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Hot Topics: Convert a thermostat to the estate

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

vandacca Member

I'm trying to replace an 8- wire 120v Honeywell thermostat with a 24v Nest Learning (third edition) thermostat. I understand that I will probably need 2 RC840T Transformer / Relays (one for heat, one for cool) to turn off the voltage. Or can I get by with 2 RC840 Transformer / Relays? Correct me if this can be done with just one transformer / relay or if there is a better dual transformer available.

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


1; L1 (Hot): Black wire

– L2 (N): White wire

– Heating: 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 guess L1 and Heat are connected to an RC840T (eg RC840T-h) and L1 and Cool are connected to other RC840T-c? Is it correct?

I also guess that below are the right wires:

RC840T-h output R -> Nest RH

RC840T-h output W -> Nest W1

RC840T-c output R -> Nest RC [19659006] RC840T-c output W -> Nest Y1

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

L2 (N) -> Nest C ??

Can anyone review and let me know what are the correct wiring and if it is possible to use all three fan speeds with the Nest thermostat? For example, do I need an extra fan control module?

Thank you!

Astuff Member

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

However, just need a transformer to power everything.

vandacca Thread Starter

Thanks @Astuff. Any tips on how to connect everything? I guess I can cut a corner and only use one or two fan speeds? Wouldn't I need some type of controller to translate a fan signal to three?

Astuff Member

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

Use a multi relay card like https://bravocontrols.com/shop/mecha…ng-and-4-gang/ and a 24vac transformer (verify 120 or 208/240 volt supply) . You have to see what you can physically fit.

You may also need to customize the wiring because Nest only accepts small wires in its connections.

vandacca Thread Starter

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

Some follow-up questions. I guess my "Com" purple thread is a common ground for the three fan speeds. Is it also used as usual for other non-fan cables?

The big thing I don't understand is how to connect the Nest thermostat (with a single G-terminal) to three fan relay switches on the multi-relay board. For example, I assume that the line "Com" will be used as one of the two contacts over 3 relay switches (eg K1, K2, K3). I also assume that the existing leads I, II, III would end the pair to K1, K2, K3.

It is the entrance that confuses me. I guess I connect the G-terminal to the input 1, 2 or 3 of the multi-relay card. I don't know how Nest handles / controls multiple fans.

I still don't really understand the other cables, but let's just start with the fans at the moment.

Astuff Member [19659006] "Com" purple is probably not a ground but a 120V power source. "Com" here means a common relay terminal. You should be able to use a multimeter to control.

Nest uses additional fan speeds by re-using existing terminals so you can run three relays. G feeds the first velocity, Y2 feeds the second velocity and * feeds the third velocity. In the installation, you configure each connection for the fan. See page 17 of the Nest Pro instructions.

vandacca Thread Starter

Thanks @Astuff. That document was a good source of information. Now I just need a Nest Pro ID to configure the thermostat. I wonder if Nest Support will give me a …

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

Is that all I will need to complete this project ?:

120v to 24v Transformer

Relay Board

– 5 x 24V SPST Relay Switch [19659006] Can you provide an example of a suitable relay switch for this type of application?

Astuff Member

This relay board comes with either four or eight relays that are already soldered to the board. So all you need is the transformer, the cable and the mounting hardware. This is what makes the relay board so appealing. The only problem is that the connections are not large enough to fill several large wires.

The Pro IDs are not actually needed to configure Nest. There is more to support and warranty.

PJmax Group Moderator

I see how to connect and configure for the three speed fan situation but how do you select a speed? It looks like the fan speed will only change based on temperature.

Astuff Member

Nest is intended to automatically control fan speed as steps 1, 2 and 3 of heating and cooling. When you manually turn on the fan it asks what speed. Hopefully these links work: https://nest.com/support/article/Lea…s#how-it-works and https://nest.com/support/article/ How … spring thermostat

PJmax Group Moderator

I love the nest links … they go around in circles. The reason I checked the issue of fan speed is that you do not want more than one speed activated at a time.

Astuff Member

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

Did a test with Nest. Y2 configured as G2 and * configured as G3. When you select the fan speed to run it correctly, only one is operated at a time. When low, only G1 was warm. Only medium G2 was just hot and high G3.

With that, Nest should work properly to run simple OP relays.

vandacca Thread Starter

@Astuff, does your latest post mean I might also need a 33ZCRLYBRD card? Or if I go with the eight relay board, would I be fine? Anyway, I completely understand the physics of electricity, it's only the implementation of ovens that I don't understand, so I have some questions.

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

Second, I confirmed that L1: L2 is ~ 120VAC and COM: Fan is ~ 37VAC. Does it look ok?

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

 thermostat notes

I can "I express my gratitude for your help.

Astuff Member

No, you need not a 33ZCRLYBRD since Nest properly sequences the fan speeds.

Yes, normal AC The fan voltage you measured is strange but is still a connected connection FAN I, II, III can go to electronic circuit so get that kind of reading. be curious to see what COM: L1 and COM: L2 measure

Schematic looks good.

vandacca Thread Starter

I measured COM: L1 yesterday just by curiosity and I got ~ 72VAC. It didn't make any sense to me, so I just ignored it. The only theory I could come up with is that they might share power three or four ways, similar to how they have phased power in most homes.

