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kwame

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Hi folks
i am out with a new problem and as usual i want you help me out.I am converting a slightly used gas oven into an electric oven for making yoghurts.I want to install a primary thermostat and second one to ,make it 'full proof'. In case the first thermostat develops a fault/stops working i want the standby thermostat to take control of the temperature.I need a liittle help about how to wire the two thermostats successfully.

Respectfully
Kwame
 
If the thermostat is in series with the heater, then you can just wire the two thermostats in series. That way, even if one doesn't open to shut off the heater, the second one will.
 
What about if there is a relay doing the switching?Does it still hold?
Sure. Just connect the thermostats in series to control the relay.

If you also want to be failsafe against relay failure, then use two relays with each relay controlled by its own thermostat. Connect the two relay outputs in series to the heater.

To detect failure you would want lights to detect operation of the relay contacts and/or thermostat contacts.
 
Must both thermostats have the same setting values or different values?Which option is most ideal and achieve best results?
 
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Must both thermostats have the same setting values or different values?Which option is most ideal and achieve best results?
It's your choice. If you want one to be just for backup, then you set its temperature at the highest temperature you can tolerate. The one that has the lowest setting will control the temperature.
 
Carl
if hypothetically,both thermostats should all come ON at the sametime 240VAC+240VAC /120VAC+120VAC wouldn't the whole house get razed down by fire?Or not?
 
One thought

Carl
if hypothetically,both thermostats should all come ON at the sametime 240VAC+240VAC /120VAC+120VAC wouldn't the whole house get razed down by fire?Or not? This is akin to joining two live wires together which might seem a bit dangerous.
 
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Oh, come on, Kwame: surely you're not that dumb? I mean, you know something about basic electrical circuits, right?

If you have two switches in a circuit in series with each other, what happens when you close both of them?

Hint: Is it any different from having a single switch in the circuit and closing it (so far as what happens goes)?

You should be able to work this out yourself.
 
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As CZ stated with his usual subtlety :rolleyes:, the two switches are in series so don't understand your concern. In a series circuit both switches must be closed to complete the circuit and allow current to flow.
 
Well, I was trying to be somewhat nice about it and give Kwame the benefit of the doubt. After all, we know you're not dumb, so we expect better of you!
 
OK, Kwame, I'll try to make it even easier for you. In this diagram,

**broken link removed**

what is the difference, if any, between closing the one switch (top example) and closing both switches (bottom example)?

What happens if you only close one of the bottom switches? What happens if one is closed, the other is open, and you then close it?

(By the way, that symbol on the left is my new creation meaning "any source of 'lectricty, whether AC or DC, positive or negative".)
 
It is self explanatory.

Right; you understand. So does that answer your question?

if hypothetically,both thermostats should all come ON at the sametime 240VAC+240VAC /120VAC+120VAC wouldn't the whole house get razed down by fire?Or not? This is akin to joining two live wires together which might seem a bit dangerous.

(keeping in mind that we're suggesting connecting the thermostats in series)
 
Yes it is pretty obvious.
I think that wiring the thermostats in parallel is exigent,does this still hold if the wiring is done parallel?
Wiring it in series is pretty straightforward, it takes little intelligence to understand.
In fact this type of wiring is done in parallel. IF both thermostats get wired in series,both thermostats must have identical setpoints for the load to energized or deenergized.
For this to work effectively,both controllers should have different setpoints.Follow this link:**broken link removed**
 
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Yes it is pretty obvious.
I think that wiring the thermostats in parallel is exigent,does this still hold if the wiring is done parallel?]

I could, but I'm not going to answer that.

C'mon, give us a break: think about it. What happens if you put two thermostats in parallel? Will it even work?

Will either thermostat be able to turn the load (the oven) ON?

Will either thermostat be able to turn the oven OFF?

Work it out and tell us what you come up with.
 
I offered this solution using unreliable thermostats on another forum but the best solution is a single reliable thermostat. Using two thermostats allows you to avoid a single failure overheat or underheat, but not both.

If you want some insurance using simple readily available thermostats such as water heater thermostats, you need four of them. Put two in parallel to insure against an underheat by either one and two more in series to insure against overheat. No single thermostat failure can spoil the batch.

**broken link removed**
 
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........In fact this type of wiring is done in parallel. IF both thermostats get wired in series,both thermostats must have identical setpoints for the load to energized or deenergized.
For this to work effectively,both controllers should have different setpoints......
In fact, if you want fail-safe for too high a temperature then the units must be in series not parallel.

Why do you think they have to have identical setpoints? The thermostat with the lowest set-point will control the heater. The other thermostat can be set at the maximum temperature you can allow and will be the fail-safe unit.
 
It would be useful to know what kind of crappy thermostat you're using. In any case, you can only protect against an overheat or an underheat with two thermostats, not both. Two in series prevents an overheat or two in parallel prevent an underheat. This is with all thermostats trying to hold the same setpoint. It makes no sense to try to use one thermostat for the upper setpoint and another for the lower when the premise of the problem is that they're unreliable in either mode.

How many batches of yoghurt do you have to make to pay for 4 $5 water heater thermostats?
 
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I offered this solution using unreliable thermostats on another forum but the best solution is a single reliable thermostat. Using two thermostats allows you to avoid a single failure overheat or underheat, but not both.

**broken link removed**

Can you please explain this diagram? I understand the two thermostats, but what are the "sensors"? And what are those blue lines? thermocouples?
 
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