Staged Auxiliary Heat

Staged Auxiliary Heat

Available on firmware 4.4.11+ — Coming Soon

Overview

Staged Auxiliary Heat lets your Mysa Smart Thermostat automatically engage auxiliary heating when your heat pump can't reach the setpoint on its own. Instead of manually switching to Aux/Emergency mode on cold days, the thermostat handles it for you based on temperature and runtime thresholds you configure.

This feature is off by default and only appears for system configurations that support auxiliary heat. The feature can be enabled and thresholds set via the Device Settings > Thermostat Configuration Menu.

Supported System Configurations

The feature is available for system configurations with 1 or 2 stages of heat plus auxiliary back-up heating (configuration codes ending in C, D, K, L, O, P; most common configurations being 31P/L and 86P/L.

• System type 1 — Compressor Stages: 1 stage of heat pump — Auxiliary Heat Type: Electric
• System type 2 — Compressor Stages: 2 stages of heat pump — Auxiliary Heat Type: Electric
• System type 3 — Compressor Stages: 1 stage of heat pump — Auxiliary Heat Type: Fossil fuel (gas/oil)
• System type 4 — Compressor Stages: 2 stages of heat pump — Auxiliary Heat Type: Fossil fuel (gas/oil)

Note: The auxiliary heat type determines whether the compressor and aux can run simultaneously. Electric auxiliary heat allows simultaneous operation. Fossil fuel (gas/oil) does not — the compressor must shut off when the furnace engages.

How It Works

The thermostat monitors two things:

1. Temperature difference — How far the room temperature is from your setpoint
2. Runtime — How long the heat pump has been running trying to reach the setpoint

When either threshold is exceeded, auxiliary heat engages automatically.

For electric auxiliary heat: The compressor keeps running alongside the aux heat.

For gas or oil auxiliary heat: The compressor stops and aux runs alone. When thresholds are met and aux heating turns off, there's a 2-minute delay before the compressor restarts, however the fan continues running during this time.

Upstaging Logic (Single-Stage Compressor)

For system types 1 and 3 (one compressor stage):

1. Stage 1 heat (compressor) engages when room temperature falls below the heat setpoint by the base differential (defaulted to 0.6°C / 1°F).
2. Auxiliary heat engages when room temperature falls further below the setpoint by the Auxiliary Heat Delta value, or when the compressor has been running longer than the Auxiliary Heat Time Delay without reaching the setpoint.

Upstaging Logic (Two-Stage Compressor)

For system types 2 and 4 (two compressor stages), the thermostat manages three heating levels:

1. Stage 1 heat (compressor) engages when room temperature falls below the heat setpoint by the base differential (defaulted to 0.6°C / 1°F).
2. Stage 2 heat (compressor) engages when room temperature falls further below the setpoint by the 2nd stage heat Minimum Temperature Difference value, or when Stage 1 has been running longer than 2nd stage heat Minimum Temperature Difference without reaching the setpoint.
3. Auxiliary heat engages when room temperature falls further below the setpoint by the Auxiliary Heat Delta, or when Stage 2 has been running longer than the Auxiliary Heat Time Delay without reaching the setpoint.

Note: Compressor heat stages can only engage once any Minimum Compressor Off Time (defaulted to 5 minutes) and maximum cycles-per-hour (3 cycles/hr for heat pump compressors; 12 cycles/hr for aux heating) limits are satisfied, preventing rapid short cycling.

Delta Stacking (Two-Stage Systems)

For two-stage systems, the temperature thresholds stack relative to the heat setpoint:

Heat Setpoint
└── -differential (0.6°C) → Stage 1 compressor ON
└── -minimum temperature difference (user set) → Stage 2 compressor ON
└── -auxiliary heat delta (user set) → Auxiliary heat ON

Downstaging/Reverse Logic (Two-Stage Compressor)

As the room temperature approaches the setpoint, the thermostat automatically downstages to avoid overshooting:

• Auxiliary heat drops out as soon as the room temperature recovers back up to the Stage 2 thresholds (i.e., within Minimum Temperature Difference of the setpoint for two-stage systems, or within the base 0.6°C differential for single-stage systems).
• Stage 2 compressor drops back out when temperature recovers to within the base 0.6°C differential.
• Stage 1 compressor continues running until the setpoint is met.

