The answer is ... it all depends.
It depends on how it is installed. Many times a two stage furnace will be installed with a single stage thermostat. When that is done a timer in the board (or on some furnaces exterior to the board) cliks off about ten minutes between the stages as long as there is a continous call for heat. Sometimes a two stage thermostat is used and then when the temperature in the house drops a degree or two from set point the second stage will kick in.
My favorite way is to use a two stage thermostat and run it through an outdoor thermostat to keep the second stage from kicking in above say 45 - 50 degrees.

Any way that it is set up, proper performance depends on proper furnace sizing and proper installation

 

its not the furnace its the stat unless someone uses a singel stage stat on a two stage furnace. what you have said is usually one of the cases you have listed but also some if the furnace doesnt satisfy in a certain time period to the set temp then it also will fire in the second stage because that wouldnt be very efficiant if the furnace ran untill it dropped 3 degrees in the house then decided to to fire up the 2nd stage then some drop down to low fire right before the setpoint is reached

 

Some furnaces have dip switches that allow the furnace to operate as two-stage with single stage stat. Outdoor stat is not required with 2-stage unless it is dual fuel or thermidistat.

 

3 degree is pretty extrem. I have a Trane XV90 w/ a VP t-stat, and it cycled 2nd stage every now and then, but the temp did not drop 3 degrees.


I think the 3 degrees depends on what t-stat is used.

If you do go two stage, DO NOT let the dealer talk you useing the single stage stat.

 

I believe most digital 2-stage thermostats look at the temp in 10ths of a degree and the time it takes to move each 10th of a degree and if it is loosing ground with differant algorithms to determine when 2-stage is needed.. That is how my thermidastat operates and also how my dad's honeywell operates, only may see a drop of one degree sometimes but most times they kick into high stage without seeing a drop..

Old mercury t-stats had to have a temp drop before 2-stage would initiate.. But with these high tech digitals and the use of algorithms they can operate to a closer margin..

JMHO
J

 

The answer is ... it all depends.

My favorite way is to use a two stage thermostat and run it through an outdoor thermostat to keep the second stage from kicking in above say 45 - 50 degrees.

It seems like one may have significant sizing issues,
if the second stage is actually needed > 45'F
(~40% of design capacity @ design temp of 10'F).

Set point-Current O.A. Temp/(Set Point- Design Temp)
70'-45'/(70'-10')=
25'/60' = ~40%

Or
70'-45'/(70'-20')=
25'/50' = 50%
makes 45'F lock-out seem a bit more reasonable
in a more mild climate.