Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 39 years


• Article 17 - May, 1996


Programmable Differential Temperature Controllers©


This is the fourth in a series of several articles that will summarize the programming capabilities of most of the differential temperature controllers used in the Colorado area. In may last article, I said I would continue this month with more of the Independent Energy controllers. I have, however, recently received a request to include some information on the Heliotrope line of controllers. So I've decided to alternate between the Independent Energy and Heliotrope controllers until they have all been covered.


In researching the Heliotrope product literature (my file goes back to the late '70s), I found reference to over 6 dozen different models. Most of these, however, have very little, if any, programming capabilities. Still, I think it important to at least list these models and tell you what their differences are.


DTT-Model Numbers according to ON/OFF differentials *


9° / 3° F

4 ½° / 1 ½° F

15° / 5° F

15° / 1 ½° F


DTT -80










DTT -100





DTT -200





DTT -290





DTT -400





DTT -690





DTT -3410





DTT -70

Differential ON Adjustable from 3° F to 24° F with OFF fixed at 1.5° F

1,2 & 4

DTT -71

1,[2 & 4]**

DTT -72

2,[1 & 4]**

DTT -704

4,[1 & 2]**

DTT -790




1 & 4


The DTT-600 series is similar to the DTT-690 series. The -600 series is hardwired rather than plug-in and the ON differential is adjustable from 5° to 20°F with a fixed OFF differential of 2 1/2°F.


DTT-90T thru 93T, 290T thru 293T, 690T thru 693T, and 790T thru 792T models have the same characteristics as shown above except that they include a digital thermometer which-displays the collector and storage temperature.


* Taken from product literature dated 7-79. Later product literature dated 3-81 lists first three as 9° / 2 ½° F, 6° / 2 ½° F, and 18° / 2 ½° F. There isn't a listing for the fourth column.


** Optional Modes that can be incorporated with appropriate sensors.

*** Hi-Limit OFF adjustable 60°- 200°F

**** The 790 series ON/OFF differentials are from the 3-81 product literature.


The addition of -ST at the end of a model number indicates side terminals for the sensor wire hookup.


Mode 1: The 100 series is OFF when the collector is below 80° F. This feature prevents the pump from turning on at night when a large amount of heat has been extracted from storage.


Mode 2: Known as the "Freeze-Recycle" or "Recirculation" method of collector freeze protection, the 200 and 290 series turn the pump on at 38° F and off at 42° F. This pushes a small amount of water through the collector to prevent freezing. This model utilizes a third thermistor sensor for the freeze detection.


Mode 4: The 400 series is a High Limit tank protection and does not allow the pump to activate once the storage is above 180° F. A third thermistor for this sensing allows this temperature detection to be made at the top of the storage tank while the differential sensor is at the bottom.


Mode 6: The 690 and 790 series have two separate outputs. For the 690 series, one output controls the pump and incorporates the OFF above 160° F of the Mode 4, and the second output controls valves for collector freeze protection. This method, known as "Freeze-Dump" or "Drain-Down" protection, electrically holds valves in a circulation position. Upon reaching 42° F or a power failure the valves are de-energized and return to a position which shuts off the circulation and drains the collector. They are re-energized at 52° F. For the 790 series, both output receptacles are wired together.


The above models are no longer manufactured and are probably no longer available except from a few used equipment dealers. There is a newer series of DTT-74/94 controllers that is used for Drain down systems. These controllers contain timing circuitry that holds the pump on while the drain down valve is closing. More on this controller series in a later article. Also, more on the later models that are currently being manufactured and sold today in succeeding newsletter issues.


I've taken all of the above information from the manufacturer's product literature which, I assume to be correct. If I've made a mistake or have left out some important information, or you otherwise take exception to what I have written, please write to me (c/o CoSEIA or to my address listed in the CoSEIA membership directory) and we will include your comments in the next newsletter.


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