Appendix F
Furnace Staging Algorithm
On initial thermostat call for heat after power-up, furnace staging algorithm provides 16 minutes of low heat provided SW-1 and SW-2 are off.
If call for heat still exists after operating for 16 minutes in low heat, furnace switches to high heat until thermostat is satisfied.
During subsequent calls for heat:
•
Low Heat run time is calculated based on previous heating cycle.
•
High Heat on time is not calculated. High Heat is energized 1 of 3 ways and runs until thermostat is satisfied.
1. After Low Heat has run for 16 minutes and call for heat is still present.
2. When furnace staging algorithm (using previous cycle information) determines High Heat is necessary for entire thermostat cycle.
3. After calculated Low Heat on time has elapsed and call for heat is still present.
•
The following flowchart shows how furnace staging algorithm calculates Low Heat on times. To predict low-heat operation, the times in Low
Heat (LH) and High Heat (HH) during previous cycle must be known. The furnace staging algorithm uses these values to determine how much
low-heat operation will occur on next call for heat.
•
Four examples are shown below to illustrate what 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
run time
=
5600
−
5398
122
= 1.6
⇒
Next cycle, furnace will run for 1 to 2 minutes in Low Heat and then switch to High Heat if call
for heat 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 minutes.
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 furnace staging
algorithm calculated 13 minutes based on a 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
run time
=
5600
−
4714
122
= 7.26
⇒
Next cycle, furnace will run for 7 to 8 minutes in Low Heat and then switch to High Heat if
call for heat 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 call for heat is
still present.
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