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Solar DHW + Space: System Controls

This section covers all of the controls for the system. 


The controls are very simple, consisting of one control to basically start the circulation from the tank through the collector when the sun is out, and and two controls to turn on the space heating when the house is cold AND the tank is hot.  That's all there is.


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A Steca differential controller is used to control the collector pump.  The controller uses a temperature sensor mounted in the collector and a temperature sensor mounted in the tank near the bottom.  When the collector temperature goes above the tank temperature by an amount that you can set, the controller turns the pump on and circulates water through the collector to heat the tank.  When the collector temperature drops below the tank temperature, the controller turns the pump off.

This controller also has a tank overheat feature that allows you to limit the maximum temperature of the tank water to a value that you set.  I use this feature to limit the storage tank temperature to 140F -- this insures a long an happy life for the EPDM liner, HDPE pipe coil, and the pump -- its an important feature.


The Steca differential controller on top
controls the collector pump operation.
Its display shows storage temperature,
collector temperature, and when the
pump is operating.
The lower unit is the remote control
for the Tagaki tankless water heater.
It will display inlet and outlet temperature,
as well as flow rate.

Wiring the Steca differential controller.
The two orange wires go to the
temperature sensors -- this is all
you have to hook up.
The wiring for these can be two conductor
18 gage, but I did not have anything
that light around, and used
heavier wire.


The wiring is very simple. 

Install the temperature sensor in the collector.


Install the temperature sensor in the tank (look for one that can just be immersed right in the water).


Hook the two temperature sensors up to the differential controller.


Plug the pump into the controller (it probably has a 120VAC outlet for this).


Plug the controller into a wall outlet.


That's all there is to it.



A Problem with the Steca Controller

I have replaced the Steca controller with another brand after having two failures. 

The failure is that the controller will run the pump all the time.  Even when the display indicates that the pump is turned off, the controller continues to run the pump.  I've tried all the usual remedies as in checking switch positions, cycling power, ... to no avail.   I believe that the relay that turns the pump on has stuck on.

The first unit was replaced under warrantee, and the replacement worked for about another month or two and failed in exactly the same way.  While they likely would have been willing to replace the unit again, I decided that I would just change to another brand.

This seems particularly strange in that the pump only draws 13 watts -- you would think that it would be very easy on the controller output circuit? 

I've written a note to Steca, and they believe that the problem is that the DC pump is powered by a switching power supply.  Its the switching power supply that is plugged into the Steca, with the pump then plugged into the switching power supply.  The switching power supplies can apparently have very high startup current surges, and they believe that this is what caused the relay to stick with the contact closed.  I am inclined to believe this explanation.

One potential cure for this if you want to use the same type of DC pump I use is to power it with a "linear" power supply.  These power supplies (I'm told) do not have nearly as high a startup power surge, and should work OK.  These linear supplies are widely available and not expensive -- Digikey and Jameco are two places that handle them.

The new controller is a Caleffi.  It is actually quite similar to the Steca with the added benefit that it displays cumulative pump hours.  Its a bit more expensive.  So far (2 months) it has worked well.
It hooks up in the same way as the Steca, and uses the same kind of temperature sensors. 

I got mine here:

After hearing the Steca explanation, I am inclined to believe that the Caleffi controller may also be susceptible to power startup surges from the switching power supply, and if I were going to run the DC pump in the long term, I would change to a linear supply, but see the update just below.

Gary October 10, 2009

Update: April 23, 2010 -- I have changed the pump to a Grundfos AC pump as a part of expanding the system to do both water heating and space heating.  This change does not in any way indicate a problem with the SwiftTech pump, which performed without a hitch for more than a year.  I just needed somewhat more flow and startup head than the SwiftTech is rated for with the new larger and taller collector.

If a PV panel is used to run the pump, then the PV panel itself acts as the controller.  When there is sun on it, it pumps, and if not it does not pump.  It may be hard to find a pump that has enough startup head and can be run from a modest size PV panel.  But, the March pump I am using now works ok with a PV panel.   In order to keep the storage temperature from going over 140F, a thermal snap switch can be installed in series with the pump and mounted just under the tank liner.  The snap switch wants to to to open when it reaches 140F, and be closed at below 140F.  One caution on using PV to drive a drain back system is that the PV must spin the pump fast enough on startup or after some clouds go by to pump the water all the way up to the top of the collector.  This did not seem to be a problem whit the March pump I am using and a PV panel that is about 15 watts, but I'm not sure this is always going to be true -- particularly if you collector is well above the tank.




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