Steca differential controller

[sfriedri]

For those of you considering differential control of charging your storage tank, I thought I'd provide some info on the controller than I am using for my system. I have a solar DHW system and use Goldline GL-30 differential controllers for this system, but I wanted a little more flexibility for my boiler system. The controller I am using is the STECA SETR 0301U, and I see it has been mentioned briefly in a few other posts. It sells for anywhere between $125-$150. Here is one listed on Ebay: (broken link removed)

Even though it is designed for a solar DHW system, it has been working well for my boiler system. It has several advantages over the GL-30 controller:

- a nice animated visual display
- three inputs for temperatures, which can be also be displayed on the screen
- a hidden menu not shown in the manual which allows further control of the different controller parameters, including adjusting both the on and off differential settings

The only limit with the controller is that its maximum storage temperature can be set up to a maximum of 205F. So in the unlikely event you can get the temperature at the bottom of your storage tank above 205F, the circulator will shut down.

Since the controller has three inputs and can display temperatures, it eliminates the need for three separate analog temperature gauges. I have my controller measuring the boiler output and the midpoint of the storage tank, which is the temperature differential it controls, and I have the third auxiliary input measuring the top of my storage tank so that I have an idea of stratification.

I'm sure everyone who is thinking about using a differential controller understands their advantage over a simple aquastat control: you are only charging your storage tank when the boiler output is hotter than the tank. This avoids potential heat loss of trying to charge the tank when the boiler is hot but the tank is hotter.

 

[Mushroom Man]

Thank you so much for that post. I have been concerned about how to control flow to and from storage. It sounds like this controller is the answer. Do I understand its function correctly? I visualize 1 pump and 1 HX (in the storage loop) with a motorized 4 way valve controlling whether the tank is in charge or recovery mode. The Steca controls the motorized valve and possibly the pump too.

Does that have the principle right?

 

[sfriedri]

Tim,

Most differential controllers that I know of simply provide on-off control. If the temperature difference that it measures is higher than the "on" differential setting, then it closes a circuit. Once the temperature difference drops below the "off" differential setting, it opens the circuit. The circuit it opens and closes can be anything (within its maximum power range), and in my case it turn off and on the circulating pump that charges my storage tank from the boiler. So this basically means that the storage tank in only being fed water from the boiler when the boiler water is hotter than the storage water, rather than just when the boiler water is hotter than a certain temperature. This eliminates the possibility of, for example, the circulating pump trying to charge your storage tank when your storage is at 180*F and your boiler is only at 160*F at the end of a firing session. This is what you would get if you just used an aquastat that turned on the circulating pump whenever the boiler was above 160*F. This could actually cause your storage temp to drop at the end of the firing session as the boiler cools down.

I'm not exactly sure how you are using your 4-way valve in your system, but presuming the 4-way valve + pump power requirements are below the limit for the controller, they could both be powered on and off at the same time using a differential controller. The Steca controller has a max power rating of 0.5 HP at 110V, or approximately 370 watts.

In my system I can charge the storage tank and supply heat to my loads at the same time or just do one or the other. Each has its own control system, and act independently.

Hope this helps.