Hot Topics: Converting a Thermostat to Nest

hand tinkering with heating panel wiring

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Original Post: 8-wire 120v Honeywell conversion to 24v NEST Learning

vandacca - Member

I'm trying to replace a 8-wire 120v Honeywell Thermostat with a 24v Nest Learning (3rd Edition) thermostat. I understand that I'll likely need 2 RC840T Transformer/Relays (one for heat, one for cool) to step-down the voltage. Or can I get by with 2 RC840 Transformer/Relays? Please correct me if this can be done with only one transformer/relay or if there is a better dual-transformer available.

Otherwise, I have the following 8 wires on my existing Honeywell Thermostat:

- L1 (Hot): Black wire

- L2 (N): White wire

- Heat: Orange wire

- Cool: Yellow wire


- Com: Purple wire

- Fan I: Red wire

- Fan II: Blue wire

- Fan III: Brown wire

an open thermostat with wiring

I assume that L1 and Heat are wired to one RC840T (eg. RC840T-h) and L1 and Cool are wired to other RC840T-c? Is that correct?

I'm also guessing that below is the correct wiring:

RC840T-h output R -> Nest RH

RC840T-h output W -> Nest W1

RC840T-c output R -> Nest RC

RC840T-c output W -> Nest Y1

FAN-III -> Nest G (Is there a better way to utilize all 3 fan speeds?)

L2(N) -> Nest C??

Can someone review and let me know what the correct wiring is and if it's possible to utilize all three fan speeds with the Nest thermostat? For example, do I need an additional fan controller module?


Astuff - Member

It is a bit more involved. You actually would need five relays. One for heat, one for cooling, and one for each fan speed.

Only need one transformer to drive it all, though.

vandacca - Thread Starter

Thanks @Astuff. Any advice how to wire it all up? I assume I can cut a corner and only use one or two fan speeds? Wouldn't I need some kind of controller to translate one fan signal into three?

Astuff - Member

Yes, you skip fan speeds but shouldn't need to.

Use a multi relay board like and a 24vac transformer (verify 120 or 208/240 volt supply). You need to see what you can physically fit.

Also may have to adapt the wires as Nest only accepts small solid wires in its connectors.

vandacca - Thread Starter

Thanks again @Astuff. My system is definitely 120V. I'm not concerned with space, as I can always add a second box and plaster around it.

A few follow-up questions. I assume that my "Com" purple wire is a common ground for the three fan speeds. Is it also used as a common for the other non-fan wires too?

The big thing I don't understand is how to wire the Nest Thermostat (with a single G terminal) to three Fan relay switches on that multi-relay board. For example, I assume that the "Com" line will be used as one of the two connectors across 3 Relay switches (eg. K1, K2, K3). I also assume that the existing wires I, II, III would finish the pair to K1, K2, K3.

It's the input that perplexes me. I assume I connect the G terminal to the 1, 2, or 3 input of the multi-relay board. I don't know how Nest handles/controls multiple fans.

I still not quite understand the other wires too, but let's just start with the fans for now.

Astuff - Member

The "Com" purple is most likely not a ground but 120v power source. "Com" here means common relay terminal. You should be able to use a multi-meter to check.

Nest uses additional fan speeds by re-using existing terminals so you can drive three relays. G feeds the 1st speed, Y2 feeds the 2nd speed, and * feeds the 3rd speed. In the setup you configure each connection for fan. See page 17 of the Nest Pro instructions.

vandacca - Thread Starter

Thanks @Astuff. That document was a great source of info. Now all I need is a Nest Pro ID to configure the thermostat. I wonder if Nest Support will provide me one...

I'll have to test the existing wires this weekend to verify voltages/connections and come up with a wiring diagram/plan.

Is this everything I'll need to complete this project?:

- 120v to 24v Transformer

- Relay Board

- 5 x 24V SPST Relay Switch

Can you provide an example of a suitable relay switch for this type of application?

Astuff - Member

That relay board comes with either four or eight relays already soldered to the board. So the only other thing you need is the transformer, wire, and mounting hardware. That's what makes the relay board so appealing. Only issue is that the connectors aren't big enough for stuffing multiple large wires in.

Actually the Pro ID is not needed to configure the Nest. It's more for support and warranty.

PJmax - Group Moderator

I see how you're wiring and configuring for the three speed fan situation but how do you select a speed? It looks like the speed of the fan will only change based on temperature.

Astuff - Member

Nest is supposed to control fan speed automatically like stage 1, 2, and 3 of heating and cooling. Also when you manually turn on the fan it asks what speed. Hopefully these links work: and

PJmax - Group Moderator

I love nest's links.... they go around in circles. The reason I was checking on the fan speed issue is because you don't want more than one speed activated at a time.

