USDT 3004 Solar Differential Controller

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The USDT 3004 is a microprocessor based multi-functional integrated unit for solar heating systems as well as HVAC systems. It operates based on the temperature difference between the solar collector and system return (from tank) temperatures. The unit display presents two temperatures simultaneously (e. g. collector inlet and outlet temperatures). Symbolic icons give information about the function and operation of the controlled system.









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At the factory, USDT 3004 is set to control a standard solar water heating system with an auxiliary relay to divert surplus heat. The auxiliary relay can be used to maintain the tank temperature, protect the system from overheating or provide another heat source to heat the storage tank. The controller has inputs for four PT1000-sensors. The fourth sensor may be used as for an additional temperature reading or for a flow meter.. A 3-key-panel below a large LCD serves as the user interface. Simple icons give information about the function and operational mode of the controller and the system performance.

  • System status and diagnostic displays
  • 4 inputs for temperature sensors
  • Suitable for type PT1000 sensors
  • Input for flow meter
  • BTU capability
  • Auxiliary relay for overheating protection and supplement heating system
  • Adjustable (4 o F to 34 o F) temperature difference (delta T)
  • Manual override of pump for system testing
  • Symbolic display of all parameters
  • Concurrent display of two temperatures
  • Indications of current state of pump
  • Solar operating hours counter
  • Has Drain Back Operation Option

Principle of Operation:

The collector sensor (S1), positioned at the solar collector, picks up the rise in temperature. The return sensor (S2) is  positioned such that it measures the temperature of the heat transfer liquid returning to the collector. The difference between these two temperatures is referred to as Delta T (DT). When DT is set to 4 (recommended setting), the collector temperature (S1) must rise at least 8 degrees F above the return temperature (S2) before the pump will operate. The collector temperature (S1) must be above the minimum collector temperature set point (CN), before the pump will operate.

The third sensor can be used to activate a (gas, oil, ...) boiler pump if hot water demand is higher than solar system can deliver.

In the normal mode of operation, an Auxiliary (A2) relay can divert surplus heat to another tank or operate an auxiliary heater for boosting the tank temperature. This function is totally independent in its operation to the solar controller and has its own separate sensor (S3).

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