The HVAC Smart Relay Switch (HSRS) is an energy saving device designed to automatically shut off air conditioning or heating when a monitored door or window remains open for a preset period of time. Available in both wired & wireless packages, the system uses compatible door/window sensors – two included. For the wireless edition, choose one of the following product generations:


  G3-Mini G3-Basic G2-Classic
RF Range 50 ft. max
50 ft. max
50+ ft.
Battery CR2032 CR123A AAA (x2)
Dimensions 2.2″ x 1″ x 0.25″ 3.1″ x 1.6″ x 1.2″ 3.25″ x 1.5″ x 0.5″
Wired input
No Yes
Mounting tape tape+screws tape+screws
Feature Specs


  • GEN2 – Our second generation (classic) sensors feature a usable wireless transmission range of 50 ft. or more. Requiring two (2) AAA batteries (not included), they are auto-addressable (up to 65,000+ possible IDs) and measure 3.25″ x 1.5″ x 0.5″. Foam mounting tape and screws are supplied. (GEN2 sensors do not store a permanent address and therefore must be initialized and re-registered whenever batteries are replaced.)
  • GEN3 – Our third generation sensors are available in two models: mini and basic. The mini sensor has a wireless transmit range of 50 ft. (max.) and operates on a single CR2032 lithium coin-cell (included). The basic sensor has similar range and includes a CR123A cell. Each is pre-programmed with a unique, permanent address (over 16 million possible IDs). The mini sensor measures 2.2″H x 1″W x 0.25″D and includes foam mounting tape (no screws).
  • The basic sensor measures 3-1/16”H x 1-9/16”W x 1-3/16”D and includes foam mounting tape and screws.



Frequently Asked Questions:


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  • Notes:
    • The two product generations are not cross-compatible as they operate on different radio frequencies. Therefore, a 2nd generation controller will not accept a 3rd generation sensor. Nor will a 3rd generation controller recognize a 2nd generation sensor.
    • G2-classic sensors feature a copper coil antenna for excellent signal transmission (50 ft. or higher).
    • Both G2-classic and G3-basic sensors provide wired contact screw terminals for optional external wired switch (N/C) or closed contact loop. Unless you plan to utilize this input, it must be disabled to avoid inadvertent trigger. To disable, simply apply a short jumper wire across the wiring terminals.
    • RF range varies for every installation. In general, wireless signals can be diminished or impeded by RF interference, concrete walls, large appliances, etc.

The HSRS is fully configurable using onboard DIP switches, allowing you to establish specific operational parameters. For instance, configure the device to temporarily disable the air conditioning or heating system when any door or window has remained open for 10 minutes and then re-enable when the door has remained closed for 1 minute. (Requires installation by a qualified hvac technician.



  • Automatically shuts off air conditioning or heating when a monitored door or window remains open too long
  • Accepts wired and/or wireless sensors types – two (2) included. (For additional sensors, see “Related Products” at the bottom of this page.)
  • Configurable delay period using onboard DIP switches, up to 10 hours
  • Expandable (accepts up to 4 wired zones and/or 8 wireless sensors)
  • Ideal for split (central) a/c systems
  • Compatible with most existing thermostats

(Note: Installation by a qualified hvac technician is recommended.)



What’s Included:

  • HSRS controller with mounting flange
  • Two (2) door/window sensors. (For additional sensors, see “Related Products” at the bottom of this page.)
  • Power converter (24 vac)
  • Integrated RF receiver (wireless systems only)
  • Whip antenna (wireless systems only)
  • Warning decals – see below:



  • Though installation is not difficult, a qualified HVAC technician is recommended.
  • System includes two (2) door/window sensors. (Add-on sensors also available.)
  • Wireless sensors are battery-operated.


Video Tutorials:


Additional Info:




Full User Manual:


Remote Notifications via Text & E-mail:


Using Wireless Sensors on Patio Doors:


Other Resources:


For Hotel & Vacation Rental Owners:


Ask a Question


Contractors / Resellers

Weight 3 lbs
For HVAC system

Central (split, ducted), Mini-split (ductless), Packaged (rooftop, etc.), Fan coil, PTAC, Window unit, Other (pls contact us)

Door/window sensor

Wireless classic (GEN2), Wireless mini (GEN3), Wireless basic (GEN3), Wired

Sensor quantity

2, 4 (+$85), 4 (+$59)

2 reviews for HVAC Smart Relay Switch (HSRS)

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Showing 2 of 2 reviews (5 star). See all 2 reviews
  1. Dan (Verified Purchase)

    I have used a similar system in a couple locations over the last ten years in vacation rental condos. They really work to reduce your utility bills. The thing I like about the Kadtronix system is it allows you to use any thermostat you want. The other systems require you to use their t-stat. I like the combination of door sensors to shut off HVAC when doors are left open. Very good people to work with.

