Fan Coil Unit with Three Wires Connected to Two Terminals

[apisdorf]

Hi. I am trying to use a Nest 3 thermostat with a 2-pipe fan coil unit. The old thermostat had three wires connected to it; red to Rc which was bridged to R, green to Y, and yellow to W. I was informed that since the fan coil system was essentially just a fan from the thermostat's perspective, the green and yellow wires were both connected to the same terminal at the fan coil unit. I have verified that the 24v side of the fan coil only has two terminals (one currently connected to the red wire and one connected to both the green and yellow wires, I believe).

The old thermostat was able to be set to either heat or cool and would be able to turn on the fan as appropriate. 

When I connected the Nest, I connected the red to Rc, the green to Y1, and the yellow to W1. When in cool mode, the unit functions fine, turning on the fan when the temp rises above the temp set on the unit. When in heat mode, however, the fan is cycling on and off rapidly. 

if I disconnect either the green or yellow wire and only use the other in W1, I can use the heat mode and when I connect only the green or yellow wire to Y1, I can use the cool mode. 

What do I have to do to be able to use either heat or cool without having to move a wire at least twice a year? 

[apisdorf]

I wanted to add that the power readings are as follows:

Battery: 3.862 V

Voc: 33.69 V

Vin: 31.36 V

lin: 20 mA (i)

[apisdorf]

Anybody have any thoughts?

[maedacortez]

Hi apisdorf,

Thanks for reaching out. I understand that you need to disconnect one wire to heating or cooling work properly . I'm happy to help!

It's possible your thermostat isn't detecting all the wires. Here's how to check:

1. Go to Settings > Equipment.
2. You should see the wires connected to your thermostat. Verify if it detects all three wires.

If the Nest thermostat does not detect all three wires I recommend you update your wire configuration Follow the Next steps.

Nest app

1. Open the Nest app tap Settings .
2. Tap Thermostat Wiring Update wiring.
3. Update any incorrect wiring information.

Let me know if you have any questions.

Regards.

Manuel

[apisdorf]

Hi. Thanks for responding. When I connect the third wire, the Nest shows it as connected. 

[maedacortez]

Hi apisdorf,

Thanks for replying! Just to confirm, is your thermostat functioning normally now?

If you're still having trouble, however, I'd be happy to help further. To diagnose the wiring, could you send me a clear picture of the wires connected to your thermostat base? This will help me check for any loose or incorrect connections that might be causing the problem.

Regards,

Manuel 

[apisdorf]

The wiring configurations in the first four pictures (with either the green or yellow wires connected to either Y1 or W1) all work -- the Y1 for cool only and the W1 for heat only).  The last two, with all three wires connected, only works on cool. On heat, in either of the last two configurations, the fan cycles on and off every few seconds  

 

 

 

 

 

 

[maedacortez]

Hi apisdorf,

Thanks for replying! Based on the picture of your wiring, everything seems connected correctly.

To help me diagnose the issue further, I have a few questions:

1. Have you tried moving the RH wire and connecting it to the RC terminal? 
2. Is your HVAC system a dual-fuel system  or a single-fuel system?
3. Do you recall selecting the correct system type during the initial setup of your Nest thermostat?

I will waiting for your response.

Regards,

Manuel

[apisdorf]

I just tried moving the red wire to RC. The exact same behavior occurred -- works with either the yellow or green wire connected to either Y1 or W1 for only cool or only heat but with both yellow and green connected, cool works but heat cycles on and off. 

The system is a two-pipe fan coil unit. It will only send either cold air or warm air depending on the time of year. I performed the same tests while heat was being sent up. The results were the same. 

The choices regarding the type of system did not include a fan coil system. I have tested both gas and electric and then both forced air and radiator. The behavior does not change. 

[apisdorf]

I wanted to post a follow-up. I bought a 24v transformer in order to test if the issue was purely power-related. I connected the wires of the transformer to the C connector and to the Rh connector which also has the red wire (that part took some experimenting and watching several videos).  I also reconnected both the green and yellow wires  to the W1 and Y1 connectors. The thermostat now can control both heat and cool without any short-cycling. 

My next step was to connect the transformer wires at the fan coil rather than at the thermostat since I did not want to have to have wires hanging out of the thermostat. There was an unused fourth (black) wire between the fan coil and the thermostat so I connected one wire from the transformer to the black wire and the other to the red wire. That worked the same as what worked at the thermostat. 

I am not sure this is the best approach so I am open to any ideas you might have. 

[maedacortez]

Hi Apisdorf,

Thanks for reaching out and for the additional information. Based on what you've described, it seems your HVAC system isn't providing enough power to operate when all the wires are connected. This explains why it works when you remove one.

In this case, I recommend contacting a technician to install a C wire on your system. This will provide the necessary power for all the wires to function properly with your Nest thermostat. If you do not have your personal tech you can contact a nest pro to install the C wire you can tap on the link to contact them.

For the US: https://goo.gle/2VIWGw5

For Canada: https://goo.gle/30Z70nv

Regards,

Manuel

[apisdorf]

Manuel,

Thank you for the information. I will do that if I find any issues with my current approach (mentioned above).

[maedacortez]

Hi apisdorf,

Sounds good! Here's a link to learn more about the C wire and the common issues it can fix when installed in your Nest thermostat. Please visit Learn about the common or c wire.

I hope this helps!

Regards, 

Manuel

[apisdorf]

Manuel,

Thank you. 

[Patrick_Caezza]

@apisdorf 

A 2-pipe fan coil unit does not change from cool to heat. it just turns on a valve that allows cool or hot water to enter the coils. The water source is what changes the temperature of the water in the system.

Do you have a picture of the original thermostat's wiring?

What is the make and model number of the 2-pipe fan cool unit?

Have you looked to see how the thermostat wires are connected in the fan coil unit?

[apisdorf]

Patrick,

Understood re: how the two-pipe fan coil works. I did not mean to imply that I was trying to be able to get either heat or cool at the same time of year from the unit; just that I wanted to set the thermostat to either heat or cool without having to move a wire from W1 to Y1 a couple times each year (when the building switches from heat to cool).

The Honeywell control board on the fan coil has only two terminals on it. One terminal has both the green and yellow wires attached to it and the other terminal had only the red wire attached to it. The new change I mentioned above was to add a 24v transformer with one wire also attached to the red wire terminal and the other wire connected to an unused black wire (there are four wires going from the fan coil to the thermostat -- the black wire was unused before) that is now attached to the C connector on the back of the Nest. 

Attached is a picture of the original thermostat's wiring.