diff --git a/homeassistant/components/google_assistant/smart_home.py b/homeassistant/components/google_assistant/smart_home.py
index 0faa9bdc484..f9d6135107b 100644
--- a/homeassistant/components/google_assistant/smart_home.py
+++ b/homeassistant/components/google_assistant/smart_home.py
@@ -151,7 +151,7 @@ def query_device(entity: Entity, units: UnitSystem) -> dict:
     if entity.domain == climate.DOMAIN:
         mode = entity.attributes.get(climate.ATTR_OPERATION_MODE).lower()
         if mode not in CLIMATE_SUPPORTED_MODES:
-            mode = 'on'
+            mode = 'heat'
         response = {
             'thermostatMode': mode,
             'thermostatTemperatureSetpoint':
diff --git a/tests/components/google_assistant/test_google_assistant.py b/tests/components/google_assistant/test_google_assistant.py
index 3b9ad7f3ef7..500e8ece143 100644
--- a/tests/components/google_assistant/test_google_assistant.py
+++ b/tests/components/google_assistant/test_google_assistant.py
@@ -215,7 +215,7 @@ def test_query_climate_request(hass_fixture, assistant_client):
         'climate.ecobee': {
             'thermostatTemperatureSetpointHigh': 24,
             'thermostatTemperatureAmbient': 23,
-            'thermostatMode': 'on',
+            'thermostatMode': 'heat',
             'thermostatTemperatureSetpointLow': 21
         },
         'climate.hvac': {
@@ -263,7 +263,7 @@ def test_query_climate_request_f(hass_fixture, assistant_client):
         'climate.ecobee': {
             'thermostatTemperatureSetpointHigh': -4.4,
             'thermostatTemperatureAmbient': -5,
-            'thermostatMode': 'on',
+            'thermostatMode': 'heat',
             'thermostatTemperatureSetpointLow': -6.1,
         },
         'climate.hvac': {