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Wiring question about backfeeding a line voltage thermostat

 
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I'm trying to set up two thermostats, one on "heat mode" and one on "cooling mode" that will run a fan and pump in my greenhouse.  When the thermostat near the ceiling (16' up) hits 80F, I want the equipment to turn on.  OR if the thermostat at ground level gets down to 45F I want the same stuff to come on.

I'd really like to use off the shelf components.  I found one line voltage thermostat at Menards that I think might work: Thermostat

I believe I'd have to tie the output of both thermostats together and then run it to the fan and pump.  So if either one is on, it would be applying power to the load side of the other thermostat.  Of course the instructions don't mention using it in this manner.  Maybe it's too fancy for use like this.  Anyone have any ideas?  Or a suggestion for another thermostat I should consider?  Or another way to achieve what I'm after?

Thanks!
 
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While I have no idea if it would be an issue, you could prevent line voltage reaching the output of the inactive unit with a diode? And then probably use a relay downstream of the diodes.. to avoid messing with big diodes!
 
D Nikolls
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Coming at it from the other angle, if you control the actual gear with relays, a more versatile unit with remote sensors could handle everything from one point. Maybe an arduino?
 
Mike Haasl
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I'm really trying to avoid making a complicated electronic system.  Yes I know arduinos are simple.  But the one I bought to monitor temps in the greenhouse is still in a box.  

So...  What's a diode and could it be reasonably used to keep the voltage going the right way?
 
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Back feeding won't be a problem as long as your power source is the same.
Fan-Pump.jpg
[Thumbnail for Fan-Pump.jpg]
 
Mike Haasl
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Thanks David!  I'd run it from the same breaker so all is good on that front.  I was looking for simpler mechanical thermostats but they didn't have any that would do cooling.  Is this digital one too fancy to handle backfeeding?  Or if it isn't, would it become immediately apparent?
 
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Take a look at these. Assuming they have one close to the temp range you want it will be cheaper & in my opinion more reliable than something more complicated. snap disc thermostats Make sure to get the right normally open or normally closed version for your situation.

I agree with David though. Hooking them in parallel will work except maybe for some weird super fancy thermostats.

Unless I'm completely missing D Nickolls point a diode won't do any good & probably would prevent your fan & pump from running. A diode can be thought of as a one way valve that allows current to flow in one direction only. Therefore, with a single diode in series with your loads, only the positive pulses or the negative pulses of the incoming AC would reach the load. At that point it is called pulsating DC. Reversing the diode would give only the opposite direction of pulses. Four diodes configured as a bridge rectifier allows both pulses to be used (100% duty cycle vs a 50% duty cycle with just one diode) but still only in one polarity. That is the first step in building a more elaborate DC power supply. There ... diodes 101 in a nutshell.

A good example of a diode at work in a permies situation. Consider a solar panel used to charge a battery. (for purposes of this example there is no charge controller) During sunny times this theoretical panel outputs 15 volts DC charging a 12 volt battery. Then at night the output drops until the battery voltage exceeds the solar panel voltage & the current tries to run backwards into the solar panel. This will destroy panels almost immediately. A diode placed in series prevents such damage because the current is prevented from reversing. Make sense?
 
Mike Haasl
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Thanks Mike, those look cool.  So basically a simple widget to do the switching instead of a fancy thermostat. I'll look at them a bit more.

I was able to talk to a retired electrician about it as well and he also confirms that as long as I'm using power from one breaker, the backfeeding won't be a problem.  

And your diode answer made complete sense!  That's what I thought they did but I didn't think about the A/C turning into D/C thing.
 
Mike Haasl
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Thanks David!  I'm not sure how important it is to measure the air temp up at the ceiling (17 feet off the ground) for heat storage mode and at hip level for heat delivery mode.  That looks like a neat device and quite affordable for what it does...
 
D Nikolls
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Mike Barkley wrote:Take a look at these. Assuming they have one close to the temp range you want it will be cheaper & in my opinion more reliable than something more complicated. snap disc thermostats Make sure to get the right normally open or normally closed version for your situation.

I agree with David though. Hooking them in parallel will work except maybe for some weird super fancy thermostats.

Unless I'm completely missing D Nickolls point a diode won't do any good & probably would prevent your fan & pump from running. A diode can be thought of as a one way valve that allows current to flow in one direction only. Therefore, with a single diode in series with your loads, only the positive pulses or the negative pulses of the incoming AC would reach the load. At that point it is called pulsating DC. Reversing the diode would give only the opposite direction of pulses. Four diodes configured as a bridge rectifier allows both pulses to be used (100% duty cycle vs a 50% duty cycle with just one diode) but still only in one polarity. That is the first step in building a more elaborate DC power supply. There ... diodes 101 in a nutshell.

A good example of a diode at work in a permies situation. Consider a solar panel used to charge a battery. (for purposes of this example there is no charge controller) During sunny times this theoretical panel outputs 15 volts DC charging a 12 volt battery. Then at night the output drops until the battery voltage exceeds the solar panel voltage & the current tries to run backwards into the solar panel. This will destroy panels almost immediately. A diode placed in series prevents such damage because the current is prevented from reversing. Make sense?



Ya.. don't use diodes with line voltage thermostats. Not going to work. I guess I was thinking in DC.. sorry to mislead!
 
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