Astuff - Member

Good point. Bad to assume with Nest that they follow a standard. I will run a test with fan speeds to check if they are cumulative or not. Some fan coil controls might be OK with it but better safe than sorry. So if Nest doesn't isolate then need different relays. I remember that Carrier made a fan coil relay board just for this issue. (found it: 33ZCRLYBRD)

Did a test with Nest. Y2 configured as G2 and * configured as G3. When selecting fan speed to run it properly only powered one at a time. When on low only G1 was hot. Medium only G2 was hot and high only G3.

With that Nest is should work correctly driving simple relays for the OP.

vandacca - Thread Starter

@Astuff, does your last post mean that I may also need a 33ZCRLYBRD board as well? Or if I go with the eight relay board, I should be fine? Anyway, I totally understand the physics of electricity, it's just the implementation of furnaces I don't understand, so I have a few questions.

Firstly, I assume that all HVAC wiring (both 24v and 120v) is AC, correct?

Secondly, I verified that L1:L2 is ~120VAC and COM:Fan is ~37VAC. Does that look ok?

Thirdly, can you review my wiring and verify that I understand the workings and got it right.

thermostat notes

I can't express enough my gratitude for your assistance.

Astuff - Member

No, you don't need a 33ZCRLYBRD as Nest properly sequences the fan speeds.

Yes, normally AC. Fan voltage you measured is strange but is still a switched connection. FAN I,II,III may go to electronic circuit so get that type of reading. Would be curious to see what COM:L1 and COM:L2 measure

Schematic looks good.

vandacca - Thread Starter

I measured COM:L1 yesterday just out of curiosity and I got ~72VAC. It didn't make any sense to me, so I just ignored it. Only theory I could come up with is that maybe they split power three or four ways, similar how they have phased power into most homes.

Astuff - Member

The FAN wires might also go direct to the motor windings. That would function as a transformer so can see odd voltages similar to phases. Interesting to find out but hopefully doesn't matter for your application.

That said, the relay contacts on the MRB are rated for 12 amps which is good for most control applications. Relays used for direct motor control are usually higher - 20 or 30 amp rated. How good are your electronics skills and tools? Can you measure the current going through the FAN Com wire? Most standard multi-meters are fused at 10 amps so need to use a clamp style meter. Also could check rating of old thermostat. Don't expect that it was rated more than a few amps of switching.

vandacca - Thread Starter

I can definitely check the amperage. My MM is rated to 20A (unfused), so hopefully that's high enough. My brother has a 10A (fused) MM. If I blow my MM, then definitely won't be able to use that MRB.

To test, do I simply need to somehow wire the MM in series, reconnect the thermostat and then turn on the fan. Is there an easier way? I'm guessing I'll need to add a 2nd wire to the COM terminal and somehow expose both the COM wire and my 2nd wire outside the thermostat. Then I can connect the MM, turn on the fan and measure the current.

Astuff - Member

Put the MM at the highest AC amp range then you can just use the MM's leads and touch the FAN Com and the I terminals. This should trigger the fan at low speed. Then try Com to II and III.

vandacca - Thread Starter

That's genius. I wish I had thought of that.

Unfortunately, I don't think it worked. I don't here the fan coming on and it's only registering 13uA. That seems much too low to power a fan.

Is it possible that the heat or cool need to be activated for the fan to kick in?

Astuff - Member

Possible. Could also explain the voltages you were reading on the fan terminals. It might have been phantom voltage. The fan Com may float until a heat or cool call so running in parallel with the hot picks up some voltage. Use an analog meter to re-measure. Don't notice it very often with control circuitry as most wires have a relay coil loading them. Low voltage circuits can have a resistor to ground as well.

vandacca - Thread Starter

Interesting. How would you recommend I test this further before installing equipment? If I turn on the heating/cooling, then remove the thermostat unit, will the heating/cooling immediately turn off? That is, does the thermostat send a message to the HVAC to turn on/off or does HVAC stay on only while the signal is maintained?

If the thermostat needs to maintain the signal for the HVAC to run, can I simply expose a couple of wires to measure voltage/current while the thermostat is connected and the system is running?

Astuff - Member

With the current thermostat can you turn the fan from auto to on without turning on heat or cooling?

The thermostat is a "analog" with relays so everything stops with the face is removed. To test you can place a jumper from L1 to heat. See if the voltage on Fan Com then goes to 120v.

vandacca - Thread Starter

Thanks @Astuff. I'll give that a try on Saturday. If all looks okay (current within acceptable levels), I'll go ahead and install the transformer/MRB/Nest.

vandacca - Thread Starter

So, it turns out that the Heat/Cool needs to be on for there to be power on the COM:FAN lines. Once the heat/cool was switched on, the COM:FAN lines had ~117V / ~1.23A applied. Well below the 12A rating of the relays, so all good here.