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  2. Tara Urban (Verified Purchase)

    Verified reviewVerified review - view originalExternal link

    I found Kadtronix through a google search when I was looking for an HVAC switch to help me control how energy is used in our vacation home when guests leave the external doors open while the central air is on. My HVAC tech asked me to purchase and he would install. I made a few calls to Kadtronix and spoke with Ken who was extremely helpful and patient. At one point he even took the information I had, so that he could research compatibility with the products he sells. He also ensured after I ordered, that the items I purchased showed up in the time frame I needed them. I would recommend Ken and his company over and over. It’s hard to find good prompt customer service and Ken has it mastered. Look forward to future business with this company.

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What is your shipping policy?

UPS ground is the default shipping method. Generally speaking, our goal is delivery within 3 business days or less. If necessary, we may upgrade your order to 3-day select or 2nd day air in an effort to reduce transit time.

Is there a remote monitoring option? I want to receive text and/or e-mail alerts.

While the HSRS does not currently provide remote monitoring capability, a cost-effective DIY solution is available. Receive instant alert texts and/or e-mails when HVAC shut-off occurs due to door/window violation. No monthly monitoring fees! Click here for details.

HSRS Wiring Installation

The HSRS is ideally suited for use with split (central / ducted) systems. We recommend installing the HSRS controller near the air handler, and routing a length of low-voltage wire into the HVAC unit for direct access to the 24vac power. (Most types of low voltage wire will suffice such as thermostat, telephone, intercom, or alarm wire.) For wiring details, including sample schematic click here.

Overhead doors & garage doors?

While HSRS door sensors are designed for residential doors & windows, there is nothing to preclude their use with other openings and door types. However, custom shims may be necessary for proper alignment of the sensor & magnet to ensure they are on the same plane.

For wired applications, you may optionally consider a specialty switch from a third party supplier. A few examples are listed below:

- Seco-Larm SM-226L

- Honeywell 958 Overhead Door Contacts

- Baomain Rotary Lever Sw. #WLCA12-2-Q

Note: Most any wired switch type can be used with the HSRS, as long as it is designed for normally-closed circuits.


Will it work with a smart thermostat?

Our HSRS is fully compatible for use with most conventional thermostats, including smart wi-fi models such as Nest and Ecobee. Keep in mind that when the HSRS shuts off the HVAC system (due to door or window violation), your thermostat may display an error indicating lost power. The error should clear once the HSRS restores normal operation.

What is the transmit range of wireless door sensors?

Signal range can be difficult to assess for a wireless device when applied indoors. This is because wireless signals can be adversely affected by a number of factors including interior walls, large appliances, RF interference, etc. Range specifications provided by the device manufacturer are typically measured under favorable conditions, typically in open air and with no obstructions. But actual usable range will be much lower and can vary from one installation to the next.

Our available wireless sensors are listed and compared below in terms of signal range.


Operating at 345 MHz, these sensors feature an open-air transmit range up to 200 ft (ideal conditions). However, you should presume a usable range of roughly 50 ft.


Our GEN2 classic sensor operates at 310 MHz and features a copper coil antenna, making it less susceptible to signal degradation issues. It has an open air transmit range of 100 ft., with an usable range of approximately 50 to 75 ft.

Comparison Chart:

The chart below compares transmit range for each product generation:

    GEN3   GEN2
  • Open air range : 200 ft.
  • Typical (usable):  50 ft.
  • Open air range : 100 ft.
  • Typical (usable): 50 to 75 ft



Will it work with a communicating thermostat?
(not to be confused with a conventional smart thermostat or wi-fi thermostat)

Our HSRS may be compatible with some communicating thermostats. To be clear, a "communicating thermostat" does not refer to a conventional smart thermostat (such as Nest or ecoBee). While conventional thermostats may possess smart features and even provide wi-fi user interfacing, they generally have no high level communications capability with the HVAC system, instead employing traditional low-voltage control wiring (Rc, Rh, C, Y, W, etc.)

Communicating thermostats interact with a control board located within the HVAC system. And unlike a conventional thermostat, it does not directly control the peripheral equipment. Rather, it sends data & command signals to the control board which, in turn, engages the equipment as necessary. Since a communicating thermostat operates through the transfer of data signals, it generally employs fewer wires or may even utilize a wireless interface such as Bluetooth. In some instances, the HVAC control board may provide typical wiring inputs for traditional HVAC control (Rc, Rh, C, Y, W, B, etc). If this is the case, you may the following options for applying our HSRS system:

  • Option 1: Condensate float switch: If the control board provides an input for a condensate float switch or similar shutoff limit, you may be able to wire our HSRS relay contacts to the specified inputs. (If a float switch or other normally-closed shutoff switch already exists, simply wire HSRS relay contacts in series with it.)
  • Option 2: Compressor/cooling wire: Break the "Y" wire (compressor / cooling) at the control board and connect to HSRS relay contacts.


  • HSRS relay contacts are: COM & N/C (terminals 12 & 13)
  • Do not attempt HSRS shutoff by disabling power to a communicating thermostat

Will a metal door interfere with wireless sensor operation?

