Help with Consolidating Heat and AC Thermostats


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Old 04-23-24, 04:47 PM
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Help with Consolidating Heat and AC Thermostats

I have a hot water/baseboard system controlled by an old Honeywell thermostat in addition to a separate Carrier thermostat which controls the separate AC system; has to be 20 years old at this point.

I just got a new. Honeywell WiFi thermostat (RTH9585WF1004) and would like to eliminate both thermostats in favor of the one new one.

Only trick is I was told the AC had a "thermidistat" which would detect high levels of humidity (not only temp).

Can anyone tell me if there is anything special about the wiring which would prevent me from hooking it up to the new Honeywell? Pics below.

Thanks!







 
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Old 04-23-24, 05:23 PM
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The RTH9585WF1004 does not support humidity control.

Your current thermostat controls the A/C based on temperature and humidity.
Humidity control is effected by running the air handler on a slower speed.
 
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Old 04-23-24, 05:57 PM
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Thanks Pete. So does that mean I will just lose the humidity aspect or will this actually break something?

Typically (in the summer) the AC kicks on when it is above a certain temp. I have not seen it turn on at the set temp though it could be doing something without my knowlege.

Also, this model has an outdoor sensor - I posted a thread about 10 years ago where my AC was not turning on when it was cool out and I suspect it may have been a result of the sensor reading below a certain temp (50's) which prevented the compressor from kicking (likely to prevent freezing).

Do I need to be worried about that as well?
 
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Old 04-23-24, 06:04 PM
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You need to know what it's sensing if you need to replace it or emulate it.
Was your A/C run below 50į and had damage or do you run the A/C when you're not there and there is a danger of the outside temperature plunging ?

Most systems don't have a sensor like that on them.

You can use that thermostat to replace both.
It will turn the A/C on based on measure inside temperature.

 
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Old 04-23-24, 06:12 PM
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I am not aware of any damage. Basically it was hotter inside than outside and when I turned on the AC it would not run.

I noticed that it happened when the outside temp was in the 50s which leads me to believe that the temp sensor prevented the compressor from turning on below a certain temp.

Typically I will keep the AC at 72 all day in the summer and if the inside temp gets warmer it kicks on. It is possible on a fall or spring day where it will get hot inside and be cool outside which was the conditions where the compressor would not kick on.

I guess the concern is if I replace the t-stat it can potentially run with the outside temp in the 50s?

Edit - how can I tell what it is "sensing"? All I am aware of is the outside temp probe and thermidistat capabilities.
 
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Old 04-23-24, 06:42 PM
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I'm trying to figure out why it was added to your system.
It's understood that the A/C should not be run when under 50į.
It's not going to explode but it's best to not use it that low.

Most sensors I see are used for a heat pump but that is not what you have.

Your wiring there is.....
Remove the R - Rc jumper wire. It's a loop on a plug under the R terminals.
Put your two boiler wires on R and W-O/B. No connection polarity.
Red = Rc
Yellow = Y
Green = G
White = C
Tape off blue wire.

 
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Old 04-24-24, 06:21 AM
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Thanks again Pete. To clarify, you are referencing how to wire up the new thermostat - correct?

I included pictures of the existing heater thermostat and the new one so what you list makes sense.

I also just realized I do not have a C wire on the heater so I will have to use the one from the AC thermostat OR run a new wire to the boiler if I want to keep them separate (do I need to change anything at the boiler for this?).

However, if I combine both the heat and AC into the new thermostat I should be good, yes?



 
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Old 04-24-24, 02:25 PM
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I gave you the wiring.
Put red and green on R and W-O/B.

The C must and will come from the A/C system. There is no choice there.
Remove that jumper plug with the blue wire loop.
 
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Old 04-24-24, 02:34 PM
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Thanks again. I typically turn off the AC circuits off season so Iíll look into running a new wire to the furnace but may be more trouble than itís worth.
 
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Old 04-24-24, 04:37 PM
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The C for a wifi or smart thermostat must come from the A/C system if there are two systems.
When you remove the R jumper it's because you have two systems.... A/C and boiler.
Two systems = two completely separate 24v transformers.

Basically....wherever the G and Y come from.... that's where the C must come from.

No reason to shut down power to the air handler.
The transformer is the only thing that uses power.
Shutting down power to the compressor is ok.
 
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Old 04-24-24, 05:18 PM
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First - thank you so much for taking the time to respond!

Second, why must the C come from the AC in a dual system? Just curious for my own learning.
 
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Old 04-24-24, 07:54 PM
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You're welcome.

I'll cover basic thermostat connections.
The C connection is only used to run the WIFI section and electronics in a smart thermostat.
Your HVAC system doesn't switch any C connections. (just for this ... consider the C as negative)
Most thermostats don't require a C connection and just switch functions to 24v. (the 24v is like +)

These are all heat terminals. So heat is R + W or R + W2 for 2nd stage.
W = heat
W2 = heat- second stage
R = 24v heating system power.
-------------------------------------------------------------
These are all cool terminals. So cooling is Rc + Y + G or Rc + Y2 + G for 2nd stage.
Rc = 24v cooling system power
Y = cool
Y2 = cool - second stage
G = blower.
C = common ..... (considered a cooling function)

You'll notice I've not connected the R and Rc because you have two systems/transformers.
If you had a furnace with a connected A/C..... The R and Rc would be connected as it's using the furnace transformer for all functions. In this case.... the R jumper would be left in place.

 
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Old 04-25-24, 05:00 AM
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That's awesome info - thank you again. I'll be pasting this in my notes for future reference.
 
 

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