How does one calculate the power requirement for the transformer? I’m sure my transformer at 40Watts (very large and beefy) is more than enough, but should I be concerned it’s not sufficient? Do I need a ~150Watt transformer? I can’t imagine the size of such a transformer. Should I be measuring the amps on L1:L2 to calculate the transformer power requirements? I assume it’s also ~1.23A.

At the moment, I’m mounting everything to a wood board so that I can install it inside the wall. Last thing I need is to source is a 6”x10” access door.

Thanks again.

vandacca - Thread Starter

Here is the final schematic. Works great.

thermostat diagram

Stevce - Member

Hey Dan! Does this still work?

I want to hookup a 24VAC Venstar ColorTouch at my parents condo with a 120V Honeywell T6069B POS.

Venstar Tech Support says the conversion can’t be done but your solution looks like it works.

Your schematic is very clear (Great Work!) on the relay board but what 120V to 24VAC transformer/power convertor did you use?

Do you have a final photo of your setup you can post?

PJmax - Group Moderator

At the top of the diagram is shown a 24vAC transformer. That runs the stat and the relays.

Stevce - Member

Thanks. Just wanted to be sure.

So just need the 24VAC Transformer from Amazon and the 8 relay board to be able to use my 24VAC Venstar on my 120V condo HVAC system replacing my old Honeywell 120V fan coil thermostat?

Just want to be sure since I’m converting line voltage to low voltage and don’t want to burn anything down or out.

vandacca - Thread Starter

Hi Stevce,

Yes, this set up still works. The actual transformer I used was this one from Amazon:

Honeywell AT140A1000 40Va, 120V Transformer - 60 Hz.

It may have been overkill and you might be able to get away with a smaller transformer.

Just make sure you don't wire anything backwards and you should be fine.

thermostat wiring with light

Stevce - Member

Awesome! Thanks Dan!

I just ordered that transformer and the 8 relay board. Only difference is my T6069B thermostat is a 6 wire. No blue fan common wire like yours & no white neutral wire either.

Any thought how your schematic would change without that blue fan Common wire? Does that alter the schematic or I just omit that jump?

Mine is as follows:

Brown = High Fan

Blue = Med Fan

Red = Low Fan

Yellow = Cool Valve

Orange = Heat Valve

Black = L1/Hot


(I can’t upload an image from my iPhone it seems)

shimps33 - Member

HI, I have the same situation as Stevce, I have a honeywell T4039 line volt in my condo and its a six wire setup too. Different colors.


Blue-Fan Low

Blk-Fan Med

Yellow-Fan High



I have followed this whole thread and understand what needs to be done but I have Nest E and it only has these inputs:

Its has a Y1,G,R on the left side and W1,C,*OB on the right. If anyone here by any chance understands the wiring for that, it would really help me. Thank you for your time.

Stevce - Member

Hey Shimps33, the colors don’t matter.

Each wire regardless of color in both our setups address the same 6 functions so the solution would be the same. ????????

Here is my photo:

thermostat with wiring and details

Stevce - Member

...I mean ‘our’ solution for shimps33 & I would be the same if vandacca and others can help advise on a 6 wire schematic for us. Please. ????????

Question: Since we only have six wires could we get away with a 4 relay board?

relay board with inputs and outputs

PJmax - Group Moderator

You only need as many relays as you have functions. The Nest E will not give you three speed fan control. You need heat, cool, fan. Three mandatory relays.

Four relays would give you the possibility of two fan speeds.

vandacca - Thread Starter

Hi Stevce,

I'm not a HVAC guy or electrician. I have a degree in Physics, so I understand theory well. So, I spent time completely understanding the circuit in my situation before proceeding. I did a lot of voltage/current tests with the original system to see how the current was flowing to understand how to wire the relays. I then tested the theory by manually controlling the furnace. I simply connected the Hot+Neutral+Com wires directly to the Hot/Cold+Fan wires and when the system came on as expected, I knew I had a good grasp of how it worked.

I don't understand how the electricity flows with your 6-wire system, specifically the return flow. Test it or get some advice from others (in this forum or an HVAC electrician). But not having that COMMON (blue) and NEUTRAL (white) wires will definitely change how things are wired, you'll need to understand your setup first.

