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





The Heliotrope DTT-81 is a basic ON/OFF differential temperature controller from the late 1970s and early 1980s and provides only the basic "ON" and "OFF" differential functions. This controller uses a relay rated at 10 amps or 1/3 HP @ 120 VAC as the final output device that switches power to the pump. The controller incorporates a solid state operational amplifier to determine when the pump should be turned ON and OFF.


A July, 1979 Heliotrope product sheet shows the DTT-81 solar thermal controller as having turn ON and OFF differential temperatures of 4.5 ºF and 1.5 ºF respectively, i.e., ON/OFF delta Ts = 4.5º/1.5º.


This controller was designed to operate with two 3 K ohm thermistor sensors for the ON/OFF differential function. During my experience with this controller I've

found that the ON/OFF differential function works just as well with 10 K ohm thermistor sensors as with 3 K ohm thermistor sensors. However, they must be one value or the other, i.e., don't mix the sensors by having a 3 K ohm sensor for the collector and a 10 K ohm sensor for the storage sensor.


If you are looking to replace your defective DTT-81 controller or just want one as a backup, order this one today as it is the only one left. Prices shown do not include postage.  Colorado sales will be charged the appropriate sales tax. Yes, I accept credit card payments through PayPal, checks, and money orders.  For questions or to place an order, send me an email (see top of this page) stating what you are interested in along with your zip code.  I'll check the postage to your zip code and send you a return email with the total cost.



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This page was last changed on 4-28-2015