120v Honeywell (7-8 wire) conversion to 24v NEST

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Old 03-12-20, 08:59 AM
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Question 120v Honeywell (7-8 wire) conversion to 24v NEST

Guys, I have something like this below, and it appears that it is a 120V system with 7 Wires, and not 8.



I used my multimeter and was able to confirm that
1)with L(hot) and N(Neutral) I get 120V
2)with L(hot) and W/Y(HEAT) I get 0V
3)with L(hot) and Y/A(AC) I get 0V
4)with L(hot) and GL(FAN LOW) I get 120V
5)with L(hot) and GM(FAN Medium I get 120V
6)with L(hot) and GH(FAN High) I get 120V

What that means is that I can literally use L as a common instead of Blue wire as shown in the below image. Because there is no Blue common wire that I have in my circuit and the one I have is for Fan Medium which I don't think is the hot wire.


Please let me know. I have already bought the below products and already have Nest 3rd Gen Pro Thermostat.

MRB-8-24VAC RELAY: https://bravocontrols.com/shop/mecha...ng-and-4-gang/
Honeywell AT140A1000 40Va, 120V Transformer - 60 Hz. https://www.amazon.com/dp/B000K2EC7K..._c1KAEbD6WB66V



Separated from: 8-wire-120v-honeywell-conversion-24v-nest-learning.
 

Last edited by PJmax; 03-12-20 at 09:59 AM. Reason: started new thread
  #2  
Old 03-16-20, 08:20 PM
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Yup, that sounds about right and should work.

You may want to check if the blue wire might actually be there (left in the wall). I've seen a case where they upgraded the thermostat and wired the COM + L together (probably for convenience). But they left the extra Blue wire unconnected in the wall.

--Dan
 
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Old 11-04-20, 06:32 AM
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Just want to say thank you for posting how to do this. I searched for a long time before finding this post, an electrician gave me a $600 quote to do the same thing! I have the exact same setup as op HERE. I've done exactly what you both have done with the diagram, relay, and transformer. I only used the one fan speed though.

However, the fan by itself would not turn on unless the system was actively in heat/cool mode. I live in an efficient building so I rarely need to actually turn on the heating system and mostly just use the fan to circulate air. This was a problem. From my understanding, when the heat is activated, power is sent through the fan common wire so that the fan turns on. So I replaced the common on the rely with L1 for the fan as well. I have L1+Cool, L1+Heat, and L1+ Fan III. With this wiring, I was able to turn on the fan without having to activate heat or cooling.

I have noticed that when I turn the settings on the thermostat completely off, there is a tiny amount of heat seeping through the vent that is closest to the fan coil. Any thoughts on why this is? The only difference between my wiring and yours is that I replaced the common wire for the fan with a wire from L1.
 
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Old 11-04-20, 08:31 AM
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Line voltage thermostat

Hi, what model number is this Honeywell line voltage thermostat?

Geo 🇺🇸
 

Last edited by Shadeladie; 11-05-20 at 06:00 AM.
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Old 11-04-20, 10:04 AM
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The original is T6575D1009, which was switched to RTH6580WF using a transformer and relay.

Here is the original wiring....
 

Last edited by PJmax; 11-04-20 at 10:51 AM. Reason: added enhanced pic from link
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Old 11-04-20, 10:56 AM
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Welcome to the forums.

I have noticed that when I turn the settings on the thermostat completely off, there is a tiny amount of heat seeping through the vent that is closest to the fan coil.
Check the fan coil. If it's hot.... you're probably getting gravity flow.

 
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Old 11-04-20, 12:27 PM
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As in this has nothing to do with my wiring, thermostat, or relay? I had not noticed this happening before I did the switch which is why I found it a bit odd.
 
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Old 11-04-20, 12:47 PM
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I don't know if your coil shut off before that stat change or not. There is usually a solenoid valve in the water supply line but not always. If you have a valve on the water line and the coil is staying hot.... the valve isn't closing.
 
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Old 11-04-20, 12:58 PM
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Okay. Is this valve something that would be controlled by the stat? I am trying to figure out if I caused this or if it just randomly happened.
 
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Old 11-04-20, 01:07 PM
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Your heat/orange wire would likely go to the valve.
 
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Old 11-04-20, 01:42 PM
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Gotcha. I will check the fan coil and see if there is a valve. Any thoughts on a fix other than just putting the old stat back on?
 
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Old 11-07-20, 02:52 PM
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Alright, I checked the fan coil. You were right, fan coil is warm. It is warm even with the power off at the breaker. There is a valve with a Honeywell actuator. I tested operation with the relay and new stat, the actuator pushes down when the heat is called for by the stat. When the stat turns off the heat, the actuator rises back up again. Doesn't seem to be anything wrong with the actuator. I took the actuator off and the fan coil was still warm. I noticed that when the system called for heat, it was noticeably warmer.

Do you think the valve has gotten stuck?
 
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Old 11-07-20, 04:15 PM
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I did some more troubleshooting, the only way I get the fan coils to turn off is if I have the thermostat calls for cool with the fan on. I am on a 2-pipe system.
 
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Old 11-20-20, 07:33 AM
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Good day all
Orangelime did your manage to connect your Honeywell RTH6580WF to be able to control the 3 speeds?
if so what terminal did you use for G2 and G3?
I have a 2 pipe system with 120V and I am ready to do the transfer but don't really like to buy the Nest and cant get the right info from honeywell if I can reconfigure the terminals.
Thanks
Much appreciated.
 
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Old 11-21-20, 03:47 PM
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Welcome to the forums.

That Honeywell stat will not control multiple speeds. That would need to be addressed separately.
 
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Old 11-24-20, 06:22 PM
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Thanks for the quick reply PJmax
I am in the process of changing the 120V T-stat to 24v Nest for 3 speed fan coil.
there is 4 wires coming from the JB inside the unit to the T-stat
White - Line
Black - High speed fan
Blue - Med speed fan
Red - Low speed fan

from the JB the neutral goes straight to the motor.
however I don't quite understand how the Nest will switch between cool and heat as there is no W or Y wires currently.
Also given the pictures and the info provided am I ok to do the conversion with:
1. 20VA Transformer
2. 3 x 24V plug in relays rated at 5Amps for High (3.7A inrush 2.6A running), Med.(2.75A/2.1), Low fan speed (2.5/1.7A). and
3. Nest Learning thermostat.
I want to install them in a Pvc box inside the fan coil unit in case there is any moisture ever - pretty dry now.
should I keep the transformer out of the box to prevent overheating?
much appreciated!




 
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Old 11-24-20, 07:20 PM
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I forgot to mention the system I have is 2 pipe where there is no actuators or control valves
the water is flowing constantly and is controlled by the condo corp. for the change over, hot in winter and cold in summer.
 
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Old 11-30-20, 09:33 AM
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cityel, sounds like you have no need for Heat/Cool wires since your condo controls it. You only have control over the fan speeds. I know that Nest uses the W / Y lines for control *AND* for power. So, you'll need to connect 24V power to the W terminal, even if it doesn't actually do anything. I don't have any experience with your kind of setup, so hopefully someone else can help you with any follow-up questions.
 
 

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