Heat pump or resistance furnace - which one do you want to turn on?
A heat pump provides heat two ways:
1. The mechanism is essentially an air conditioner with the ability to work backwards. To heat a house it literally compresses the heat from outdoors and brings it inside. When it's too cold outdoors the heat pump becomes less efficient, and eventually unable to provide heat. So,
2. There is a resistance furnace to help it. The resistance furnace is a big toaster. Electric coils heat up and air blows over them to heat the house. The resistance heat is 100% efficient, but much more expensive than heat pump heat.
To turn on the heat pump the thermostat button should be set on "HEAT." You can see in the photo that the button here was set on "HEAT." When in that position only the green light should come on.
If the heat pump breaks the resistance furnace can be turned on so the house can be heated while one waits for the compressor to be fixed.
In that case the thermostat button would be set to Emergency Heat, sometimes called Supplemental Heat.
When the button is set in that position the red light should also come on.
But when the button is set on "HEAT" both lights should NOT come on! In this house both lights did turn on, as indicated by the yellow arrows.
The resistance furnace provides much warmer heat than the heat pump alone would blow from the ducts.
So, is the problem here that the old, analog thermostat is broken and both lights are merely coming on, or is the resistance furnace actually turning on?
Mighty Mo saw that the heat in the duct just over the unit was very warm, too warm for just the heat pump to be providing heat alone.
The heat pump compressor was operating at this time, so it did turn on as demanded. And we see that the resistance furnace must also be on as the temperature just above the unit is so warm.
My recommendation: this was an older unit and an older thermostat - the plaque said 24 years. The heat pump compressor's life span is about 12 - 15 years. But that is not hard and fast. Their life span depends on maintenance and care. When the compressor is broken heating the house is very expensive! And many homeowners would not know. I have inspected houses in the past where the resistance furnace was always operating at the same time as the compressor and find out that one reason the people were selling the house was because the electric bills were so high!