The algorithm in a 2-stage stat is much more sophisticated than what
the furnace board can hope to achieve. The furnace can only take into
account how long the thermostat called for heat and how long the
furnace spent in low and high fire mode to satisfy that call, and
perhaps how long it has been since the last call. As you note, the
thermostat can take all that into account as well as the rate of
temperature change.

Here's your answer from the horse's mouth:

http://www.whiterodgers.com/homeowne...Set_80_faq.htm

"1F8X-XXX Series Multi-stage and Heat Pump thermostats are designed to
determine the optimum time to activate the second stage. Simply
raising the temperature in heating or lowering it in cooling will not
always force the thermostat to bring the second stage on quickly.
There is a time delay from 0-25 minutes. The time element (0-25
minutes) of the delay varies depending on the performance of the first
stage. In Heating for example, assume for the last 2 hours the
thermostat is set on 70 and the room temperature is 70 with the
equipment using only the first stage of heat. The thermostat
calculates (internally) the performance of the system. Since the
equipment is keeping the temperature within 1 degree of setpoint the
thermostat will delay second stage for a longer time if you manually
raise the temperature or if the room temperature quickly changes. Once
the second stage comes on it will come on sooner the next time there
is a difference between the setpoint and the room temperature. The net
effect of the staging program is when the first stage is capable of
making temperature the second stage will delay longer. When the
thermostat calculates first stage cannot make temperature in a
reasonable time the second stage will come on sooner. This is a built
in function that automatically optimizes the use of additional stages
of heat or cool. Most thermostat models have a method to bring the
additional heat or cool stages on quickly for testing purposes listed
in the installation instructions under the heading of "Testing Your
Heat/Cool System."

%mod%

(Dave) wrote in message . com...
I recently had a Trane two-stage variable speed furnace (model
TUY060R9V3)installed along with a White Rodgers 1F80-261 single-stage
thermostat. My installer noted that the single-stage thermostat,
properly jumpered, works as well as using a White Rodgers 1F81
two-stage thermostat, as the 1F81 model operates by timer in the same
way as the single-stage, properly jumpered, would (i.e., ten minutes
on low fire, then switch to second stage heat).

Based on what I've read and discussions I've had with others, it seems
that a two-stage thermostat would in fact take into account
temperature differentials, etc., in determining when to switch to high
fire, not just time.

Is anybody familiar with this issue, specifically with respect to
White Rodgers thermostats? Apologies in advance if this is an obvious
question to some of you. I am no expert on heating, but it seems that
my two-stage system should be running with a multi-stage thermostat.

Thanks in advance for any comments.
Dave