The GS366A-MWAC is compatible with most 24 VAC residential heating and cooling systems commonly used in North America. This includes many gas, oil, electric, conventional furnace and air-conditioning, heat-pump, multi-stage, and dual-fuel systems.

Before installation, use the Siterwell Compatibility Checker or follow the compatibility guidance provided through the Siterwell Link app.

The GS366A-MWAC is designed for low-voltage 24 VAC HVAC systems. It should not be installed on 110 V or 220 V high-voltage systems, millivolt heating systems, DC-powered HVAC systems, many proprietary communicating systems, or solid-fuel heating systems using wood, coal, biomass, or similar fuels.

Common signs of an unsupported high-voltage system include terminals marked L1 or L2, voltage markings such as 110 V or 220 V, or thermostat wires joined using wire nuts. If your existing thermostat uses proprietary terminals such as V+/Vg, D+/D−, 1/2/3/4, or A/B/C/D, verify compatibility before proceeding.

The GS366A-MWAC requires continuous 24 VAC power. Most installations use either an existing C wire or the Siterwell GS136A-WCAN Power Extender Kit, also known as the PEK, on compatible HVAC systems.

If there is an unused thermostat wire behind the wall, it may sometimes be used as a C wire if the same wire can be connected to the C terminal at the HVAC control board. Always verify both ends of the wire before using it as C.

The GS136A-WCAN Power Extender Kit is designed for compatible HVAC systems that do not have a C wire at the thermostat.

It allows the thermostat to receive the continuous power it requires while using the existing thermostat wiring. The PEK connects to the HVAC control board and should always be installed according to the official GS136A-WCAN installation diagram.

No. The PEK should only be used with compatible HVAC configurations.

It is not designed for dual-transformer systems where heating and cooling are powered by separate transformers. If your thermostat has two separate power wires entering the wall and connected independently to Rc and Rh, verify the HVAC configuration before installing a PEK.

Do not transfer jumper wires from the old thermostat to the GS366A-MWAC.

Older thermostats often use a short jumper wire or metal staple between Rc and Rh. The GS366A-MWAC manages this connection internally when required.

Only connect wires that come from the wall. If your system has only one R-family wire labeled R, Rc, or Rh, it normally connects to the Rc terminal.

Some thermostats display one set of terminal labels for conventional systems and another for heat-pump systems. Before connecting the GS366A-MWAC, determine which type of HVAC system you have.

Do not determine the system type based only on the physical side of the connector where the wire is located. Useful indicators include whether the outdoor unit provides both heating and cooling, the HVAC equipment model number, manufacturer documentation, O/B or auxiliary-heat terminals, and the wiring configuration shown at the HVAC control board.

Wire color alone is not a reliable way to identify the system.

The most reliable method is to check the model number and documentation for your outdoor HVAC unit.

A system may be a heat pump if the outdoor unit provides both heating and cooling. Other possible indicators include an O/B reversing-valve wire, AUX or AUX/E wiring, or electric auxiliary heat inside the air handler.

Some packaged HVAC systems use different configurations, so these indicators should not replace proper equipment identification.

Do not guess the wire functions.

Turn off power to the HVAC system and take a clear photo of the existing thermostat wiring. Locate the HVAC furnace or air-handler control board and trace the thermostat wires to their terminal connections. Record the terminal label connected to each wire before installing the GS366A-MWAC.

Wire color may help trace a wire, but the terminal at the HVAC control board determines its actual function. If the wires cannot be positively identified, contact an HVAC professional.

No.

Thermostat wire colors are not standardized.

Always identify a wire using the terminal it is connected to.

For example, a red wire should only be treated as an R wire when it is actually connected to an R-family terminal.

Short jumper wires connecting two terminals should not be entered into the Siterwell Compatibility Checker.

Two wires entering the wall and connecting separately to Rc and Rh may indicate a dual-transformer HVAC system.

