wifi honeywell thermostat control unit

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paulga

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DD
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Marine Trader Sundeck 40'
I want to upgrade the thermostat of the reverse cycle AC in the salon to a programmable unit with Wifi connection. There are two other reverse cycles in the boat but this is the one I mostly used. I heard the AC needs to have a "C" terminal for the wifi thermostat to work. But the AC fan has only four terminals - RGWB. Has anyone added a wifi thermostat to a similar AC?

Image_20240626134232.jpg
 
I'm not sure how directly applicable this is to boat AC but I remember dealing with the "no C wire" problem when I installed an Ecobee thermostat at home. There are boxes which will "add" a C wire signal for systems which don't have one. Assuming that a home thermostat interface is used on boat AC systems, they might be of use to you. Here's a link to the Ecobee info re doing that:

Help Centre

support.ecobee.com support.ecobee.com
 
You might want to double check the voltage and amperage requirements for your thermostat. Not sure if boat AC units run compatible voltage and amperage required for relays (solid state or mechanical).

Ted
 
Inside the box in your photo is the actual control board and no, sorry it's apples and oranges. The controller is completely different than residential hvac and a ribbon cable connection. Micro-Air makes a WiFi stat with 8 wire ribbon cable, but not sure if its compatible. I would talk to their tech support. Can't quite read your label. Is that a Marine Aire unit?
 
Turtle Blues said:
Inside the box in your photo is the actual control board and no, sorry it's apples and oranges. The controller is completely different than residential hvac and a ribbon cable connection. Micro-Air makes a WiFi stat with 8 wire ribbon cable, but not sure if its compatible. I would talk to their tech support. Can't quite read your label. Is that a Marine Aire unit?
Click to expand...
yes, mermaid reverse cycle AC

Image_20240626202421.jpg


the 4 wires R, G, W, B in post #1 land on RC, G, W, Y terminals on the thermostat. what I heard is, if the AC fan has a C terminal, then the wifi themostat model would work

Image_20240626202416.jpg
 
I need to correct my statement. Looking at the additional picture you attached you do indeed have an older unit that uses a HVAC t'stat but not standard color identification on the wires. The "C" wire that Honeywell refers to is 24vac to power the stat. I assume per your photo is the Red wire connected to the RC and RH terminals, take a meter and confirm what the voltage being controlled is at the stat base.
Hopefully I have successfully attached the install manual, wiring is about page 14
 

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  • M9INSTALL.pdf
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According to that manual, that thermostat will work. It says the thermostat wiring is the standard 24VAC used in home systems. Your problem, just like in home systems, is that older thermostats were just wired as a switch, they didn't have dedicated wires for power.

The solution is to find where the "R" wire connects to the 24VAC transformer, and run a wire to "C" from the opposite terminal off that transformer. You might try calling Mermaid, I suspect they've heard this question many times before and probably have an answer ready. If not, a few minutes taking off covers and chasing wires should get you there. Post pics from inside that box with the RGWB terminals if you need some help.
 
I am no expert and do this at your own risk. The good side is that both home units and yours run on 24v A.C. looking at page 41.

I believe R would be your common or known as your C wire. But you will need to bring a wire up from the other side of the transformer to power the Tstat. But I am thinking there are other controls on the boat's tstat controller.
 
CaptTom said:
According to that manual, that thermostat will work. It says the thermostat wiring is the standard 24VAC used in home systems. Your problem, just like in home systems, is that older thermostats were just wired as a switch, they didn't have dedicated wires for power.

The solution is to find where the "R" wire connects to the 24VAC transformer, and run a wire to "C" from the opposite terminal off that transformer. You might try calling Mermaid, I suspect they've heard this question many times before and probably have an answer ready. If not, a few minutes taking off covers and chasing wires should get you there. Post pics from inside that box with the RGWB terminals if you need some help.
Click to expand...
The thermostat is powered by two AA batteries. if the purpose is a dedicated wire for power, why not use a jumper wire to connect C and RH on the thermostat terminals?
 
Yes, to the above comments. You will need to bring up a wire from the other side of the 24vac transformer to the stat base. This provides stat power independent of the switching functions which is what the Honeywell needs.
 
The batteries only power memory and clock. The older stat only provided switching functions.
 
Turtle Blues said:
Yes, to the above comments. You will need to bring up a wire from the other side of the 24vac transformer to the stat base. This provides stat power independent of the switching functions which is what the Honeywell needs.
Click to expand...

​

Got it.
If all it requires is constant 24vac, could the power also be sourced from an outlet using this adapter?
 
Don't know the internal pathways on the Honeywell board well enough to answer.
See a lot of multiple transformer control systems but have also seen boards smoke when plugged in on startup.
 
