tekmar 664 Snow Detector & Melting Control Installation - Manual (Page 8)

8 of 36 © 2009 D 664 - 07/09
STAGING
The 664 controls up to two stages in order to supply the required temperature. After the first stage is turned on in the firing
sequence, the control waits a minimum amount of time before turning on the next stage. After the minimum time delay between
stages has expired, the 664 examines the control error to determine when the next stage is to fi re. The control error is determined
using Proportional, Integral and Derivative (PID) logic.
Proportional – compares the actual temperature to the target temperature. The colder the temperature, the sooner the next stage
is turned on.
Integral – compares the actual temperature to the target temperature over a period of time.
Derivative – determines how fast or slow the actual temperature is changing. If the temperature is increasing slowly, the next
stage is turned on sooner. If the temperature is increasing quickly, the next stage is turned on later, if at all.
– Each stage has a minimum on time, and a minimum off time.
Minimum
On Time
Minimum
On Time
Minimum
Off Time
Boiler Contact Closes
Boiler Fires
Fire
Delay
Fire
Delay
Time
Heating Load
On
Differential
Off
Time
FIRE DELAY (FIRE DLY)
The FIRE DLY is the delay time that may occur between the time that the
664 closes a stage contact and the burner fi res for that stage. This delay
is usually the result of burner pre-purge or other forms of time delay built
into the burner’s safety circuits.
BOILER MASS (Boil MASS)
The Boil MASS setting allows the 664 to adjust to different types of heat sources depending on their thermal mass.
Light (LITE)
The LITE setting is selected if the boiler(s) that is used has a low thermal mass. This means that the boiler(s) has a very small
water content and has very little metal in the heat exchanger. A boiler that has a low thermal mass comes up to temperature quite
rapidly when fi red. This is typical of many copper fi n-tube boilers.
Medium (MED)
The MED setting is selected if the boiler(s) that is used has a medium thermal mass. This means that the boiler(s) either has
a large water content and a low metal content or a low water content and a high metal content. This is typical of many modern
residential cast iron boilers or steel tube boilers.
Heavy (HEVY)
The HEVY setting is selected if the boiler(s) that is used has a high thermal mass. This means that the boiler(s) has both a large
water content and a large metal content. A boiler that has a high thermal mass is relatively slow in coming up to temperature. This
is typical of many commercial cast iron and steel tube boilers.
DIFFERENTIAL (DIFF)
An on / off heat source such as a boiler must be operated with a differential in order to prevent short cycling. With the 664, either a
fi xed or an automatic differential may be selected.
Fixed Differential
The differential is centered around the target temperature. If the
temperature drops 1/2 the differential below the target temperature, the
664 closes the Boiler contact(s) to fi re the boiler(s). If the temperature
rises 1/2 of the differential above the target temperature, the 664 opens
the Boiler contact(s) to turn off the boiler(s).
Auto Differential (AUTO)
If the AUTO Differential is selected, the 664 automatically adjusts
the Differential setting under the current load conditions to avoid
short cycling.
STAGE 1 AND 2 (STAGE)
The Stage 1 and 2 setting may be selected to AUTO or OFF. When AUTO is selected, the stage is activated and the control
operates the appropriate boiler. When OFF is selected, the control does not fi re the stage.