Typically:

• Rc supplies cooling power
• Rh supplies heating power

A short jumper between Rc and Rh with only one R wire entering the wall does not normally indicate a dual-transformer system.

For a true dual-transformer installation, the C wire must come from the cooling-side equipment.

The GS136A-WCAN PEK should not be used with a dual-transformer system.

Professional installation is recommended for these configurations.

Some systems may use terminals such as:

• S / S1 / S2
• L
• G2 / GM
• G3 / GH
• W3
• Y3
• Separate O and B

Some of these functions are not directly supported or may require additional HVAC configuration.

Unused wires should be individually insulated and secured so they cannot contact another terminal or conductor.

Professional HVAC assistance is recommended when uncommon wiring controls additional fan speeds, heating stages, or proprietary equipment.

Many standard residential installations can be completed in approximately 45 minutes or less.

Before starting, confirm HVAC compatibility, turn off power to the HVAC system, photograph the existing thermostat wiring, label every connected wire, and follow the installation instructions provided with the thermostat or through the Siterwell Link app.

Professional installation is recommended for advanced, unidentified, high-voltage, proprietary, or dual-transformer systems.

Before switching off power, change the thermostat temperature enough to make the heating or cooling system start. During heating season, raise the setpoint. During cooling season, lower the setpoint.

Then turn off HVAC power at the breaker or system power switch. Wait approximately five minutes and confirm that heating or cooling does not start, no conditioned air is coming from the vents, and HVAC equipment indicator lights are off.

If the equipment still appears powered, locate and switch off the appropriate breaker before continuing.

After HVAC power has been turned off, carefully remove the thermostat display from its wall plate.

Take a clear photo of all connected wires before disconnecting anything. Label each wire according to its terminal, then disconnect the wires while preventing them from falling back into the wall.

Some older thermostats may include an additional plate that must be removed before the terminal labels become visible.

Turn off HVAC power before removing the thermostat.

Grip the thermostat securely and pull it straight away from the wall. Avoid twisting or tilting the thermostat while removing it from the base.

If you need to remove the backplate itself, photograph and label the wiring first, release and disconnect each wire, remove the mounting screws, and then lift the backplate away from the wall.

If the thermostat screen remains black, first confirm that the power connection and C wire are secure at both the thermostat and the HVAC control board. If a spare wire has been reassigned as C, verify that it is connected correctly at both ends.

Next, remove the thermostat and make sure excess wiring is not preventing it from sitting fully against the base. Reinstall the thermostat and confirm that it is securely seated.

Also confirm that the HVAC breaker is on and that furnace or air-handler access panels are fully closed. Some HVAC systems use safety switches that interrupt thermostat power when an access panel is open.

Some systems also use a 3 A or 5 A control-board fuse. If that fuse is blown, replace it only with one of the same amperage.

If the thermostat previously worked and suddenly lost power, also check for a tripped breaker, blown fuse, or HVAC condensate safety issue.

Verify the complete PEK installation against the official GS136A-WCAN wiring diagram.

Check the PEK connections at the HVAC control board, thermostat-side connections, loose wiring, HVAC control-board power, the control-board fuse, and whether the thermostat is fully seated on its backplate.

If the problem continues, provide Siterwell Support with clear photos of both the original thermostat wiring and the GS366A-MWAC installation.

Reconfiguration may be required after changing HVAC wiring, heating or cooling stages, Y2 or W2 connections, system type, equipment configuration, or other installation-related settings.

Before changing physical wiring, turn off HVAC power and photograph the current configuration.

After reinstalling the thermostat, allow it to complete startup before continuing with system configuration.

A reset can be useful when you intentionally want to clear schedules, preferences, or the complete device configuration.

A reset should not automatically be the first troubleshooting step for HVAC performance or connectivity problems.

Reset Schedule and Preferences removes the existing schedule and restores changed preferences to their default values.

