A1.2
Differential (delta-T) Controllers
Differential controllers, or delta-T controllers, are used in the collector, pool/spa, DHW
recirculator, and boiler options. They all have a high temperature sensor, and a low
temperature sensor. The over temperature (OVT) or high limit function comes from the
low sensor on the tank.
1. Set the switch to the ON position. If the controller does not operate, remove
the sensor wires and repeat the test. If it still does not operate it is bad and must
be replaced. If it does operate, then there is a fault in the wiring or sensors.
2. To test the controller by itself, remove the sensor leads from the controller. A
set of resistors is used to simulate the temperature values needed to cycle the
controller through its modes. One simulates a cold STO tank sensor (25k
Ω
).
One simulates a hot COL collector sensor (3k
Ω
), and the third simulates the very
hot OVT condition (1k
Ω
). Set the controller switch to AUTO and connect the
STO, COL, and OVT resistors to the terminals in the patterns indicated in Table
A1.1. Note the operation of the controller.
Table A1.1 Delta-T Controller Tests
COL(Hi Temp)
STO(Lo Temp)
RESULT
terminals
terminals
Collector Pump is
COLD
COLD
OFF (no solar)
HOT
HOT
ON (proper run)
HOT
OVT
OFF (Hi limit)
COLD = high R (25k
Ω
~ 40ºF)
HOT = low R (3k
Ω
~ 130ºF)
OVT = V low R (1k
Ω
~ 189ºF)
If no fault is found, test the sensor wiring by reading the resistance of each circuit with a
digital ohmmeter. The leads must be disconnected from the controller for these
measurements.
Determine the temperature reading from the temperature vs. resistance chart of the
sensor. If the circuit is open (infinite resistance) or shorted (zero resistance), the wiring
is probably at fault. If the resistance is out of the range of the values on the chart, the
sensor is probably at fault or the wiring has too much resistance.
HoloTech
tm
Solar Installation Manual
21