CaptTom said:
According to that manual, that thermostat will work. It says the thermostat wiring is the standard 24VAC used in home systems. Your problem, just like in home systems, is that older thermostats were just wired as a switch, they didn't have dedicated wires for power.

The solution is to find where the "R" wire connects to the 24VAC transformer, and run a wire to "C" from the opposite terminal off that transformer. You might try calling Mermaid, I suspect they've heard this question many times before and probably have an answer ready. If not, a few minutes taking off covers and chasing wires should get you there. Post pics from inside that box with the RGWB terminals if you need some help.
Click to expand...
I opened the box and snapped some more photos

Barrier terminal - RGWB
IMG20240627222741.jpg


Box inside overview 1
IMG20240627222905.jpg


A closer look of inside
IMG20240627223939.jpg


The lower red wire on terminal position "R" is traced to the one labeled with a green arrow
IMG_20240627_223835.jpg


Where should the "C" wired connect to?
 
The transformer is in the lower right corner of the last 2 photos. Should have 4 wires, the 120 supply and the 24v output. The 2nd wire of the 24 is what you are looking for. Is the wiring diagram on the inside of the cover?

Page 42 of the manual has a schematic. You are looking for the wire that goes from the transformer to the relays. That is what you want to attach to.
 
Last edited:
What Turtle said.

Assuming the wire from the transformer goes to a spade terminal on the relay, you'll want one of these to allow a second connection for your "C" wire:
31sq61ZJe-L._AC_.jpg
 
Turtle Blues said:
The transformer is in the lower right corner of the last 2 photos. Should have 4 wires, the 120 supply and the 24v output. The 2nd wire of the 24 is what you are looking for. Is the wiring diagram on the inside of the cover?

Page 42 of the manual has a schematic. You are looking for the wire that goes from the transformer to the relays. That is what you want to attach to.
Click to expand...
Thanks for the pdf manual.
I checked page 44 but cannot map the diagram to the physical wiring.


There are four wires on the left side of the transformer - black, white, red, orange. The red and orange wires are both capped in the end, the black and white wires connect to the top right box


IMG_20240628_132420.jpg

IMG_20240628_132938.jpg


Two wires on the right side of the transformer - green and red
IMG20240628131421.jpg

The green wire also goes to the top right box while the red goes to a lower box on the left. Is the red or green the second 24v wire?
IMG_20240628_133405.jpg
 

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    IMG20240628131238.jpg
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Yes the line drawing is only that. It may be helpful to write your wire colors on it to make it more comprehensible to you.
Reading the label on the top of the transformer, it has multiple voltage capability. Common White and 120v Black. 208V Red and 240V Orange being the unused and capped connections. 24vac output being the Red and Green, the two wires you are looking for to make your circuit. Currently one of those wires is common to your relays and the other runs to the stat for switching. It is the wire common to the relays that must be brought to the stat base to provide the power circuit for the stat
 
Turtle Blues said:
Yes the line drawing is only that. It may be helpful to write your wire colors on it to make it more comprehensible to you.
Reading the label on the top of the transformer, it has multiple voltage capability. Common White and 120v Black. 208V Red and 240V Orange being the unused and capped connections. 24vac output being the Red and Green, the two wires you are looking for to make your circuit. Currently one of those wires is common to your relays and the other runs to the stat for switching. It is the wire common to the relays that must be brought to the stat base to provide the power circuit for the stat
Click to expand...

I cannot tell which one is common to the relay.

I noted the red wire, as arrowed in the pic below, exits the transformer and connects to the bottom terminal of a box. There are four terminals on this box where the two in the middle are jumped with a piece of white wire. The top terminal connects a red wire that goes to the outside R terminal that is wired to the thermostat base. What's the function of this box?

IMG_20240628_133405.jpg
 
Following the wiring diagram on page 42 the red wire is the left side of the transformer to the R terminal at the Tstat base . The manufacturer might not have followed his own diagram in that it is looped to a relay before it gets there. The diagram shows the right side wire of the transformer going to the relays and then the terminal block on the Tstat base. In your photo this is the green wire which appears to immediately land on a power relay above it. Whatever the deviations are these are the 2 24vac wires you need for the stat power circuit
 
Turtle Blues said:
Following the wiring diagram on page 42 the red wire is the left side of the transformer to the R terminal at the Tstat base . The manufacturer might not have followed his own diagram in that it is looped to a relay before it gets there. The diagram shows the right side wire of the transformer going to the relays and then the terminal block on the Tstat base. In your photo this is the green wire which appears to immediately land on a power relay above it. Whatever the deviations are these are the 2 24vac wires you need for the stat power circuit
Click to expand...

so the green wire is common to the relay, and the red wire runs to the stat for switching?

honeywell customer service told me to use the c wire adapter thp9045a to save the effort of running a long wire to the thermostat. so I would fork the above green wire to the C terminal on the lower barrier of thp9045a?

