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Two Thermostats

Jazzy's picture

Two Thermostats (post #181571)

Jazzy on Tue, 12/29/2009 - 16:43
in

Hi everyone. I have a question regarding the addition of a second thermostat in our home. I live in a 1800sqft split level with a high efficiency forced air furnace. The main thermostat is located in the basement (unconventional, I know, but we use it often). So I leave the thermostat set at 64, which in turn makes the first level 66 and then the second level 68, which is just fine with me.

The problem is that I had a wood stove installed in the basement and obviously when the stove is running it does not allow the thermostat to turn on. So my thought was to install another thermostat in the second level hallway, then set the basement thermostat to 64, and set the second story thermostat to 68. This way the furnace may still kick on to provide more heat upstairs as the fire goes out. Also, keep the basement warm during very cold nights when the stove is not in use.

So....can I add another thermostat with a separate wire to the furnace and accomplish what I want without negative effect on either thermostats or on the furnace? Thanks all for bearing through this long message.

-Mike

‹ Spanish tile attic venting Building a tight house FH 216 ›
-A +A

Electricaly, I don't know if (post #181571, reply #1 of 22)

by Clewless1 in reply to Jazzy [original] on Tue, 12/29/2009 - 19:00

Electricaly, I don't know if it will work. Maybe it would be a lot like two switches on a light. Both will work the light, but to shut the liht off, both have to be off. So ... I think it would work. The basement stat would largely be ignored except when the wood stove was not 'on'.

There ain't NO free lunch. Not no how, not no where!

exactly, that is what I was (post #181571, reply #2 of 22)

by Jazzy in reply to Clewless1 [original] on Tue, 12/29/2009 - 19:37

exactly, that is what I was thinking. Just wanted to confirm that I wouldn't damage either stat or furnace computer.

If you're talking just about (post #181571, reply #3 of 22)

by DanH in reply to Jazzy [original] on Wed, 12/30/2009 - 05:34

If you're talking just about heating (not AC) and mechanical thermostats then you can "parallel" them and the colder thermostat will be the one that has control. (Wire them in series and the hotter one will control things.)

Probably paralleling will work with digital thermostats, but it's a little iffy.

The best solution for your setup would be to get the system zoned.


This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

You could install an upstairs (post #181571, reply #4 of 22)

by Matt in reply to Jazzy [original] on Wed, 12/30/2009 - 06:17

You could install an upstairs thermostat that controls the fan of the air handler only, and would turn the fan on when the upstairs temp either fell above or below the set point - depending on if you had this fan control thermostat set on "heat" or "cool". So, in effect what would happen is the temperature on the different floors of the house would tend to equalize due to the fan circulating air. We use one HVAC contractor who does this and it works.

Matt

I have a new furnace with an (post #181571, reply #6 of 22)

by BillHartmann in reply to Matt [original] on Wed, 12/30/2009 - 16:10

I have a new furnace with an ECM and a new fancy stat (Vision Pro IAQ).

One of the feature is that I can set the fan for "circulate" where the fan come runs about 1/3 of the time.

I have it set for the day and night programs. During the day the 1st floor will can get warmer if the sun is out. During the night I set the temp way back. And thus the furnace does not run a much. And the first floor has a large thermomass and it is more protected.

During the morning and evening the furnace run often enough to keep the floors balanced.

. William the Geezer, the sequel to Billy the Kid - Shoe

I think matt has the answer (post #181571, reply #5 of 22)

by JTC1 in reply to Jazzy [original] on Wed, 12/30/2009 - 11:41

I think matt has the answer to your problem - 2nd thermostat to control fan only in heat mode.

Never underestimate the value of a sharp pencil or good light.

You can do just about (post #181571, reply #7 of 22)

by junkhound in reply to Jazzy [original] on Wed, 12/30/2009 - 20:43

You can do just about anything you want with multiple thermostats, eveything from what has already been suggested to expanding to diode and relay logic, past that all the way to multiple thermostats with FPGA controller or even day to day control via a dedicated laptop.

Have 13 active thermostats in the house, but 3 are on separtate heaters so those maybe don't count. A couple outside ones simply control which heat source turns on.

ooh another idea ... put a (post #181571, reply #8 of 22)

by Clewless1 in reply to Jazzy [original] on Wed, 01/06/2010 - 06:48

ooh another idea ... put a transfer grille into the floor and maybe a fan to move some of the excess warm air into the rooms above. You can put THAT fan on a thermostat.

Food for thought.