Reset All returns the thermostat to factory settings. This also removes the thermostat from the Siterwell Link account, clears device settings, clears HVAC configuration, and requires the thermostat to be set up again.

HVAC configuration may need to be restored carefully after a full reset.

Available system modes depend on the HVAC configuration.

Heat mode allows the thermostat to call for heating only. Cool mode allows it to call for cooling only.

Heat/Cool Auto mode allows the thermostat to automatically switch between heating and cooling based on separate heating and cooling setpoints.

Aux mode is available for compatible heat-pump systems configured with auxiliary heat. Only auxiliary heat operates while this mode is selected.

Off mode prevents the thermostat from calling for heating or cooling, although the fan may still operate depending on the selected fan settings.

Auto mode is useful when outdoor conditions may require heating during one part of the day and cooling during another. The thermostat uses separate heating and cooling setpoints. If the indoor temperature falls below the heating setpoint, the thermostat calls for heat. If the indoor temperature rises above the cooling setpoint, it calls for cooling.

Comfort Settings store preferred temperature and fan configurations for different situations. The thermostat includes default Comfort Settings for Home, Away, and Sleep. These settings can be used within your weekly schedule or activated temporarily using a Comfort Setting Hold. Additional Comfort Settings can be created through the Siterwell Link app.

The GS366A-MWAC supports a seven-day schedule. Each scheduled activity determines when a selected Comfort Setting begins. You can edit schedules from either the thermostat or the Siterwell Link app. Activities can be added, edited, removed, or copied to other days. A schedule must always contain at least one activity. If an activity is deleted, the previous activity continues until the next scheduled activity begins.

A Hold temporarily overrides the normal thermostat schedule. A Manual Hold lets you select a temporary temperature, while a Comfort Setting Hold lets you temporarily activate a saved Comfort Setting such as Home, Away, or Sleep. While a Hold is active, the normal schedule is temporarily ignored. When the Hold expires or is cancelled, the thermostat returns to its normal schedule.

Available Hold behavior includes temporary timed Holds, a Hold that remains active until the next scheduled activity, a Hold that remains active until you manually change it, or an option that lets you select the duration each time a temperature adjustment is made. Your preferred Hold behavior can be configured through Device Preferences.

When a Hold is active, its status appears on the thermostat home screen. Select the Hold status and cancel it to return to the normal schedule. Some setting changes may not take effect as expected while an existing Hold remains active.

The GS366A-MWAC allows the HVAC fan to operate in several ways. In Auto mode, the fan follows normal HVAC operation. In On mode, the fan can run continuously while the selected Comfort Setting or fan control is active. Minimum Fan Runtime can be used to circulate indoor air for a minimum amount of time each hour, even when active heating or cooling is not required. A temporary Fan Hold can keep the fan running independently for a selected period. A Fan Hold may remain active even when System Mode is set to Off.

The thermostat temperature display can be changed through Device Preferences. You can choose either Fahrenheit or Celsius according to your preference.

Yes. Heating and Cooling Ranges allow you to restrict the available temperature setpoints. The source configuration lists a default heating range of 45–79°F, or 7–26°C, and a default cooling range of 65–92°F, or 18.5–33.5°C. This can help prevent unintended temperature changes.

Temperature Correction allows you to compensate for a consistent difference between the thermostat reading and a trusted reference thermometer. The temperature reading can be adjusted by up to ±10°F or ±5°C. Use this feature when the thermostat consistently reads slightly higher or lower than a reliable comparison thermometer. If the difference changes significantly over time, the cause may be environmental rather than a consistent sensor offset.

The thermostat home screen displays the current HVAC operating status. Depending on system configuration and operating mode, it may indicate heating, cooling, fan operation, Auto mode, auxiliary heat, or an active Hold. The Equipment Running screen can provide additional information about active HVAC equipment. For multi-stage systems and heat pumps, it may also display the active stage or heating source.

