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



The goal of this web site is to provide those solar pioneers who installed solar heating systems back in the 1980s and 90s, and those who inherited these systems, a means to keep their systems running.  Failed controllers often are the cause of such systems being shut down.  Look through this site to see if what's here can help you get your solar heating system back up and operating.


Conifer Solar Consulting supports the solar industry by repairing vintage active solar thermal system controllers.


Conifer Solar Consulting (Jon Klima, Owner) is a life time member of the Colorado Solar Energy Industries Association (CoSEIA)


For more general consumer information go to the CoSEIA web site,  This site also contains a very extensive list of solar electric and solar thermal web site resources.


MENU --- Click on any of the links below for more information.


Controller retail repair costs


Used/rebuilt controllers for sale


New (Old Stock) DTT-692 controllers for sale


New (Old Stock) DTT-692T controllers for sale


Generic solar heating system descriptions


Controller/Sensor matches


Index to selected controller articles from past CoSEIA newsletters


Index to selected controller tech & installation/application manuals



Controller question? Maybe I can help, send me an email at


I've had quite a few people ask me how a differential controller (sometimes called a differential thermostat) works.  Since this is an often asked question, I've copied three pages from Section 2 of the The Solar Controls Book© that I wrote back in 1981 which gives a basic description of a simple 1980s or '90s differential controller operation.  You can download this 80 KB PDF file by clicking here.  Most new controllers sold today use digital technology and most have more complex options than the 1980s and '90s controllers.  But the basic ON-OFF operation is the same. 



Unsolicited Testimonials


You stated that I should be all set now, indeed I was. That has only come about through your knowledge, kindness and assistance. Initially being a stranger seeking help on the internet Jon came forth by seeking out my problem and offering multiple solutions. Through numerous calls, e-mails and electronic testing Jon diagnosed and resolved my solar dilemma. Again Jon, thanks ever so much. Warren/New Hampshire



...I would again like to thank you for your professionalism! You did a better job diagnosing my problem from afar than my local solar guy did. From RK in Hawaii.



I just wanted to thank you again for your help with my solar system. I bought 2 DTC-30 from you. Then with your help and your website it was able to locate the correct thermistors and now everything is working!!! From BT in Florida.



Prompt, courteous, professional service. No extra or hidden charges. Rapid turn around. I expect Jon will be able to handle anything else I may come up with. I won't need to bother searching all over, or anywhere else. 30 years of Solar assisted DHW, back on track, thanks to Jon. A "five star" operation, in my opinion. What a pleasant surprise! BG in Castle Rock, Colorado



A Solar Domestic Hot Water Troubleshooting Program ---

I've put together a solar domestic hot water system troubleshooting program, called SDHW-PROB, to help novices and homeowners troubleshoot their SDHW systems.  This is available to use online at no charge.  If this is of interest to you, just click here to get started.  The first two pages tell you about the program and how to use it.


A Few Controller Maintenance Hints ---

Every service technician should carry a can of electrical contact cleaner such as that found at Radio Shack. Often a controller problem can be fixed by simply spraying a bit of the contact cleaner into the ON-AUTO-OFF switch (or other additional switches).


Check for tightness of all sensor and controller terminal board wire terminations before assuming the controller is bad.


Make sure the controller is not mounted under any pipes such that, if a leak were to occur, the controller would be exposed to the dripping fluid.


Make sure the controller is not mounted on any surface that is exposed to high heat or vibration, such as pump motor housings or the side of an air handler unit. Such a mounting will often lead to cracked solder joints on the printed circuit board.


Although many sensors are installed using ordinary thermostat wire between the sensors and the controller, the better installations will use twisted shielded wire, such as Beldon 8762 (or equivalent), for the interconnecting wire. When using shielded wire, terminate the shield on the ground screw at the controller only. Do not terminate the shield at the sensor end, let it float.


Carry a tooth brush with you. It comes in handy to clean corrosion off of a controller printed circuit (PC) board. If the corrosion hasn't eaten away a trace, sometimes just cleaning the corrosion (which is conductive) off of the board will solve the problem.


Did you know the Heliotrope DTT-80 (and other non-display models) can be used with 10 K ohm sensors rather than 3 K ohm sensors?  However, to do this, either the high limit (gray wires) sensor wires must not be used (not connected) OR a simple modification to the controller circuitry must be made.  I can make this modification for a $10 modification fee.  If you make such a modification, I'd strongly suggest that you place a note on the controller indicating it's been converted for usage with 10 K ohm sensors.


Testing sensors --- For this test, it's best to use a digital meter - just a little easier since you shouldn't need to do any meter calibrating. You will be measuring the resistance of each sensor. If you need a 3 K ohm or 10 K ohm temperature vs. resistance chart, click here and download these charts.  This will give you the resistance values for the sensors at various sensor temperatures. For those wanting to test 30 K ohm sensors, you can use the 3 K ohm chart but you need to multiply all resistances by 10.  So, the resistance of a 30 K ohm sensor is 30 K ohms at 77 °F.  An alternative is to use the 10 K ohm chart but multiply the resistances by 3.


First, you will need to estimate the temperature of the storage sensor which should be located near the bottom of your storage tank. Remember, the water at the bottom of the tank will be cooler than the water at the top.  If you can reach in and touch the tank where the sensor is mounted, that will also give you an idea as to what the temperature is. Now place the meter on the resistance/ohms setting. Disconnect the two storage sensor wires at the controller. Place one of the meter's probes on one of the sensor wires and the other probe on the second sensor wire. Make sure that you don't touch the metal part of both probes at the same time. Doing so will interfere with the resistance reading. Write down the resistance reading. Compare the resistance reading on the temperature vs. resistance chart and find the sensor's temperature. If it's close to your estimated tank temperature, then the sensor is probably OK. If it's OK, reconnect the sensor to the controller.  If it's not OK, check to make sure the sensor wire is good between the controller and the sensor.  If the wire is OK, then you need to replace the sensor.


The collector sensor resistance measurement is best taken when the sun has not been on the panels for at least an hour or so. It's just hard to estimate the panel's temperature when the sun is shining on them. So, do this either when it's very cloudy or first thing in the morning when the sensor temperature should be close to the ambient air temperature. Disconnect the collector sensor at the controller. Now follow the same steps for measuring the collector sensor as you did above for measuring the storage sensor. As above, compare the resistance reading on the temperature vs. resistance chart and find the sensor's temperature. If it's close to your estimated collector temperature, then the sensor is probably OK. If it's OK, reconnect the sensor to the controller. If it's not OK, check to make sure the sensor wire is good between the controller and the sensor.  If the wire is OK, then you need to replace the sensor.


Links --- I've had several people ask what new solar heating controllers are presently available. Here are several links to check out.


GoldLine GL-30 Differential Temperature Controller, uses 10 K ohm temperature sensors, both controller and sensors available at ENERGY SUPERMARKET,


Heliotrope Delta-T controllers, DTT-84 (now DLTA 001 000) and DTT-94 (now DLTA 000 000) use 10 K ohm temperature sensors. The DLTA controllers are now sold by Heliodyne. See them at


Steca GmbH, Controller models TR 201, TR 301, TR 502, TR 603, TR 704, Sundra, and Santana


Tekmar line of controllers including a proportional model, use 10 K ohm temperature sensors,


IMC Instruments, Inc., Eagle line of controllers that use 10 K ohm temperature sensors,


Resol GmbH, click on the English version, then click on solar controllers,


This page was last changed on 4-12-2019