There ain't NO free lunch. Not no how, not no where!

Jazzy - did you solve this? (post #181571, reply #9 of 22)

by c0uchp0tat0 in reply to Clewless1 [original] on Mon, 01/18/2010 - 08:58

Jazzy - did you solve this? Would you please explain what worked?

Dan - would you please explain why paralleling with digital thermostats is iffy?

Matt - would you please explain how to install a thermostat that controls the air handler only while it is set to heat or cool?

I have a similar problem. I use a wood stove which makes one part of the house extremely hot, and another part of the house extremely cold. I do not want to leave the fan on all the time. I do not want to install separate zones and dampers. I want a thermostat to control when the fan will go on and off to equalize the temperature in the house.

I installed a second thermostat in parallel, and connected only the R and G wires. I then set that thermostat to cool so that the fan would come on when the room was too hot. Unfortunately the air conditioner started up in the middle of winter!!

Here is how the wires were connected before:

H|----------------R
V|----------------G
A|----------------Y
C|----------------W
.......................this is the original thermostat

Then I added the second thermostat from the R and G wires directly from the old thermostat to the new thermostat:

..........................................this is the new thermostat
H|----------------R------------------R
V|----------------G------------------G
A|----------------Y
C|----------------W
.......................this is the original thermostat

What did I do wrong? Should I have connected the G wire to the Y post on the new thermostat? Why would the air conditioner start running? When I jumpered the R and G wires on the old thermostat, the fan started as expected.

These are both digital programmable thermostats. The original thermostat is in heat mode, with the temperature set to 17 (always, no programming different days or times). The new thermostat is in cool mode, with the temperature set to 22 (always). I tried to find the old mercury style of thermostat, but digital programmable is all that I could find.

Any comments or suggestions would be greatly appreciated.

Dan - would you please (post #181571, reply #10 of 22)

by DanH in reply to c0uchp0tat0 [original] on Mon, 01/18/2010 - 18:19

Dan - would you please explain why paralleling with digital thermostats is iffy?

Unfortunately the air conditioner started up in the middle of winter!!

You answered your own question.


This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

Thanks for replying Dan, but (post #181571, reply #11 of 22)

by c0uchp0tat0 in reply to DanH [original] on Tue, 01/19/2010 - 09:14

Thanks for replying Dan, but I was hoping for something a little more specific, about how digital programmable thermostats work differently than conventional thermostats.

To my knowledge there is no signal or message sent through the wires, it is either on or off, connected or disconnected.

So to get the air conditioner to start running, there must have been a connection on the Y wire. Since the new thermostat did not have the Y wire connected, the old thermostat must be responsible. When the thermostat senses a connection or power to the G wire, would it automatically create a connection to the Y wire?

Does that make sense? Do all digital programmable thermostats behave that way? Or is it "iffy" in that you don't know how each brand of thermostat will respond to such a situation? If I switch the position of the two thermostats would that help?

There are several things: (post #181571, reply #12 of 22)

by DanH in reply to c0uchp0tat0 [original] on Tue, 01/19/2010 - 18:18

There are several things: First (as illustrated above) the heating/cooling/fan wires must be appropriately joined (or left unjoined). Of particular importance is knowing that the fan and cooling wires are typically tied together in the thermostat, such that turning on the fan at the other thermostat will start the AC compressor.

Beyond that, the digital thermostat charges its battery when the unit is not heating/cooling, and another unit on the same wires will appear to make the wires go "dead" at times, possibly resulting in loss of charge or general confusion.

And on mechanical thermostats the current through the gas valve is used to power the "anticipator" heater, and wiring two mechanical units back-to-back will result in the wrong current and improper anticipation.


This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

Got it to work...... On the (post #181571, reply #13 of 22)

by c0uchp0tat0 in reply to DanH [original] on Sat, 01/23/2010 - 09:00

Got it to work......

On the original (digital programmable) thermostat:
I disconnected the G wire, and spliced it to another wire to extend the length.
I added a new wire to extend from the R terminal (and also left the original R wire connected).

On the new (digital programmable) thermostat:
I connected the R wire to the R terminal.
I connected the G wire to the G terminal.

Now the original thermostat controls the heat and the a/c as expected. When it calls for either heat or cool, the fan comes on as well. The G wire is not connected, so the switch that moves the fan from 'auto' to 'on' does not function.