Access Control lets you protect selected thermostat functions using a four-digit security code. Protected functions can include System Mode, temperature adjustments, Vacation settings, schedules, Comfort Settings, device preferences, date and time settings, Wi-Fi settings, and installation settings. You can change which functions require the code or disable Access Control when required.

Access Control can be managed through the Siterwell Link app. If you cannot restore access through the app, contact Siterwell Customer Support for assistance.

The Siterwell Link app provides remote access to supported thermostat functions. Depending on the thermostat configuration and firmware, this can include temperature control, Comfort Settings, scheduling, Vacation settings, fan settings, device preferences, device naming, weather-related location settings, firmware information, and Access Control management.

Open the thermostat in the Siterwell Link app and go to the device settings. Select Device Name, enter the new name, and save the change. Using clear device names can make it easier to manage multiple thermostats or locations.

Vacation Mode allows you to define a temporary HVAC configuration for a period when the home is unoccupied. You can set a departure date and time, return date and time, vacation temperature, and fan behavior. Vacation settings can be created, edited, or removed using either the thermostat or the Siterwell Link app. When the vacation period ends, the thermostat can return to normal operation.

Open the thermostat in the Siterwell Link app and access the Vacation settings. Choose Set a Vacation, enter the departure date and time, enter the return date and time, select the desired temperature and fan settings, and save the vacation. A vacation can also be created directly from the thermostat's Vacation menu.

Depending on system configuration, the GS366A-MWAC can provide alerts related to unusually low indoor temperature, unusually high indoor temperature, low humidity, high humidity, heating performance, cooling performance, auxiliary heat runtime, and auxiliary heat operation at higher outdoor temperatures. Available alert settings depend on the HVAC configuration.

Yes. Thermostat display alerts can be disabled while alerts remain available through other supported notification methods. Heating and cooling equipment alerts can also be enabled or disabled through the relevant settings.

Weather information is based on the home address registered through the Siterwell Link app. The address is converted into location information and used to retrieve relevant weather data. For weather information to appear, the thermostat must be registered to a Siterwell Link account, connected to Wi-Fi, and configured with the correct home location.

Confirm that the thermostat is registered to your Siterwell Link account and connected to Wi-Fi. Also verify that the correct home address is saved in the app. If the thermostat previously displayed weather information but has lost its connection, restart the thermostat and check the connection again. If weather information still does not appear, contact Siterwell Customer Support.

Open the Siterwell Link app and review the address assigned to the home. Re-enter or correct the home address and save the change. The thermostat uses this location to retrieve weather information. If the location is correct and the issue continues, contact Siterwell Customer Support.

When connected to Wi-Fi, the GS366A-MWAC can receive firmware updates automatically. Firmware releases may be deployed gradually to eligible devices. Routine firmware updates normally do not require manual installation.

Firmware information can be viewed from the thermostat's About information or through the device settings in the Siterwell Link app. Firmware version numbers are identifiers rather than decimal values. Different devices may display slightly different versions when maintenance or patch releases are deployed.

Yes. The GS366A-MWAC supports integration with compatible Amazon Alexa smart-home environments.

Yes. The GS366A-MWAC supports integration with compatible Google Home environments.

The home screen displays the current indoor temperature prominently and provides access to temperature adjustment, system mode, weather information, indoor humidity, equipment status, Hold status, and the Main Menu. Weather information is available when the thermostat is registered, connected to Wi-Fi, and configured with a valid home location. Equipment status indicates when heating, cooling, or fan operation is active. Hold status indicates when a temporary temperature override is in effect. The Main Menu provides access to thermostat settings and configuration options.

prepare:

• A clear photo of the original thermostat wiring
• A clear photo of the GS366A-MWAC wiring
• The HVAC equipment model number, if available
• A description of the issue
• Any alert or error information displayed on the thermostat

For unusual wiring, dual-transformer systems, unidentified terminals, or HVAC equipment requiring advanced configuration, consultation with a qualified HVAC professional is recommended.