Furnace Thermostat Setup Question

skooter

Hello,

I installed a new digital, non-programmable, furnace thermostat for my 2 stage natural gas furnace. I am trying to determine which settings would be the most economical. There isn't much to setup really except for one item that I would like to find out which would be the best setting for it that would use less gas. The setting is the: SELECT STAGE FAST (ON or OFF) When its selected "ON"(which is the default setting) the second stage begins immediately when the SETPOINT is raised more than 3 degree's F in HEAT.
When its selected "OFF" the second stage will be delayed approximately 15 minutes before starting. So the question is which is the most economical setting, ON or OFF?

Furnace is an American Standard Freedom 80 and thermostat is a White-Rodgers 1F83-261

Thank you for any information.
.............skooter

HVAC Doc

If I am remembering correctly (don't have the specs near me), 1st stage I beleieve is low fire. Check your installation/owners manual to find out for sure. Most manufactures recommend low fire be set to run as much as possible because it gives you more even heat and also is more economical (60% open) for those times you do not have a high demand (sunny day 45 degrees.). vs. the wide open full throttle (100%). On high demand days, if you had a single stage stat to begin with or the installer chose the default board setting, the unit would run approx. 15 min. anyways before kicking up to high fire.

skooter

Thank you again HVAC Doc! Yes 1st stage is low fire. I am going to change the thermostats default setting from ON to OFF and let it do the 15 min thing before going to 2nd stage. I appreciate your advice sir! Best of the new year to you!

...........skooter

Comfortman

the only benefit of extending "low fire" operation is increased run time
this will usually result in more even heat throughout the house
there is no energy efficiency gain however, as an 80% furnace is always an 80% furnace
it is simply running at a lower input rate in "low fire"
if it were in "high fire" it would be burning more fuel, but the resulting shorter run time would mean the same amount of fuel would be burned to meet the demand

HVAC Doc

Actually, that statement isn't correct. As any manufacture will tell you, when using a properly sized 2 stage furnace operates at a lower capacity most days except in extremes when high fire is needed. Just because it is in low stage does not mean it "increases or doubles" its run time. You are only using the capacity needed to heat your home at that given time.