I just started with 8 wires coming out of my wall that were wired into my thermostat. My guess is that 3 of those wires (HOT, NEUTRAL and COMMON) are coming from my fuse panel and the remaining wires are going to my HVAC system. You'll need to figure out your situation (or get an experienced HVAC person to give you advice on how these things are normally wired). Perhaps you're missing power?

In my schematic, the wires on the right are coming out of my wall and were connected to my original thermostat. The wires on the left are coming out of my circuit and connecting to my new thermostat. Maybe there are 2 or 3 additional wires from your fuse panel required to make this work (just guessing here)? Sorry I couldn't be more helpful, but I started just like you with no knowledge of how this stuff works.

With regards to the smaller relay board, PJmax is right. I was considering dropping the ability to control 1 fan speed (eg. medium) so that I could use the much smaller board, but in the end I went big. But that smaller board is a really nice size. If I had to do it over again, I'm not sure if keeping 3 fan speeds is worth it. Otherwise, you have the same functions (5) as I do, so if you wanted to control all 5 (Heat/Cool/Fan1/Fan2/Fan3) you need the 8-relay board because this manufacturer doesn't make a 5-relay board, which would have been ideal. Each relay is just a controllable on/off switch.

If you are wiring a Nest E that only has the ability to control 1 or 2 fan speeds, then definitely get the 4-relay board.

BTW, when screwing down the relay board terminals onto your wires, take special care and make sure it's tight. Those relay board screw terminals don't give a lot of feedback so I found it hard to tell if the connection was good and it was tight enough. Pull hard on the wires to make sure.

Stevce - Member

Thanks Dan for the detailed reply! Much appreciated!

While I don't have a Physics degree I am tech savvy having worked with electric and electronics as a semi-advanced amateur my whole life having built circuit boards in my teens and wiring my basement fully to the box. Just a tad rusty but like riding a bike on the theory once I get going.

I will take my trusty multimeter to the wiring to see how the volts flow and I'll go with the smaller 4-relay board since I can live with 2C/2H (med+high) on my Venstar Colortouch T7850. The main thing I really needed was a way to convert 120V line flow to 24VAC to not burn out anything and looks like there are many out there that have taken this transformer/relay route with great success.

It's not for the first timer for sure without lots of measure twice (or ten) cut once theory.

I'll need to find the white/neutral wire source to complete the circuit. I would be shocked if the L1/Black wire is Common but my multimeter will confirm.

Venstar T7850 (not what I have installed yet - for illustration purposes only)

I have 6 wires coming out of wall shown in post #33 that I want to connect to this Venstar backplate.

hand working on a thermostat

Here is my Honeywell T6069B/C schematic: (left side...need to figure which one I have)

working on a thermostat

vandacca - Thread Starter

Hi Stevce,

FroM the image you provided, it looks like you have 8 wires coming out of your wall! This is probably sufficient to work with the schematic I provided. I couldn't read the labels under the wires, but I suspect that the Black wire is equivalent to my L1 and the white wire is equivalent to my L2 or N. And my guess is that the green wire is COM.

In my situation, I only used the wires from the wall/base-plate. The wires connected to your thermostat may be of different colours to the wires coming out of your walls, and so I recommend focusing only the wires/colours coming from the wall, otherwise it can confuse things during communication.

Astuff posted some useful tips in this thread. Set your Multi-Meter (MM) to measure current at the highest AC amp range then touch the MM's leads to the FAN COM (green) and the FAN-LOW terminal (which ever the blue thermostat wire connects to your base plate). This should trigger the fan at low speed. Then try FAN-COM to FAN-MED and FAN-HIGH.

You could briefly do the same with HEAT and COOL, but if the fans aren't running, it's hard to tell if the heater/cooler is running and you probably don't want to heat/cool it too long without the fans running. You could hard-wire (or short) FAN-HIGH to FAN-COM to run the fan at high speed, and then use this MM method to test HEAT/COOL and feel for hot/cold air from the vents.

Only perform this test once you're fairly certain of the wiring. Good luck and read all of the posts from Astuff. He really helped me understand how all this stuff works.

Stevce - Member

Thanks Dan.

One clarification I should have stated is that Venstar backplate photo with the 8 wires above is a stock photo I posted from the internet for illustration purposes only. My bad.

The wires coming out of my wall to my T6069B/C tstat is in post #33.

The schematic from Honeywell in post #37 above shows how it is likely connected - one of the 3 diags on the left side that I need to figure out. Sorry for my miscommunication.

vandacca - Thread Starter

Well, in that case, you'll need to figure out what is going on with your implementation. Maybe there is additional power wires required, or the previous installer did something non-standard. Do you have a backplate in your installation like the stock photo? If so, it may help to see how that is wired. Good luck.