Appendix Ffurnace Staging Algorithm - Bryant 355CAV A Series Troubleshooting Manual

On initial thermostat call for heat after power-up, furnace staging algorithm will provide 1 minute of medium heat followed by 15 minutes of low heat
provided setup switches SW1-2 and SW4-2 are off. If call for heat still exists after operating for 1 minute in medium heat and 15 minutes in low heat,
the furnace will switch to high heat until the thermostat is satisfied.
During subsequent calls for heat:
- Low heat and medium 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 medium heat has run for 1 minute and low heat has run for 15 minutes and the 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 and medium 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- and medium-heat on times. To predict low- and medium- heat
operation, the times in Low Heat (LH), Medium Heat (MH), and High Heat (HH) on the previous cycle must be known. The furnace staging
algorithm uses these values to determine how much low heat and medium 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 and operating in
medium heat, and coming on in high heat.
EXAMPLE 1:
Furnace ran for 1 minute in medium heat, 15 minutes in low heat, and 5 minutes in high heat during previous call for heat.
LH = 15
MH = 1
HH = 5
Calculate Heat Load HL: (140 x 15) + (228 x 1) + (350 x 5) = 4078
3648 < 4078 < 5600 ⇒ Low heat run for next cycle is 0 minutes. Calculate medium heat run time for next cycle.
= (5600 – 4078) / 122 = 12.48 ⇒ Next cycle furnace will run 12 minutes in medium heat and then switch to high heat, if heat call is still
MH
run time
present. Low heat run time is 0 minutes.
EXAMPLE 2:
Furnace ran for 1 minute in medium heat, 15 minutes in low heat, and 10 minutes in high heat during previous call for heat.
LH = 15
MH = 1
HH = 10
Calculate Heat Load HL: (140 x 15) + (228 x 1) + (350 x 10) = 5828
5828 > 5600 ⇒ Next cycle furnace will only run in high heat, low- and medium-heat run time is 0 min.
EXAMPLE 3:
Furnace ran for 12 minutes in medium heat during previous call for heat. This can happen if the furnace staging algorithm calculated 13 minutes based
on previous call for heat.
LH = 0
MH = 12
HH = 0
Calculate Heat Load HL: (140 x 0) + (228 x 12) + (350 x 0) = 2736
2328 < 2736 < 3648 ⇒ Calculate low- and medium-heat run time for next cycle.
LH = (3648 – 2736) / 88 = 10.36
run time
= (16 – 10) = 6 ⇒ Next cycle, furnace will run for 1 minute in medium heat, and up to a maximum of 10 minutes in low heat, before switching
MH
run time
back to medium heat for 5 minutes if the heat call is still present. After operating in medium heat for a total of 6 minutes the furnace will switch to high
heat, if heat call is still present.
EXAMPLE 4:
Furnace ran for 0 minutes in low heat, 0 minutes in medium heat, and 6 minutes in high heat during previous call for heat.
LH = 0
MH = 0
HH = 6
Calculate Heat Load HL: (140 x 0) + (228 x 0) + (350 x 6) = 2100
2100 < 2328 ⇒ Next cycle furnace will only run for 1 minute in medium heat, then switch to low heat for 15 minutes maximum, then switch to high
heat, if heat call is still present.
APPENDIX F
FURNACE STAGING ALGORITHM
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