Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 40 years





The Heliotrope DTT-400 is a basic ON/OFF differential temperature controller from the late 1970s and early 1980s and provides the basic "ON" and "OFF" differential functions along with a high limit tank protection which de-activates the pump when the storage tank temperature is above 180 ºF. A third sensor is needed to sense this temperature at the top of the storage tank. 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-400 solar thermal controller as having turn ON and OFF differential temperatures of 9 ºF and 3 ºF respectively, i.e., ON/OFF delta Ts = 9º/3º.


The default HIGH LIMIT, as delivered from the factory, was spec'ed at 180 ºF. As designed, the HIGH LIMIT sensor for this controller is spec'ed as being a 3 K ohm thermistor sensor. However, upon request, the internal circuitry can be modified to allow the HIGH LIMIT to be changed from the standard 180 ºF to 170 ºF or 160 ºF for a $10 service fee.


This controller was designed to operate with two 3 K ohm thermistor sensors for the ON/OFF differential function, one 3 K ohm thermistor sensor for the HIGH LIMIT 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. The internal circuit can also be modified (for a $10 service fee) to allow the HIGH LIMIT to operate with a 10 K ohm sensor and give the 180 ºF, 170 ºF, or 160 ºF limits. If the HIGH LIMIT function option is not needed, the HIGH LIMIT sensor input wires are simply left unconnected.


If you are looking to replace your defective DTT-400 controller or just want one as a backup, order one of these before this limited supply runs dry. 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 4-25-2019