This "reverse staging" reduces overshoot and improves efficiency by turning off the most expensive heating stage as early as possible.

Configurable Settings

You can adjust these settings and thresholds in the app. Temperature values use 0.5°F (~0.3°C) increments and time values use 10-minute increments.

Stage 2 Heating (Heat Pump)

• Minimum Temperature Difference — Description: Temperature difference that triggers Stage 2 heating — Default: 1.9°C (3.5°F) — Range: 1.1-4.4°C (2-8°F)
• Heat Stage Time Delay — Description: Runtime before Stage 2 kicks in — Default: 20 min — Range: 10–120 min

Auxiliary Heat

• Auxiliary Heat Delta — Description: Temperature difference that triggers aux heat — Default (Electric): Single Stage: 2.2°C (4°F) / Two Stage: 3.3°C (6°F) — Default (Gas/Oil): Single Stage: 2.5°C (4.5°F) / Two Stage: 3.9°C (7°F) — Range: 0.5–5°C
• Auxiliary Heat Time Delay — Description: Runtime before aux kicks in — Default (Electric): 40 min — Default (Gas/Oil): 60 min — Range: 10–120 min

Stage 2 Cooling

• Minimum Temperature Difference — Description: Temperature difference that triggers Stage 2 cooling — Default: 1.9°C (3.56°F) — Range: 1.1-4.4°C (2-8°F)
• Cool Stage Time Delay — Description: Runtime before Stage 2 cooling kicks in — Default: 20 min — Range: 10–120 min

Short Cycling Protection

To protect your equipment and improve efficiency, the thermostat enforces minimum run times and delays:

• At least a 5-minute gap between compressor cycles (Minimum Compressor Off Time = 5 mins)
• Cycles-per-hour (CPH) limits are enforced before compressor can re-engage (CPH = 3 cycles/hr for compressor; 12 cycles/hr for auxiliary heating)
• For gas/oil aux, a 2-minute reverse-staging delay before the compressor restarts after aux turns off (fan runs during this delay)
• Electric aux can run simultaneously with the compressor, so no reverse-staging delay is needed

Choosing Your Auxiliary Type

When setting up staged aux heat, the app will ask what type of auxiliary heating you have. The options are:

• Electric Resistance — Aux heat runs simultaneously with the heat pump compressor when required
• Gas — Compressor shuts off when aux (furnace) engages; 2-minute delay on reverse-staging
• Oil — Same behavior as gas
• Not Sure — Defaults to Gas/Oil behavior (compressor off during aux) as a safe fallback

Caution: Selecting the wrong auxiliary type can cause inefficient or potentially damaging behavior. An incorrect heat type selection could be detrimental when the system attempts to run HP and AUX at the same time. A warning pop-up will appear during setup to confirm your selection.

Troubleshooting

Aux didn't engage when I expected it to

Check that:

• Staged Auxiliary Heat is enabled
• Either the temperature difference OR runtime threshold was met
• The app displays rounded values for convenience, but the thermostat uses precise values internally — borderline cases may behave slightly differently than expected
• Short cycling protection timers may be preventing re-engagement if the compressor recently cycled on

Aux and compressor running at the same time (fossil fuel system)

If you have a gas or oil furnace and the compressor is running simultaneously with auxiliary heat, the auxiliary type is likely set incorrectly to "Electric Resistance." Change the setting to the correct fuel type immediately to prevent potential equipment damage.

Manual Aux/Emergency Mode

If you manually set the thermostat to Aux/Emergency mode (from the device or app), automatic staging defers to your manual selection. The automatic staging logic won't override manual emergency mode.

Note: The EMER indicator on the thermostat display only appears when you've manually engaged Aux/Emergency mode. When in Aux/Emergency mode, EMER appears on the screen any time you interact with the device as a reminder the system is using the aux heat back-up only and not the heat pump compressor. Automatic staging doesn't show this indicator — it works in the background.

Need help? Contact our support team at support@getmysa.com for questions about staged auxiliary heat or thermostat configuration.