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-92 is a basic ON/OFF differential temperature controller from the late 1970s and early 1980s. It was considered a basic workhorse controller for the solar water and space heating industry. It uses a relay rated at 10 amps or 1/3 HP as the final output device and incorporates a solid state operational amplifier to determine when the pump or fan should be turned ON and OFF.


A July, 1979 Heliotrope product sheet shows the DTT-92 solar thermal controller as having turn ON and OFF differential temperatures of 15 ºF and 5 ºF respectively, i.e., ON/OFF delta Ts = 15º/5º. I've tested these differential temperatures and found them to typically range from 13.5º/5º at a storage temperature of 77 ºF to 17º/5.5º at a storage temperature of 154 ºF.


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 and 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 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.


This new (old stock), un-used controller is priced at $40, plus postage. An August, 1984 Heliotrope Thermal Products Price List shows the list price of these controllers, with two SAS-3 sensors, to be $92.33. Each controller has been tested to verify proper operation and carries a six month warranty. The controllers are being sold without sensors since many buyers already have sensors from a previously installed system. If you wish to buy new (old stock) 3 K ohm sensors, they are available for $16 each. I do not have any new 10 K ohm sensors.


If you are looking to replace your defective DTT-92 or just want one as a backup, order this one today as it is the only one I have 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 2-25-2019