My parents called me since their thermostat was stuck in a 17 deg HOLD mode.  When I looked at it, it did appear to be wonky and pulling out the batteries did not reset it.

Off to Home Depot and I picked up the cheapest Honeywell programmable one at $35.

Removing the old one, the red, white and green wires were connected to the corresponding terminals but there was a black wire connected to W2 and a blue wire tucked in behind unconnected.  Googling W2 indicated this was a second heating stage and I needed a thermostat with more than single stage control.

Back to Home Depot again and I returned with the cheapest Honeywell programmable with two stages for $80.  Checking online afterwards, the cheapest Honeywell two stage is $37 USD on Amazon (RTH6350).  A newer model, the RTH6360 is $49 and features an upgraded interface and a calendar based clock that can automatically adjust for daylight savings.

A little research indicates that single stage furnaces run fully on or off and a second stage adds a lower heat mode which theoretically saves gas and keeps the temperature more consistent.  By default, thermostats are single stage only so you need to check the specs to see if they support the second stage.  Sometimes this is marked as 2H (2 heat)

Two stages may need to be set in the system settings.  My model also allowed me to changed it to single one week programming versus the default 5-1-1 ( separate programs for 5 weekdays, Sat & Sun).

The standard wiring is

Red – 24VAC
White  – Heat control
Green – Fan control
Yellow – Cool control (air conditioning is rare in my locale)
Blue – Common (for thermostats that need power like Nest which is why it was tucked away)
Black – Wired to W2 second stage heat in my scenario