3.4
Solar Controls and Wiring
Differential controllers (called "Delta-T" controllers) are used to operate the collectors,
recirculator DHW option, and pools/spas. The temperature sensors are wired with two-
conductor 18-24 ga. bell wire. If the length of the wire is over 50 ft. use 18 ga. Be
careful when running sensor hook-up wire through the building to avoid shorting or
breaking the wires with staples, or scraping the insulation off on sharp metal edges.
Connections are made with crimp terminals or wire nuts. Seal the connectors with
silicone sealant to keep water out. Water will corrode the connections and cause
control problems.
The differential controllers used with HoloTech
tm
solar systems use two temperature
sensors which perform three functions. Several different controller models many be
used, so consult the manufacturer's literature for specific details.
The high temperature sensor reads the output of the heat source. The heat source may
be the collectors or a spa/pool heat exchanger. On the controller, the terminals for this
sensor are marked collector or COL.
The low temperature sensor reads the lowest temperature of the heat receiver, which
feeds lower temperature water to the heat source. The low temperature sensor position
is the bottom of the solar tank for collector circuits, or the return line of the pool for the
pool/spa circuits. The terminals on the controller for this sensor are marked storage, or
STO.
The low temperature sensor also performs the function of an over temperature (OVT, for
short) sensor on the heat receiver. Its function is to shut off the pump when the
maximum temperature is reached in the tank or pool.
The control logic is as follows:
When the high temperature sensor (COL) is 18-20˚F warmer than the low sensor (STO),
the controller turns the pump ON. This is called the CUT-ON DIFFERENTIAL
temperature.
When the high temperature sensor (COL) falls to 4-5˚F warmer than the low sensor
(STO), the controller turns the pump OFF. This is called the CUT-OFF DIFFERENTIAL
temperature.
When the low/OVT sensor reaches its high limit set point, usually 160F, the controller
turns the pump OFF. The circuit cannot turn on again until the OVT sensors falls about
15˚F. This range is called the OVT DEADBAND and it prevents the system from rapidly
cycling on and off, called short cycling.
All sensors must be held tightly against the surface for good thermal contact. They
must be insulated very carefully to read only the temperature of the surface, and not the
HoloTech
tm
Solar Installation Manual
15
