Appendix F - - Furnace Staging Algorithm - Carrier PG9UAA Troubleshooting Manual

On initial thermostat call for heat after power- -up, furnace staging algorithm will provide 16 minutes of low heat provided setup switch
SW1- -2 is off. If call for heat still exists after operating for 16 minutes in low heat, furnace switches to high heat until the thermostat is
satisfied.
During subsequent calls for heat:
- -Low heat run time is calculated based on previous heating cycle.
- -High heat run time is not calculated. High heat is energized in 1 of 3 ways and runs until the thermostat is satisfied:
1. After low heat has run for 16 minutes and call for heat is still present.
2. When the furnace staging algorithm (using previous cycle information) determines high heat is necessary for the entire thermostat
cycle.
3. After the calculated low heat on time has elapsed and the call for heat is still present.
— The following flow chart shows how the furnace staging algorithm calculates low heat on times. To predict low heat operation, the
times in Low Heat (LH) and High Heat (HH) on the previous cycle must be known. The furnace staging algorithm uses these
values to determine how much low heat operation will occur on the next call for heat.
— Four examples are shown below to illustrate what the furnace will do on a cold start, coming out of night setback, coming on in low
heat, and coming on in high heat.
EXAMPLE 1:
Furnace ran for 16 minutes in low heat and 5 minutes in high heat during previous call for heat.
LH = 16
HH = 5
Calculate: (228 x 16) + (350 x 5) = 5398
3648 < 5398 < 5600 ⇒ Calculate low heat run time for next cycle.
LH = (5600 - - 5398) / 122 = 1.6 ⇒ Next cycle, furnace will run for 2 minutes in low heat and then
run time
switch to high heat, if heat call is still present.
EXAMPLE 2:
Furnace ran for 16 minutes in low heat and 10 minutes in high heat during previous call for heat.
LH = 16
HH = 10
Calculate: (228 x 16) + (350 x 10) = 7148
7148 > 5600 ⇒ Next cycle furnace will only run in high heat, low heat run time is 0 min.
EXAMPLE 3:
Furnace ran for 13 minutes in low heat and 5 minutes in high heat during previous call for heat. This can
happen if the furnace staging algorithm calculated 13 minutes based on previous call for heat.
LH = 13
HH = 5
Calculate: (228 x 13) + (350 x 5) = 4714
3648 < 4714 < 5600 ⇒ Calculate low heat run time for next cycle.
LH = (5600 - - 4714) / 122 = 7.26 ⇒ Next cycle, furnace will run for 7 minutes in low heat and then
run time
switch to high heat, if heat call is still present.
EXAMPLE 4:
Furnace ran for 0 minutes in low heat and 10 minutes in high heat during previous call for heat.
LH = 0
HH = 10
Calculate: (228 x 0) + (350 x 10) = 3500
3500 < 3648 ⇒ Next cycle furnace will only run in low heat for 16 minutes maximum, then switch to high
heat, if heat call is still present.
APPENDIX F
FURNACE STAGING ALGORITHM
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