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


• Article 19 - August, 1996


Programmable Differential Temperature Controllers©


This is the sixth in a series of articles that will summarize the programming capabilities of most of the differential temperature controllers used in the Colorado area. This article will examine the Heliotrope DTT-74/794 and DTT-84/94 controllers. The DTT-74 and 794 both use 10 k ohm sensors and are both used to operate the HG spool drain down systems (this DTT-74 is a different -74 than was reviewed in the June, 1996 newsletter). The difference between the -74 and -794 is the -74 requires the 115 VAC valve and pump be hardwired into the controller. The -794 allows the 115 VAC valve and pump power cords to be plugged into control box mounted receptacles. Either model can be programmed, as described below, to provide power to a 12 VDC HG spool drain down valve instead of a 115 VAC valve. The DTT-84 and -94 controllers also use 10 K ohm sensors and are used to turn a circulator pump or blower ON or OFF at appropriate temperature differentials. The -84 requires the pump to be hard wired into the controller, whereas, the -94 provides a receptacle for the pump or blower power cord.


When I looked at my Heliotrope DTT-74/794 product literature (three separate pieces of literature), I found several conflicting numbers. But a quick call to Mike Dice at Heliotrope set things straight. So the values stated below should be OK. The-DTT-74 and -794 both possess a minimal amount of programming capability. What is unique about these controllers is the unusual valve open/close and pump ON/OFF sequences. The valve opens only after the collector sensor has reached 80° F and stays open for a minimum of 45 minutes regardless of the outside ambient temperature. There is also a 10 minute wait before the pump can come on. After the 10 minute time delay, the pump will turn on when the panel sensor is 15° F warmer than the storage sensor, and off when the difference is 3° F. However, if the storage high limit set point is reached, the valve OPEN signal is removed, the valve starts to close, and the pump continues to run for 10 minutes, regardless of the collector to storage temperature difference. The valve is also de-energized and starts to close when the panel sensor temperature drops to 55° F (after the above mentioned 45 minute time delay). Now for the programming capabilities. The controllers are shipped from the factory set to operate a 115 VAC drain down valve. The controllers can be made to operate a 12 VDC drain down valve by changing the plastic jumper on the right side of the circuit board identified as "VALVE" from the left to the right position. The other programming capability deals with the high limit set point. The controllers are shipped from the factory with the high limit set at 160° F. To change the high limit to 180° F, two jumper wires on the right side of the circuit board must be cut.


The DTT-84/94 controllers have a bit more programming capability than the DTT-74/794 controllers. The programming is controlled by six ON-OFF DIP switches located on the circuit board. Switch 1 ON enables the high limit pump shut off. This does not override the recirculation feature, if selected with switch 2. Switch 1 OFF disables the high limit pump shut off. Switch 2 ON enables freeze recirculation. This feature utilizes the collector outlet mounted sensor for freeze detection (at 42° F). Heliotrope also recommends one or more FS-5 switching freeze sensors to be used for redundant protection. Switch 2 OFF disables the freeze recirculation feature. Switch 3 ON prevents the pump from turning on at collector temperatures below 80° F. If freeze recirculation is selected (with switch 2), the pump will still be turned on at 42° F, i.e., the pump-OFF-below-80° F will be over-ridden for recirculation. Switch 3 OFF disables the pump-OFF-below-80° F feature. Switch 4 ON enables the 160° F high limit and switch 4 OFF enables the 180° F high limit. Switch 4 is used in conjunction with switch 1 (enable/disable high limit). Switches 5 and 6 both ON select 9° and 4° F as the turn ON/turn OFF differentials whereas switches 5 and 6 both OFF select 18° F and 5° F as the turn ON/turn OFF differentials.


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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