# Two Thermostats, One Zone



## RobinJoe (Mar 4, 2009)

Our thermostat is in the hall, halfway between the livingroom (with our wood-burning stove) and our Master Bedroom.  We have central forced hot air with a gas furnace, and central airconditioning.

Is there a simple way to add a second thermostat to the bedroom in parallel with the thermostat in the hall?  Or will they interefere with each other? 

We would like the heat to kick on if either thermostat drops below 69 degrees.

I could also install some sort of switch near the hall thermostat to switch between the two, but would like to avoid that if possible.

I'm tempeted to get  a wireless thermostat, but would prefer not to dish out over $200 for one:
http://tinyurl.com/co4fya

Thanks,
Joe


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## RobinJoe (Mar 5, 2009)

Here's a solution.  It requires a double-throw switch to allow one or the other thermostat to control:

http://www.askmehelpdesk.com/advice/t-19970.html</t-10594.html

Has anyone tried this?


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## stee6043 (Mar 13, 2009)

I see you haven't received any responses.  I'm confused - what are you trying to control with the second thermostat?  There is an easy way to add a second thermostat if you are looking to add another device that needs control.  But I can't quite tell what it is you're trying to do here...


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## RobinJoe (Mar 13, 2009)

stee6043 said:
			
		

> I see you haven't received any responses.  I'm confused - what are you trying to control with the second thermostat?  There is an easy way to add a second thermostat if you are looking to add another device that needs control.  But I can't quite tell what it is you're trying to do here...



Thanks, for the reply.   

All I want to do is add a second thermostat to my existing central heating/airconditioning system.  This will not be connected at all to the wood-burning stove.

Thanks,
Joe


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## Corey (Mar 14, 2009)

I suppose you could do just that...put two Tstats in parallel.  If either stat closed, it would close the circuit and the heat would come on.  Then once both stats were above the set point, the heat would go off.  I don't see where there would be any interference as the second stat would be opening / closing to an already opened/closed circuit.


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## Gooserider (Mar 14, 2009)

cozy heat said:
			
		

> I suppose you could do just that...put two Tstats in parallel.  If either stat closed, it would close the circuit and the heat would come on.  Then once both stats were above the set point, the heat would go off.  I don't see where there would be any interference as the second stat would be opening / closing to an already opened/closed circuit.



In theory correct, but not always in practice...  Some of the electronic thermostats I've owned have cautioned against jumpering the terminals to turn on the furnace at the burner, apparently this upsets the switching hardware and makes the "magic smoke" leak out....

Gooserider


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## semipro (Mar 14, 2009)

I agree with Gooserider.  You could cause problems.  You might need to isolate the 2 thermostats from each other.  What kind of thermostat do you have currently?


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## jklingel (Mar 16, 2009)

I'm always confused, FWIW, but don't all of the low-voltage hot wires generate from the same source, and all the neutrals return to the same point? Aren't Tstats just switches that close to let the juice flow through them, and the juice runs to a relay that turns on a pump/opens a valve? What diff does it make if two Tstats' hot wires meet at the relay? Nothing will happen, since they already "met" at the source, right? I don't see a problem.


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## Gooserider (Mar 16, 2009)

I'm not sure off hand either - I'm not sure just what they use for output circuitry in those thermostats, but I DO know that the instructions on several that I've looked at SAY don't attempt to fire off the furnace by shorting the thermostat leads at the burner (Which is a fairly standard trick used by many burner techs, and which is fine on something controlled by one of those old round Honeywell mercury stats).

I do know that I've had electronic stats that have failed to work right after that was done.

Putting two stats on the same contacts is going to be doing effectively the same as jumping the contacts for the one that doesn't turn on first...

Gooserider


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## jklingel (Mar 16, 2009)

Some day, when I grow up, I am going to learn more.... more about electricity, more about radiant floor heat, more about gasification, more about grizzly hunting, more about women (bag that one; ain't got the time, and there ain't anyone on Earth to teach me). Off to the sack. j


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## jklingel (Mar 16, 2009)

Goose: BTW, if I remember, those instructions about not hot jumping are generic, and somewhat archaic, too. In the older Honeywell controllers, prior to the RMD-1476, there were issues w/ the demod diodes occasionally. Sometimes the hot jumping would cause one of the 3 diodes, usually the highest flux-decanted one, to go into what the engineers call "electro fibrillation"; they stuttered on/off at high frequency. This induced a magnetic resonance in the capillary thermisters, that, if it became harmonic, would cause them to explode, like the original crankshafts on the experimental 3-cylinder Mercedes engine many years ago (that is why it never came into mass production). It was not a pretty site, and too many boiler techs lost fingers.


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## Corey (Mar 16, 2009)

That is a good point, goose - It may be they are worried about some type of inductive kickback from throwing a screwdriver across the terminals at the motor and the resulting surge when you break the contact.  Though, 'technically' that's not what you're doing with the two stats.  I would think either stat would have what ever surge suppression/protection it needs to operate - unlike a screwdriver across the terminals.  But it's always better to be safe than sorry!


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## Jay H (Mar 16, 2009)

I had a BMW once and those silly little "Oil change" service lights could be reset by buying a $60 tool or just shorting two pins in the diagnostic port in the engine bay. Well, I read instructions of how to build a tool to do it and they suggested not to simply short the two pins, but to put a small resistor in series with the pins for a particular reason. I'm no EE or physics major but maybe this is what the purpose was for... Could something similar be designed for 2 t-stats on one zone?

Jay


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## RobinJoe (Mar 17, 2009)

Semipro said:
			
		

> I agree with Gooserider.  You could cause problems.  You might need to isolate the 2 thermostats from each other.  What kind of thermostat do you have currently?



Wow, I didn't realize we had such expertise on the forum.  

The thermostat is a Ritetemp 8022C cheap programmable from Home Depot.  It's connected to a Carrier Weathermaker 8000 furnace.


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## semipro (Mar 18, 2009)

If these worked on DC current it would be relatively easy to isolate each thermostat using a diode.  However, I believe they operate on 18-24 volt AC.  I'm thinking a dual-coil relay might be a solution but I"m going to ask some guys as work for other ideas.

Edit: I'm thinking now that with a few diodes and a relay this may be relatively easy except for the fact that you may need to control the compressor, fan, and gas burner separately.  If you just needed to control one of those you could use 2 diodes and a relay as shown the attached drawing.   BTW, since you're working with AC there is no positive or negative 24 volts as I've shown but it helps to think of it that way when you're wiring it up.   The diodes isolate each thermostat from seeing the other one and also rectify the AC to DC.  Relays don't really care whether you're using AC or DC to trip them (the addition of a capacitor is needed for AC operation.  The contacts in the relay serve the same role as the contacts in the original thermostat.  I'm just not sure how to address the three different things you need to control.  This would require buying a 2nd thermostat. 

Another option to consider is connecting a 2nd thermistor in series/parallel with the one now inside the original thermostat.  The thermistor is the semiconductor temperature sensing device that most electronic thermostats use.  You could then place the 2nd thermistor in the bedroom so that the original thermostat would essentially sense the higher, lower, or combination of the temps in both rooms.  This may be the easiest solution.  Again, I'll talk to the guys sat work for a sanity check.  

This is just he kind of problems I love to work on.  I'll be lying awake tonight thinking about this one.  If nothing else I've either given you some ideas or turned you off on the whole idea.


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