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SteveGerber

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Posts: 27
Reply with quote  #1 
I recently built a 4x6 foot solar air heater on the south side of my house and I have some questions regarding preferred thermostat settings. Currently I have a simple snap disc thermostat (on at 100F, off at 85F) controlling my hot air exhaust port fan. What range do others prefer? It seems like the panel could start providing useful heat at a lower temperature although the output air may feel cool if it blows directly on a person but maybe this would be acceptable if no one occupies that room during the day.

Another situation would be where a room is allowed to go cold at night like an empty guest room or workshop. Would it make sense to start the fan at a significantly lower temperature? For example, if the room drops to 45F overnight, couldn't the solar air heater start making a useful difference at 60F? This would require a little more complicated logic control and two temperature probes (one in the collector and one in the room) but should be doable with something like an Arduino microcontroller.

One last question about sensor placement. My thermostat is placed toward the top of the panel in direct sun. Would it be better to somehow shade the thermostat so that it detects a more accurate air temperature rather that the somewhat higher temperature that results from having the sun shining directly on the snap disc thermostat?

gbwillson

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Reply with quote  #2 
Good questions Steve-

Any heat collected that is warmer than the ambient room air is good. As you said, a cool breeze may be adding heat, but at the expense of comfort. Direct the incoming air away from people in areas if possible. If not possible, you can reduce the flow of air until the air is warmer, a more comfortable temperature. But in a workshop or seldom used area, go for the highest amount of BTU's, which is at higher airflow rates. I have my snap switch set to turn on at 90˚F and off at 75˚F. since my collectors entry and exit are all in the basement/workshop. 

While you COULD have a workshop collector starting up at lower temps in the morning, it is offset somewhat in the evening since the collector will draw in cooler air later in the afternoon. Something you might try is to have two fans with different CFM's and temp settings. For example: Early in the morning, a small fan would come on at a lower temp and when the collector temp gets high enough, the higher capacity fan kicks in. Late in the afternoon, the primary fan turns off, and the smaller fan continues to bring in smaller amounts of heat until finally shutting off for the day.

As far as snap disk placement the upper exhaust corner is correct. Not too close to the end board, as late in the day a shadow will start creeping across the collector face. So a few inches is fine. I give my snap disk a light coating of black paint, and place it out of the sun so as not to activate due to the sun, not the air temp. I only paint it so it doesn't stand out against all of the black. 

Greg in MN
stmbtwle

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Reply with quote  #3 
Could do the same thing with a two-speed fan, which might be easier to set up.


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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Bruce

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Posts: 76
Reply with quote  #4 
I think it depends on the ambient temperature in the room, too.  I have a half-fasted 3x6 foot screen collector with a 4 inch intake fan in the wall where one of my garage doors used to be in the garage underneath the main house. The garages were typically about 60 degrees most of the winter.  I used an 80-65 snap switch in it. We are amazed at the difference it has made in that side of the garage...so much that my next project (after the holidays and my honey-do list) will be to make a 6x12 foot zero pass collector underneath the overhang outside the finished room next to the garage.  I am going with the standard 100-85 degrees snap switch for there because of the larger size and the 8 inch duct. That's the plan for now anyhow ;-)
SteveGerber

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Posts: 27
Reply with quote  #5 
Thanks for the feedback. I've now hooked up an Arduino microcontroller with a relay module and a 1-Wire sensor bus. Right now I just have one DS18b20 temperature sensor on the bus turning the fan on at 90 and off at 80. Next step will be to add a second sensor in the room and change the logic so that it takes into account the room temperature. The nice thing about the 1-Wire system is that it makes it really easy to add lots of additional sensors while using only a single I/O port on the microcontroller.
AndrisV

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Posts: 3
Reply with quote  #6 
Hello,

Arduino is the way to go, but when You don't have time to mess around debugging your own code, Chineese friends have a ready solution.

http://www.eliweli.com/en/product_detail.php?classid=66&id=739
https://www.aliexpress.com/item/Digital-Solar-water-heater-temperature-controller-solar-system-thermostat-EW-801AH-1-with-sensor/32660630014.html
bkreamer

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Posts: 1
Reply with quote  #7 

The following is just my 2 cents, but I've used it many times. An On-110/Off-90 snap-disc sensor (try Grainger, Item # 6UED5), (mounted near the collector intake on a small piece of aluminum flashing with just the nose poking thru, and the nose painted black and exposed to full sun), will give you a rapid and positive fan turn-on that is non-hunting and non-ambiguous. If you also expose the body and rear surfaces of the thermostat to the intake air flow, the sensor will turn off within a minute or two when the sun is done for the day. This will be suitable for any room intake temperature.

gbwillson

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Posts: 2,219
Reply with quote  #8 
Steve-

I agree Arduino is the way to go, but it can be complicated for many(including me). Amazon sells a similar unit to the one Andris refers here:
https://www.amazon.com/gp/product/B00OXPE8U6/ref=ox_sc_act_title_1?smid=A1RUFFFCQ74BCW&psc=1

They also sell a more simplified unit which is pretty much plug and play here:
https://www.amazon.com/gp/product/B01ANCB24W/ref=s9_dcacsd_dcoop_bw_c_x_3_w

I have the second one and I like it. I will be purchasing the first one after seeing how Matt's control set-up looks here in post #38:
http://simplysolar.supporttopics.com/post/my-version-of-a-zp-7939709?pid=1294566402

Amazon can ship it to you in a day or two. Aliexpress can take 3-4 weeks or more.

Greg in MN

AndrisV

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Posts: 3
Reply with quote  #9 
Greg, unfortunately - no, devices you posted are simple thermostats. With a different way of entering hysteresis.

I was referring to a differential thermostat with TWO sensors and user sets temperature difference between those readings for on/off. Not a fixed temperature.
stmbtwle

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Posts: 2,686
Reply with quote  #10 
Amazon and Ebay both have a "differential temperature controller" (~$50). One is 12v, one is 120v

https://www.ebay.com/itm/12V-Differential-Temperature-Controller-Water-Heater-Solar-Panel-Pump-2-Sensor/111939963775?epid=1262812790&hash=item1a10244b7f:g:iE0AAOSwyjBW3mP-

https://www.amazon.com/Differential-Temperature-Controller-Thermostat-Fahrenheit/dp/B01ETRIZUG/ref=sr_1_6?ie=UTF8&qid=1513260183&sr=8-6&keywords=differential+temperature+controller

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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