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APPENDIX F
FURNACE STAGING ALGORITHM
On initial thermostat call for heat after power-up, furnace staging algorithm will provide 45 seconds of intermediate heat followed by 19 minutes of
minimum heat provided setup switches SW1-2 and SW4-2 are off. If call for heat still exists after operating for 45 seconds in intermediate heat and
19 minutes in minimum heat, the furnace will switch to maximum heat until the thermostat is satisfied.
During subsequent calls for heat:
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Modulating rate is calculated based on previous heating cycle.
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Maximum heat run time is not calculated. Maximum heat is activated in 1 of 3 ways and runs until the thermostat is satisfied:
1. After intermediate heat has run for 45 seconds and minimum heat has run for 19 minutes and the call for heat is still present.
2. When the furnace staging algorithm (using previous cycle information) determines maximum heat is necessary for the entire thermostat
cycle.
3. After the calculated modulating rate has operated for 19 minutes and the call for heat is still present.
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The following flow chart shows how the furnace staging algorithm calculates the modulating rate. To predict the modulating rate for
the next heating cycle the modulating rate (MOD%), time of operation at the modulating rate (MODT), and the time of operation at
maximum heat (MAXT) for the previous cycle must be known. The furnace staging algorithm uses these values to determine the
modulating rate for the next call for heat.
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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 at an intermediate rate (rate between 52% - 71%), and returning to minimum heat operation.
EXAMPLE 1:
Furnace ran for 45 seconds in intermediate heat, 19 minutes in minimum heat (MOD% = 40%), and 5 minutes at maximum heat during previous call
for heat.
MOD% = 40%
MODT = 19
MAXT = 5
Calculate Heat Load Requirement HLR: (40 x 19) + 65 + (100 x 5) = 1325
825 < 1325 < 1965
Modulating rate for next cycle is 66%.
Next cycle furnace will run 45 seconds at intermediate heat followed by 19 minutes
at 66% of full rate and then switch to maximum heat, if heat call is still present.
EXAMPLE 2:
Furnace ran for 45 seconds in intermediate heat, 19 minutes in minimum heat (MOD% = 40%), and 12 minutes at maximum heat during previous call
for heat.
MOD% = 40%
MODT = 19
MAXT = 12
Calculate Heat Load Requirement HLR: (40 x 19) + 65 + (100 x 12) = 2025
2025 > 1965
Next cycle furnace will only run in maximum heat.
EXAMPLE 3:
Furnace ran for 45 seconds at intermediate heat, 12 minutes at 66% of full rate (MOD% = 66%) during previous call for heat. This can happen if the
furnace staging algorithm calculated a modulating rate of 66% based on previous call for heat. See EXAMPLE 1:
MOD% = 66%
MODT = 12
MAXT = 0
Calculate Heat Load Requirement HLR: (66 x 12) + 65 + (100 x 0) = 857
825 < 857 < 1965
Modulating rate for next cycle is 42%.
Next cycle, furnace will run 45 seconds at intermediate heat, followed by 19 minutes at
42% of full rate and then switch to maximum heat, if heat call is still present.
EXAMPLE 4:
Furnace ran for 45 seconds at intermediate heat, 18 minutes at 42% of full rate (MOD% = 42%) during previous call for heat. This can happen if the
furnace staging algorithm calculated a modulating rate of 42% based on a previous call for heat. See EXAMPLE 3:
MOD% = 42%
MODT = 18
MAXT = 0
Calculate Heat Load Requirement HLR: (42 x 18) + 65 + (100 x 0) = 821
821 < 825
Next cycle furnace will only run for 45 seconds at intermediate heat, followed by 19 minutes at 40% of full rate and then switch to
maximum heat, if heat call is still present.
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