10
11
Measuring Current Draw
Heat Anticipators
Heat anticipators are small resistance heaters (wired
in SERIES with the "W" circuit) and built into most
electromechanical thermostats. Their purpose is to prevent
wide swings in room temperature during system operation
in the HEATING mode. Since they are wired in series,
the "W" circuit will open if one burns out preventing heat
operation.
The heat anticipator provides a small amount of heat to
the thermostat causing it to cycle (turn off) the heat source
just prior to reaching the set point of the thermostat. This
prevents exceeding the set point.
In order to accomplish this, the heat output from the
anticipator must be the same regardless of the current
flowing through it. Consequently, some thermostats have
an adjustment to compensate for varying current draw in
the thermostat circuits.
The proper setting of heat anticipators then is important to
insure proper temperature control and customer satisfaction.
A heat anticipator that is set too low will cause the heat
source to cycle prematurely possibly never reaching set
point. A heat anticipator that is set too high will cause the
heat source to cycle too late over shooting the set point.
The best method to obtain the required setting for the
heat anticipator, is to measure the actual current draw in
the control circuit ("W") using a low range (0-2.0 Amps)
ammeter. After measuring the current draw, simply set the
heat anticipator to match that value.
Electronic thermostats do not use a resistance type
anticipator. These thermostats use a microprocessor
(computer) that determines a cycle rate based on a program
loaded into it at the factory.
Calculating The Approximate CFM
The approximate CFM actually being delivered can be
calculated by using the following formula:
KILOWATTS x 3413
Temp. Rise x 1.08
DO NOT simply use the Kilowatt Rating of the heater (i.e.
2.5, 3.4, 5.0) as this will result in a less-than-correct airflow
calculation. Kilowatts may be calculated by multiplying the
measured voltage to the unit (heater) times the measured
current draw of all heaters (ONLY) in operation to obtain
watts. Kilowatts are then obtained by dividing by 1000.
EXAMPLE: Measured voltage to unit (heaters) is 230 volts.
Measured Current Draw of strip heaters is 11.0 amps.
230 x 11.0 = 2530
2530/1000 = 2.53 Kilowatts
2.53 x 3413 = 8635
Supply Air
95°F
Return Air
75°F
Temperature Rise 20°
20 x 1.08 = 21.6
8635
21.6
= CFM
= 400 CFM
If a low range ammeter is not available, a "Clamp-on" type
ammeter may be used as follows:
1. Wrap EXACTLY ten (10) turns of wire around the jaws
of a clamp-on type ammeter.
2. Connect one end of the wire to the "W" terminal of
the thermostat sub-base, and the other to the "R"
terminal.
3. Turn power on, and wait approximately 1 minute, then
read meter.
4. Divide meter reading by 10 to obtain correct anticipator
setting.
