Need water temperature switch help

Thread Starter

jahmes143

Joined Mar 3, 2017
7
Hey guys! New college student member here. I’m working on a solar water heater project and need some help.

I have an electric water heater in a garage, and a solar water heater outside (which reaches temps of 180F). I would like to connect the two in a looped water system with a 12v circulation pump (that is, water can flow from the solar heater to the electric heater and also from the electric heater back to the solar heater).

I only want the pump to run when the solar heater temperature is greater than the garage water heater.

Is there a plug and play solution that would accomplish this? If I need to build something, what components would I need?

Be gentle…I’m not well versed in electronics terminology.
 

wayneh

Joined Sep 9, 2010
16,102
Is there a plug and play solution that would accomplish this?
Do you mean off-the-shelf, or a reasonably simple circuit? Because I have all the parts I would need on hand already, I would build my own thermostat. I'd use two LM35 thermometers, one on each side. These ICs output a voltage that is proportional to temperature. I would then use a comparator such as LM393 (4 in one package) to switch state based on the comparison of the two voltages (temperatures). The comparator output would control a MOSFET switch, which is turn would act like a relay to control the power to the pump.

If this sounds interesting at all, we can work out the details.
 

Reloadron

Joined Jan 15, 2015
4,501
I only want the pump to run when the solar heater temperature is greater than the garage water heater.
A potential problem is when the water is running in a circulatory loop things are fine and the water throughout the loop should be about the same temperature. When the pump stops my guess is the water will develop hot spots in the loop, you will not have uniform water temperature but as soon as the circulatory pump comes on the water will likely have thermal equilibrium.

The systems I have been exposed to ran the circulatory pump 24/7. A circulatory pump uses very little power since it simply moves the water in a closed loop. With the water running in a loop when a demand exist I would use electric heat as needed to supplement the solar.

Measuring water temperatures is fairly easy and to do what you originally had in mind would be done using a few temperature sensors and a few controllers with a few solenoid valves. Did you plan on any storage tank or just a loop[ using on demand?

Ron
 

djsfantasi

Joined Apr 11, 2010
5,579
.. The systems I have been exposed to ran the circulatory pump 24/7. A circulatory pump uses very little power since it simply moves the water in a closed loop. With the water running in a loop when a demand exist I would use electric heat as needed to supplement the solar.
24x7? Wouldn't the solar water heater become a radiant water cooler at night? Forcing the electric water heater to run even if there was no demand?
 

Reloadron

Joined Jan 15, 2015
4,501
24x7? Wouldn't the solar water heater become a radiant water cooler at night? Forcing the electric water heater to run even if there was no demand?
Uh Oh, I do believe you have a point there. While I have seen them I never worked with a solar heater. Actually I am not sure what they do after the sun goes down. Good point. :(

Ron
 

Thread Starter

jahmes143

Joined Mar 3, 2017
7
Guys thanks for all the helpful replies so far!

Dave - Thanks for the suggestion; I will look into those. Do they allow temperature sensors to be placed at 2 different locations (e.g., one in the solar heater, one in the garage heater)?

Wayne - I'll send you a direct message and we can discuss further.

Albert - I'll give that a read thru tonight

Ron - if I used longer probes that go deeper into the respective heaters, would there be less hot spots? I don't plan on having any additional water storage other than the two heaters. The way the system will work is that city water goes into the cold in of the solar heater, then out the hot of the solar heater and into the cold of the garage water heater, then out of the hot of the garage water heater and to the house fixtures (essentially the solar heater is acting as a pre-heater for the garage heater). Meanwhile, there will also be the previously described circulatory system. Open to suggestions if you see any issues with this setup. The solar heater is in a well insulated glass-top box (google batch water heater).
 

Thread Starter

jahmes143

Joined Mar 3, 2017
7
Do you mean off-the-shelf, or a reasonably simple circuit? Because I have all the parts I would need on hand already, I would build my own thermostat. I'd use two LM35 thermometers, one on each side. These ICs output a voltage that is proportional to temperature. I would then use a comparator such as LM393 (4 in one package) to switch state based on the comparison of the two voltages (temperatures). The comparator output would control a MOSFET switch, which is turn would act like a relay to control the power to the pump.

