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-692 is a basic ON/OFF differential temperature controller from the late 1970s and early 1980s for use with drain down systems. It was considered a basic workhorse controller for the solar water heating industry. This controller has two output receptacles, one for the drain down valve and the second for the circulation pump. Two relays rated at 10 amps or 1/3 HP are the final output devices that switch power to the output receptacles. 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-692 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 13º/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, one 3 K ohm thermistor sensor for the HIGH LIMIT function, and one FS-5 snap switch sensor for the LOW 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 the LOW LIMIT function option is not needed, the LOW LIMIT sensor input wires are simply shorted together.


These new (old stock), un-used controllers are priced at $55 each, plus postage. An August, 1984 Heliotrope Thermal Products Price List shows the list price of these controllers, with three SAS-3 sensors and one FS-5 sensor to be $133.59. Each controller has been tested to verify proper operation and carries a Conifer Solar Consulting 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. The FS-5 snap switch sensors are available for $25 each.


If you are looking to replace your defective DTT-692 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 8-4-2018