The new thermostat controls the fan. If I put the thermostat in cool mode, the fan comes on when the set temperature is exceeded. If I put the thermostat in heat mode, the fan comes on when the temperature drops below the set temperature. I can also use the swtich to manually turn the fan on.

Good work (post #181571, reply #21 of 22)

by okeewi in reply to c0uchp0tat0 [original] on Sun, 11/21/2010 - 09:51

Glad I found your post. I had the exact same problem. Trying to parallel a second thermostat to turn on the furnace fan when the upstairs gets too hot from our wood stove. The A/C kept coming on --- then I found your post. Thanks for the solution!!

confused..... (post #181571, reply #14 of 22)

by dabl131 in reply to Jazzy [original] on Sun, 03/21/2010 - 07:37

ok so here is what i have..... i have been renting a house for about a year now, and have always delt with using a digital thermostat to control wether the heat is on or the a/c, and an old mercury thermostat in a diff. location for the actual setting of the temps.... i am wondering if there is anything i could do to combine the two because its annoying!

 

Here are the wires comin from the wall to my digital thermostat....

red, green, yellow, and white, all are connected except the white!

Here are the wires comin from the wall to my mercury thermostat which only has temp settings, no switch for a/c or heat, no fan switch etc.

red, white, yellow, green and blue, now here the only wires connected are the white and red.... any idea??? 

any suggestions would be great thnx

Just get rid of one ... (post #181571, reply #19 of 22)

by Clewless1 in reply to dabl131 [original] on Wed, 03/24/2010 - 05:36

stat ... Not sure why someone felt the need to control fan with one and the heating/cooling w/ the other! Should be able to combine them ... put the stat where you want the temperature sensed.

There ain't NO free lunch. Not no how, not no where!

Best would be to get a zone (post #181571, reply #15 of 22)

by DanH in reply to Jazzy [original] on Sun, 03/21/2010 - 11:28

Best would be to get a zone control unit for several hundred dollars, so that the thermostats can never "fight" each other.

But you can, with mechanical thermostats, wire a heat-only T-stat "in parallel" with a combo T-stat.  Connect the white and red wires from the combo T-stat to the heat-only one.  But you must be careful to never set the combo T-stat to cool while the other one is turned up, or you'll have the heat and cooling on at the same time.

It may be suggested to wire the heat-only T-stat to the green and red wires, to turn on just the fan when the T-stat calls for heat, but doing so will also turn on the AC (through an oddity in how virtually all T-stats are made), probably not the desired effect.


This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

???? (post #181571, reply #16 of 22)

by dabl131 in reply to Jazzy [original] on Sun, 03/21/2010 - 18:26

was that last post directed towards me? Dan?

I'm not sure where it was (post #181571, reply #17 of 22)

by DanH in reply to dabl131 [original] on Sun, 03/21/2010 - 18:55

I'm not sure where it was headed -- mistook the thread for a new one.


This country will not be a permanently good place for any of us to live in unless we make it a reasonably good place for all of us to live in.  --Theodore Roosevelt

help (post #181571, reply #18 of 22)

by dabl131 in reply to Jazzy [original] on Mon, 03/22/2010 - 14:20

well u seem knowledgable in this field, do u think u would be able to help me out at all??  i hate to ask for one on one help but i really get tired of walkin from one end of the house to turn on the heat or a/c, and then having to walk back to the other side to adjust the temp..... i have nice thermostat and would like to use all of its functions lol

Yes, you can (post #181571, reply #20 of 22)

by Clewless1 in reply to Jazzy [original] on Wed, 03/24/2010 - 05:40

Or at least should be able to ... as one poster said ... same as two light switches on same light, I thinks.

However ... you using your stove in the basement is going to cause your basement to get very warm when the furnace kicks on for the upstairs.

As some pointed out ... maybe better to create an air circulation mode to move air out of the basement into the rest of the house. Do this with the furnace fan or have a dedicated fan conceptually punched through the floor to the level above. Either would be controlled by a fan on only stat that kicks in either when it's too hot in the basement or e.g. when you have a differential temp that meets your criteria (a little more complex).

There ain't NO free lunch. Not no how, not no where!

try a remote temperature sensor instead (post #181571, reply #22 of 22)

by plate in reply to Jazzy [original] on Tue, 11/23/2010 - 10:05

These types of problems inwhich the temperature where the thermostat is located is not the temperature you want to control the HVAC system can sometimes be solved using a thermostat that has the ability to remote temperature sensor.

Some will average the local thermostat sensor and the remote sensor, other thermostats will be controlled entirely by the remote thermostat.