If this sounds interesting at all, we can work out the details.
Wayneh - can't figure out how to message you for the life of me (using web browser on iPhone). Would you mind messaging me?
 

Reloadron

Joined Jan 15, 2015
4,501
Ron - if I used longer probes that go deeper into the respective heaters, would there be less hot spots? I don't plan on having any additional water storage other than the two heaters. The way the system will work is that city water goes into the cold in of the solar heater, then out the hot of the solar heater and into the cold of the garage water heater, then out of the hot of the garage water heater and to the house fixtures (essentially the solar heater is acting as a pre-heater for the garage heater). Meanwhile, there will also be the previously described circulatory system. Open to suggestions if you see any issues with this setup. The solar heater is in a well insulated glass-top box (google batch water heater).
I really don't know but gradients in temperature would be a concern with no flow. I never tried it with no flow? :(

If wayneh has a viable workable solution I would give it a shot. His suggestion as to measuring and reacting to temperature would work. If you want a readout of temperature then there is a bit more to it but if you just want to open and close a few valves at preset temperatures there is no need for a readout.

Ron
 

Thread Starter

jahmes143

Joined Mar 3, 2017
7
I really don't know but gradients in temperature would be a concern with no flow. I never tried it with no flow? :(

If wayneh has a viable workable solution I would give it a shot. His suggestion as to measuring and reacting to temperature would work. If you want a readout of temperature then there is a bit more to it but if you just want to open and close a few valves at preset temperatures there is no need for a readout.

Ron
Ron - thanks, yea I don't need a readout I guess. To prevent the pump from constantly turning on/off like you're saying it might, perhaps a better design would be for the pump to switch on when the solar heater is 5 degrees (10,15, some these value) warmer than the garage heater, and stay on until the two equalize in temperature. How much more complexity would that add?
 

Reloadron

Joined Jan 15, 2015
4,501
A readout or display really would not add complexity but would add cost. On the bright side I have seen various process controllers (temperature controllers) on Amazon and Ebay pretty cheap as they come in from China. Controller aside you need to choose a sensor. When I did stuff like this was some time back but usually I used a Thermocouple and mounted it in either a 1/2" or 3/4" "T" adapter. This allowed the sensor to be in the flow and the Thermocouple was a typical 1/8" sheath with a compression fitting. Nice thing about a controller is you can experiment with a few set points and see what works best.

Time to walk the dogs and chat with the wife. :)

Ron
 

hp1729

Joined Nov 23, 2015
2,304
Hey guys! New college student member here. I’m working on a solar water heater project and need some help.

I have an electric water heater in a garage, and a solar water heater outside (which reaches temps of 180F). I would like to connect the two in a looped water system with a 12v circulation pump (that is, water can flow from the solar heater to the electric heater and also from the electric heater back to the solar heater).

I only want the pump to run when the solar heater temperature is greater than the garage water heater.

Is there a plug and play solution that would accomplish this? If I need to build something, what components would I need?

Be gentle…I’m not well versed in electronics terminology.
So you need to sense the temperature in both places and comparethem? If the in-house water tank is full you can't pump more water in? Do you need to turn off the in-house system or will just adding hot water keep it off? Can you add hot water if it is full?
 

wayneh

Joined Sep 9, 2010
16,102
Ron - thanks, yea I don't need a readout I guess.
FWIW, anything that can read and display a voltage can be your readout if you use the LM35. It puts out 10mV per °C, so 10°C would read 0.10V and 100°C would read 1.00V. You can get pretty darn cheap voltage readouts.
To prevent the pump from constantly turning on/off like you're saying it might, perhaps a better design would be for the pump to switch on when the solar heater is 5 degrees (10,15, some these value) warmer than the garage heater, and stay on until the two equalize in temperature. How much more complexity would that add?
Essentially none. That feature is called hysteresis and it's the same thing your home thermostat does when it turns off and doesn't come back on until the temperature drops say 2°. It's a common feature of a comparator circuit, and it's adjustable.