Many types of metal doors, windows, and patio door frames can interfere with the operation of a wireless door sensor, severely undermining the RF signal. The sensor may operate flawlessly prior to installing the unit. But once mounted to the door or frame, there may be little, if any, detected signal at the controller / receiver.

Does the problem affect all wireless door sensors?

While most any wireless door sensor may be subject to this kind of impediment, our GEN2 "classic" sensor is designed with a copper coil antenna that makes it less susceptible, though not completely immune.

Is there a viable solution?

Fortunately, there is a relatively simple approach for combating the problem. Click here for details.

Which wireless system should I choose? (GEN2 vs GEN3)

Our HSRS wireless systems are available in two generations: GEN2 & GEN3. While both are functionally equivalent, there are some key differences to consider when making your purchase. (Bear in mind that the two generations are not cross-compatible. Therefore, you may not use a GEN2 sensor with a GEN3 controller, or vice-versa.) The two product generations are discussed below.

For a comparison summary, scroll to the bottom of the page.


Manufactured by Honeywell, these sensors are the same type found in compatible security systems. They feature sleek, modern design and long battery life (batteries included). We offer two GEN3 sensor models: mini and basic. Both operate at 345 MHz, and are advertised with a transmission range up to 200 ft. (open air). However, we have found the usable range is typically lower and can vary widely from one installation to the next. Therefore, you should presume a max range of 50 ft. GEN3 sensors store a factory-assigned ID (address) and need only be registered (i.e., paired) once at the controller/receiver. Long-life lithium battery is included, lasting 5 years or more.


Tried and true, our GEN2 classic (X10) sensor has withstood the test of time. Operating at 310 MHz and featuring internal copper-coil antenna, this device packs a wallop. Less susceptible to signal degradation issues, it yields an open air transmit range of 100 ft. (50 to 75 ft. typical). The sensor stores a random, self-assigned ID (address). Therefore, re-registration at the controller is required whenever batteries are replaced. Two (2) AAA alkaline batteries are required (not included), delivering up to 2 years of reliable operation. (Or use optional lithium cells for longer battery life!)

Comparison Chart:

The chart below compares critical features of the two wireless product generations:

    GEN3   GEN2
  • Modern design
  • Permanent ID (address)
  • Long-life lithium battery (included)
  • Copper coil antenna
  • Less susceptible to interference
  • Higher usable transmit range
  • Moderate range
  • Temporary ID (address)
  • Batteries not included

What is the current-switching capacity?

The Kadtronix family of smart relay systems are rated as follows:

  •   5A @ 125vac

Any tips for completing a proper install?

Installation is straightforward and requires no special skills other than your existing expertise with HVAC systems and basic wiring. Consider the following tips for a smooth, trouble-free installation:

Tip #1: When installing the HSRS wireless system, be careful to avoid contacting the antenna to any grounded surface such as the HVAC equipment chassis. For best RF reception, locate the antenna as high as possible (i.e., just below the ceiling). Also, ensure a minimum separation of 3 feet from any of the following:

  •   - Wireless devices (wi-fi routers, cordless phones, etc.)
  •   - Metal equipment (HVAC, laundry appliances, water heater, metal shelving, etc.)

Tip #2: If you are considering a wireless sensor to be used on a metal door, patio door, or other metal framed opening, apply masking tape to temporarily mount the sensor until confirming proper signalling with the controller/receiver. Consult the following resource for details:

Tip #3: When installing a wireless door/window sensor, consider the following:

  • - The mini sensor requires no setup other than inserting a CR2032 battery (included)
  • - The basic sensor contains two wiring screw terminals which must be wired together with a short jumper. Apply the jumper prior to inserting the battery. (The jumper can be omitted if you plan to apply a wired door switch at these wiring terminals.)
  • - The classic sensor must be properly initialized after inserting batteries. To do this, press & hold the white push-button for 5 seconds.

Tip #4: Before permanently mounting wireless door/window sensors, first complete the registration (i.e., pairing) procedure, keeping the sensors close at hand for convenience. Following registration, test to ensure the controller unit recognizes door sensor activity. Once you are satisfied the system is behaving properly, apply the sensors as desired.


Is it safe for my A/C compressor?

Absolutely. First, consider that modern HVAC systems are equipped with built-in delay mechanisms to ensure that refrigerant pressures have equalized before allowing the compressor to start. Second, our HSRS controller features its own configurable delays, described below:

Delay #1 minimizes the possibility of short-cycling by preventing shut-off unless the monitored door or window remains open for a preset time period. Configurable from 0 to 600 minutes (10 hours), the typical setting for most installations is 2 minutes. Therefore, a door must remain open continuously for 2 minutes before shut-off is invoked.

Delay #2 is a configurable re-activation timer that prevents the HVAC system from restarting until the desired time elapse. As mentioned earlier, most HVAC systems provide their own built-in delay for this purpose, so a secondary delay is typically not required. Still, we find that some customers prefer to deploy it for added peace of mind.