Solar Controller - build one!

My existing controller works pretty good.  What I have is this.  I have a normal programmable digital thermostat that I program several times during the day temperatures when I know the sun is out.  it then drives a larger relay that powers a stat power to make 110 Volts AC from the existing 12 VDC battery bank I have.  Then I plugged in a timer into the outlets on the stat power, that enables the AC to drive my floor heat water pump.  I programmed the timer for a 15 minute period (the time it seems to take to draw the heat from the panels) every two hours.

I have to still mount the stat power, and clean-up some wires.  The stat power 10TB charger is actually wired to my wind power for charging these batteries.

This seems to work very well, but I know I am wasting allot of solar heat.  So I want to build a controller that will run a separate pump for the solar panels, and store the hot water in a tank each time the panel is at a maximum temperature on the panels.

I went and bought a cheap digital thermostat from Canadian Tire.

Before I start voiding the warranty

See the blue thing, that's what we are after.  Its a thermistor and measures the ambient temperature.  You can see it here without even taking the unit apart from the top of the unit.

Back of the unit with mounting plate

Mounting plate removed

Take the screws out of the main unit once the back is off

left to right, main board and display, back, and wall mounting

In the top right corner you can see "R6" which is the blue thermistor.

A closer shot with the screwdriver pointing at the thermistor

remove the thermistor, you have to heat the bottom two contacts from the bottom and pull it out, this is pretty basic if you know how to solder.  even if you don't want to do this the leads are long enough that you can cut them off from the top, close as possible to the circuit board.

this is the main board underside, you have to remove the rest of the screws first that hold it.  when you flip it, careful of the LCD display, they want to stick to the board sometimes, just carefully remove it and place it and its spacer back into the front cover.

this area is the connector on the other side, that connects to the wall mounting plate.  it has four contacts, but I am only using two for the pump.  I wanted to make the other two not in use connect to the temperature probe (thermistor).  this way when this units installed, it only connects to the 4 screws in the wall mount plat, and I can remove it at any time.  it will still look factory.

I have circled in RED the two spots that I want to disconnect from the main board.  I just used my dremel to cut the traces.  You can just scratch a path across them with a screwdriver or whatever.  They are normally for the fan control of the thermostat.

Done cutting - Warranty? What's that? :)

Here is a higher res picture.  You can see the traces cut and then the two outside pins are wired to where the "R6" thermistor runs to on the board.  also notice the thermistor, modified for a remote mount with wires.

K, now put it back together.  I was careful of the two screws beside the display.  don't strip them, but try to get them as snug as possible on the main board.  They butt up the LCD to the main board, and if you don't have a good contact, the display will miss segments.

when you first start it up, because you have not wired the thermistor on it, it will say "Lo C".  That's only because the resistance is high due to the missing thermistor and this unit will not measure temperature too low.  I am not sure of its limit right now, but it does not matter.  I only want the cooling to turn on when the solar panels are above what ever set temperature I choose.

Mounting the probe to the panels.  I just used liquid metal.  I was thinking about setting something up here better for better heat transfer, but really, you don't need extreme accuracy.  the thermistor is not that accurate itself.  the controller says within .5 degrees.  so with that in mind, I am sure this will be fine!

all done mounting the probe.

I quickly tested the next morning with the controller, it read 10.5 degrees C.  seems to work, but I will let you know.  I need to get the DC pump operational first before I finish testing this unit.

Now the two outside pins are for the probe, and the two inside are the relay that will activate when the temperature is above the set point on the controller

I am going to use a 30amp automotive relay to buffer the power I need for my other project, 12 VDC solar pump.  it can draw up to 7 amps @ 12 VDC.

I already run my floor heat with the stat power I mentioned before, so I already have a 12 VDC battery bank that will power the relay, and the pump.

I got the wire run to the panel.  I still have to insulate the pipes that run to the solar panels before winter, but for now its good enough for testing the system for now.  I don't need any heat yet, but its nice for the colder mornings.

It seems to work!  you can only set the high point to 35 c but it should work great.  As soon as I hooked up the temperature probe the relay kicked in as it was already at 40 c.  It was towards the end of the day, so I only had one batch of heat in the panels, so it kicked on the pump and as soon as it got below 35 c it turned off.

The mess, the controller added is to the left of the black PVC pipe, middle.

I just wired it between my normal thermostat, and the pump that I already have.  I have to get the 12Vdc pump built still before adding it onto this system, but for now this will even bring the efficiency up quite a bit.  Any time the water in the panels is above the temperature you set, it will turn on to pump, this will give time for each batch to warm up. 

Right now the pump pushes the floor water, into the panels, then from the panels it goes into one hot water heater, then that feeds the floor heat, then the loop all over again.  So between the floor and the water heater, the only thing I am really missing, is it will not run to store heat if the main thermostat has no demand for heat.  It will never run to store into the hot water heater either.

This is the 110Vac pump that I have

This is where its going to the floor heat, the water heater on the left is the one that is plumbed into the system.

Manual that came with the digital thermostat

Update - Testing results

this unit works pretty good, one glitch I have found.  If the temperature is over 60 degrees, the thermostat has an error, it says "Hi c".  and it wont activate its relay at this point.

this can be corrected one of two ways.  One way is to put a resistor inline and shifting the temperature measurement by adding resistance.  I have thought about this and might try it, how it would work is to offset the measurement about 10 degrees, when the display is set to the maximum of 35 c, it would actually take 45 c to trip the relay.  having the unit read 10c lower than actual temperature, might make enough difference.

normally in the winter, the panels should never have a chance to hit the "Hi c" error.  I have only encountered this because it is hot outside, and the inside thermostat does not demand any heat, so the panels never circulate right now, therefore giving it a chance to get very hot.

The second thing that will correct the "Hi c" error, is when I get a dedicated pump just for the panels.  they will always circulate when above the set high point, storing heat into tanks, separate from the demand from the inside thermostat.  the only time it will hit a "Hi c" error is when the storage, and the panels are all the same high temperature, if they get that high in a normal day.  I don't think this will happen often though, as the nights are getting chilly, and if I am drawing off the storage tanks at night, it would be very rare to get to the temperature that is high enough during the day.

The floor heat responds very quickly.  If I set the inside thermostat up two degrees, within an hour the ambient temperature is raised to the set temperature.  the concrete sure holds heat for quite some time, if you have to cool down, it takes about 1/2 day.  Ugly for testing in the summer!

For the most part, the solar controller, not completely setup the way I want it, works very well!

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