology for precise heating and cooling, and includes two stages of electric auxiliary heat for colder weather backup. The good news is that with the right controller—such as the SmartZone 3.0—everything can be wired cleanly and reliably, giving homeowners the comfort of true zoned control without losing the performance features of their equipment.
The diagram below will show how to connect a SmartZone controller to a HyperHeat ducted system with auxiliary heat. Whether you’re a professional HVAC installer or a DIYer who’s confident working with low-voltage controls, this article will help you understand the wiring layout, terminal functions, and key considerations for making the system operate correctly.
PARTS NEEDED:
SmartZone Controller
24VAC Delay-on-make relay (like the ICM102 or Supco TD69 or Mars 32391)
A delay-on-make relay is a simple timing device that waits a set amount of time after receiving power before it closes its contacts and energizes the connected load. In other words, when the thermostat or zone controller calls for heat, the relay doesn’t switch on immediately—it holds off for a few seconds (or minutes, depending on the setting) before passing power through.
In a heat pump system with two stages of electric auxiliary heat strips, a delay-on-make relay can be wired to control the second stage. The first stage energizes right away when auxiliary heat is needed, providing an initial boost. Then, if the call for heat continues, the delay-on-make relay activates after the preset time and brings on the second stage.
This approach helps prevent all the heating strips from coming on at once, which can cause large electrical surges and unnecessary wear on components. Instead, the stages are sequenced smoothly, delivering strong heating capacity when it’s truly needed while reducing stress on the system and the electrical supply.
NOTES:
SmartZone manages single stage AUX without any additional wiring.
W2 on SmartZone does not connect to W2 on Mr. Cool
There are many variations of the time delay relay but the one we have linked below we have tested. However, any “delay-on-make” relay will work that is capable of enough current to energize the Mr. Cool input.
The “W2” or AUX Heat 2nd Stage input must draw at least 0.04 A (40mA) to engage the relay style suggested here. If it doesn’t then an additional relay may be needed to make this work correctly. See diagram below.
One of the major benefits of pairing SmartZone with a ducted Mr. Cool system is that you’re not limited by the “one-size-fits-all” thermostat controls that come with most equipment. Instead, you get precise control over which zones call for heating or cooling, when the heat pump should stay on versus when backup heat should be engaged, and how to stage auxiliary heat efficiently.
The below wiring application can be used if the above does not work with the equipment brand/model you are using. It incorporates the time delay relay with a know working relay that is offered by zoningsupply.com to act as a load for the 40mA minimum that the ICM relay requires to activate.
WHAT TIME DELAY BETWEEN AUX STAGES?
Setting the delay-on-make timer to about 10 minutes strikes the right balance between comfort and efficiency. Electric heat strips draw a very high current load (often 40–60 amps per stage), so bringing them all on at once can create large inrush demands on the electrical service. By holding off the second stage, the system reduces unnecessary peaks in power consumption.
In many cases, the heat pump plus the first stage of auxiliary heat can satisfy the thermostat within that 10-minute window. This avoids running the second stage altogether, saving energy and lowering operating costs. But if the heating demand continues past 10 minutes—such as during extreme outdoor temperatures or when several zones are calling—the delay ensures that the second stage is available to maintain comfort without excessive lag.
In short, the 10-minute setting ensures the system uses the least amount of auxiliary heat necessary, prevents electrical overloads, and still provides strong backup capacity when it’s truly needed.