1719842700287.png
 
Hmm, I can't say yes to your question, looking at your photo neither wire goes to the stat base uninterrupted, but that doesn't matter. Verify with a meter that the red wire is what eventually lands on the R terminal on the base.
Green landed on the stat base according to standard color coding would be the fan.
The module you have above is intended to save you from having to pull an additional wire.
It splits what is wired through it to derive a constant common which it identifies as K.
 
Turtle Blues said:
I need to correct my statement. Looking at the additional picture you attached you do indeed have an older unit that uses a HVAC t'stat but not standard color identification on the wires. The "C" wire that Honeywell refers to is 24vac to power the stat. I assume per your photo is the Red wire connected to the RC and RH terminals, take a meter and confirm what the voltage being controlled is at the stat base.
Hopefully I have successfully attached the install manual, wiring is about page 14
Click to expand...
"...the Red wire connected to the RC and RH terminals, take a meter and confirm what the voltage being controlled is at the stat base"

How to do this test? The volt meter has two probes, where should the probes touch?
 
This step is overcome by your later photo confirming a 24vac transformer. Some older stats controlled line voltage and I wanted to check against that.
In general I have some long jumper wires made up to facilitate testing. Just clips and connectors on a section of wire, nothing fancy. My first checks would be continuity (without power) so I know what the wires are back at the control box if there is any question.
On your stat base: To test for voltage connect 1 probe to the R terminal then (if necessary due to distance use a jumper wire) go to the green wire off the transformer with the other probe. That should be the continuous 24vac you have been searching for.
Just as an explanation, if you jump the R terminal to the G on the stat base it should energize the blower relay turning on the fan.
Using the Honeywell splitter will probably make a lot of this unnecessary so I apologize if its getting ponderous.
 
Turtle Blues said:
This step is overcome by your later photo confirming a 24vac transformer. Some older stats controlled line voltage and I wanted to check against that.
In general I have some long jumper wires made up to facilitate testing. Just clips and connectors on a section of wire, nothing fancy. My first checks would be continuity (without power) so I know what the wires are back at the control box if there is any question.
On your stat base: To test for voltage connect 1 probe to the R terminal then (if necessary due to distance use a jumper wire) go to the green wire off the transformer with the other probe. That should be the continuous 24vac you have been searching for.
Just as an explanation, if you jump the R terminal to the G on the stat base it should energize the blower relay turning on the fan.
Using the Honeywell splitter will probably make a lot of this unnecessary so I apologize if its getting ponderous.
Click to expand...
I did the continuity test. It shows the red wire from the transformer is wired directly to the thermostat.
The two connectors, as marked in green circles below, are 3 way quick disconnectors.

IMG_20240701_215523.jpg


However the volt reading b/t the red and green is 0. I tapped one probe to the R terminal outside of the control box, the other probe to the green wire. For easy access I detached the green wire from the relay thing for this test. Before detach, the volt is also zero. The breaker for the reverse cycle was on.

IMG_20240701_215839.jpg
 
Ok, next step is to switch to 120vac testing. Follow incoming power supply verifying voltage toward the transformer. You are trying to determine if the transformer has power or if not identify the relay that is holding it open.
If it has 120 on the high side and no 24v it is bad but if it was working previously I suspect there is a relay open.
 
Turtle Blues said:
Ok, next step is to switch to 120vac testing. Follow incoming power supply verifying voltage toward the transformer. You are trying to determine if the transformer has power or if not identify the relay that is holding it open.
If it has 120 on the high side and no 24v it is bad but if it was working previously I suspect there is a relay open.
Click to expand...

I just tested, the reverse cycle unit is working as expected.

With the thermostat off, there is 120v ac b/t the two screws marked in blue circles. I can see one black and white wire each connects directly from one screw to the transformer. So there is not a relay in the way.
The dc volt b/t the green and red wires from the transformer is always 0.

IMG_20240702_212216.jpg
 
So you verified 120vac in and nothing out. This means the transformer needs to be replaced. The label on the top should give a size rating. You don't need the multi voltage capability of the existing one
 
Turtle Blues said:
So you verified 120vac in and nothing out. This means the transformer needs to be replaced. The label on the top should give a size rating. You don't need the multi voltage capability of the existing one
Click to expand...
if the transformer is bad, why is the air conditioner working fine?
 
Thats a conundrum.
It's 24v powered relay logic. The relays have a diagram on top. I'm assuming powered closed.
So if closed they are getting 24v.
Verify your meter settings and retest.
 
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