Active solar hot water systems use pumps and differential controllers to
automate the process of collecting and storing solar heat so storage
tanks may be located in basements far from the area of heat collection.
Active systems do consume some power BUT the value of heat collected is
great compared to the power used to circulate water. Active systems use differential controllers to regulate pumps and
collect solar heat. Thermistors are commonly used to sense temperatures
between 0*F and 250*F.
NTC (Negative Transfer Coefficient) thermistors are made from sintered
semiconductor metal oxide chips that change resistance with temperature.
10K NTC thermistor sensors are commonly used with solar hot water
applications. As the temperature increases the thermistor resistance decreases.
When the collector probe is the same temperature as the storage probe
the resistance of both probes is the same and the voltage at the common
junction is half the supply voltage. This is how differential
temperatures may be sensed.
Once thermistor sensors are made into thermistor probes they are ready
to collect temperature data by changing resistance with temperature. At
75*F 10K NTC thermistors have a resistance of 10,000 ohms. NTC means
Negative Temperature Coefficient. In other words their resistance
decreases as the temperature increases.
S is the storage temperature thermistor. See how
one side of S is connected to ground. C is the collector temperature thermistor.
See how one side of C is connected to a +5 volt supply. Now notice
that both S and C have a common junction. The voltage at the common
junction expresses the difference in temperature between the probes.
When both probes are the same temperature the common junction voltage is
half the supply voltage. In this case that voltage would be 2.5 Volts.
Most commercial differential controllers have separate adjustments for the
pump on and pump off duration as well as other unnecessary whistle and bell
adjustments such as the high the temperature shut off. A pump should never
be shut off when the temperature is too high. An empty collector quickly
reaches stagnation temperatures above 200*F. It’s better to add additional
storage tanks if storage temperatures
are over 140*F. The most important feature of a differential controller
involves responding to a difference in temperature.
This rugged, simple, safe, easy-to-use Basic Differential Controller may
be used to regulate an AC or DC pump. The relay can handle up to 300
Watts of power even though the controller itself only requires 1 watt of
DC power between 10VDC and 20VDC. A PV panel with a battery
back-up is sometimes used to power both controller and pump. A
protection diode has been installed incase the user mistakenly reverses
polarity. The red heat available LED helps the user make the best
If you decide to automate the heat collection process and isolate the
area of heat storage a differential controller may be used to regulate a
pump or fan and bring heat where you need it when you need it. Remember
a pump or fan only consumes a fraction of the energy that it collects. Differential controllers provide a bridge between the intermittent
nature of sunlight and the need for a dependable supply of heat and hot
water. To purchase a BDC click on the illustration.