How big a pump are we talking about, and where does its power come from?
 

shteii01

Joined Feb 19, 2010
4,667
Two temperature sensors. LM35 is probably OK if water temperature never comes close to freezing temperature.

Arduino Uno board. (LM35 connect to ADC pins, there are six of them so you can have 6 temperature sensors.)

Motor/pump relay connected to Uno.

Uno has regulator so the whole thing can be powered from 12 VDC source.

Uno software for all the individual pieces is free, just need to combine the pieces to make Uno do what you want.
 

Thread Starter

jahmes143

Joined Mar 3, 2017
7
FWIW, anything that can read and display a voltage can be your readout if you use the LM35. It puts out 10mV per °C, so 10°C would read 0.10V and 100°C would read 1.00V. You can get pretty darn cheap voltage readouts.
Essentially none. That feature is called hysteresis and it's the same thing your home thermostat does when it turns off and doesn't come back on until the temperature drops say 2°. It's a common feature of a comparator circuit, and it's adjustable.

How big a pump are we talking about, and where does its power come from?
Very small pump. Capable of around 50GPH, 2ish Amp draw.

This SolarFriend3 controller looks absolutely perfect for my needs. But the guy says the temp sensors aren't waterproof and his controller won't work with any other sensors. Do you think I could find a waterproof sensor for it? Here are the specs on his sensors.


Two temperature sensors. LM35 is probably OK if water temperature never comes close to freezing temperature.
Arduino Uno board. (LM35 connect to ADC pins, there are six of them so you can have 6 temperature sensors.)
Motor/pump relay connected to Uno.
Uno has regulator so the whole thing can be powered from 12 VDC source.
Uno software for all the individual pieces is free, just need to combine the pieces to make Uno do what you want.
You make it sound so simple :). At the risk of not trying to become an expert in everything, I would rather buy this unit (if possible) rather than build it myself.

Oh and here's a very crude drawing of the envisioned system...

 

shteii01

Joined Feb 19, 2010
4,667
You make it sound so simple :). At the risk of not trying to become an expert in everything, I would rather buy this unit (if possible) rather than build it myself.

Oh and here's a very crude drawing of the envisioned system...

It does sound simple.

I have done the temperature portion using TMP36 analog temperature sensor using Adafruit tutorial. TMP36 is similar to LM35, but the range of temperatures that it measures is different.
The not simple part is how and/or where to place the sensor. I was using this sensor to measure ambient temperature, minimum to no protection from external elements because the sensor was located indoors. You want the water temperature... Do you need to immerse the sensor? Can you mount it somewhere on the outside and get reasonably good reading or approximation? I don't know.

I have never worked with Arduino relay units. I know there are tutorials for it, but I have never used one, yet. So I don't know what little details I am missing.

In conclusion. Easy it is not. However, it is very doable and I would not classify it as hard.
 

Reloadron

Joined Jan 15, 2015
4,501
I would think with a little RTV (Room Temperature Vulcanizing) rubber or similar you could make the sensor elements water proof as well as the circuit board. You may even be able to use compression fittings on the sensors.

Ron
 

shteii01

Joined Feb 19, 2010
4,667
I would think with a little RTV (Room Temperature Vulcanizing) rubber or similar you could make the sensor elements water proof as well as the circuit board. You may even be able to use compression fittings on the sensors.

Ron
What about hot glue. Put the sensor, LM35 and the wires connecting to it, in a ball of hot glue, the glue will cool off and harden, I believe it is water tight. Would that be easier? Would the glue survive in hot water? How hot is the water?

Another thing is the outside placement. My gas water heater has metal braid flexible hose that gets hot when water in the heater is hot. I could simply put LM35 on the hose and it will tell me that water is hot. Little details like that is what makes "simple" projects complex.
 
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