Astuff Member

FAN wiring rna can also go directly to the motor windings. It would work 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. How good are your electronics skills and tools? Can you measure the current flowing through the FAN Com cable? Most standard meters are fused with 10 amps, so you have to use a clamp meter. Can also check the classification of old thermostat. Don't expect it to have received more than a few amplifiers.

vandacca Thread Starter

I can definitely control the current. My MM is rated at 20A (unmelted), so hopefully it is high enough. My brother has a 10A (molten) MM. If I blow my MM, I will definitely not be able to use that MRB.

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

Astuff Member

Put MM on the highest AC amplifier, you can only use the MM wires and touch the FAN Com and I terminals. This should trigger the fan at low speed. Then try Com to II and III.

vandacca Thread Starter

It's genius. I wish I had thought about it.

Unfortunately, I don't think it worked. I do not have this fan coming and it only registers 13uA. It seems too low to operate a fan.

Is it possible that the heat or cooling needs to be activated for the fan to come in?

Astuff Member

Possible. Can also explain the voltages you read on the fan connections. It may have been phantom tension. Fan Com can float until a hot or cool call so that it runs parallel to the hot picks up some voltage. Use an analog meter to measure. Do not notice it often with control circuits as most wires have a relay coil that charges them. Low voltage circuits may also have resistance to the ground.

vandacca Thread Starter

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

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

Astuff Member

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

The thermostat is an "analog" with relays so everything stops with the face removed. To test, place a stirrup from L1 to heat. See if the voltage at Fan Com then goes to 120v.

vandacca Thread Starter

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

vandacca Thread Starter

So, it turns out that Heat / Cool must be on for it to be current on the COM: FAN lines. When the heat / cold had been turned on, the COM: FAN lines had ~ 117V / ~ 1.23A applied. Well below the relays 12A for the relays, so all good here.

How to calculate the power requirement of the transformer? I'm sure my 40Watts transformer (very large and beefy) is more than enough, but should I be worried that it's not enough? Do I need a ~ 150Watt transformer? I can't imagine the size of such a transformer. Should I measure the amplifiers on L1: L2 to calculate the transformer power? I guess it's also ~ 1.23A.

Currently, I mount everything on a wooden board so that I can install it inside the wall. The last thing I need is to source is a 6 "x10" access door.

Thanks again.

vandacca Thread Starter

Here is the final schedule. Works well.

 thermostat diagram

Stevce Member

Hi Dan! Does this still work?

I want to connect a 24VAC Venstar ColorTouch to my parents' apartment with a 120V Honeywell T6069B POS.

Venstar Tech Support says that the conversion cannot be done but your solution looks like it works.

19659006] Your schedule is very clear (Great Work!) On the relay card but which 120V to 24VAC transformer / power converter did you use?

Have a last photo of your installation that you can post?

PJmax Group Moderator

A 24vAC transformer is shown at the top of the diagram. It runs state and relays.

Stevce Member

Thank you. Just wanted to be safe.

So you only need the 24VAC transformer from Amazon and the 8 relay card to use my 24VAC Venstar on my 120V condo HVAC system which replaces my old Honeywell 120V fan coil thermostat?

to be safe because I convert line voltage to low voltage and don't want to burn anything down or out.

vandacca Thread Starter

Hi Stevce,

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

Honeywell AT140A1000 40Va, 120V Transformer – 60 Hz. https://www.amazon.ca/dp/B000K2EC7K/…_ZCNQDbWEZW71V

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

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

 thermostat cables with light

Stevce Member

Fantastic! Thanks Dan!

I just ordered that transformer and 8 relay card. The only difference is that my T6069B thermostat is a 6-conductor. No blue fan common thread like yours & no white neutral thread either.

Any thought how your schedule would change without the blue common wire fan? Does it change the schematic or do I just leave out that hope?

Mine is as follows:

Brown = High fan

Blue = With fan

Red = low fan

Yellow = cooling valve

Orange = heating valve

Black = L1 / hot

[img] blob: https://www.doityourself.com/e0e633ef-36d1-4829-b2b6-59aad28459ab

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

shimps33 Member

HI, I have the same situation as Stevce, I have a honeycomb T4039 line volt in my apartment and there is a six wire installation as well. Different colors.


Blue-Fan Low

Blk-Fan With

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:

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

Stevce Member

Hey Shimps33, the colors don't matter.

Each thread regardless of color in both of our settings addresses the same 6 functions so that the solution would be the same. ????????

Here is my photo:

  thermostat with wiring and details

Stevce Member

… I mean "our" solution for shimps33 [19659036] & I would be the same if vandacca and others can help advise on a 6-wire schedule for us. Please. ????????

Question: Since we only have six wires, can we get away with a four relay card?

 Relay board with inputs and outputs

PJmax Group moderator

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

Four relays would give you 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 the theory well. So I spent time fully understanding the circle in my situation before continuing. I did a lot of voltage / current tests with the original system to see how the current flowed to understand how to connect the relays. I then tested the theory by manually checking the oven. 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 grip on how it worked.

I do not understand how electricity flows with your 6-conductor system, especially the return flow. Test it or get advice from others (in this forum or a plumbing electrician). But not having the common (blue) and neutral (white) cables will definitely change how things are connected, you must first understand your installation.

I just started with 8 wires coming out of my wall that were connected to my thermostat. My guess is that 3 of those wires (HOT, NEUTRAL and COMMON) come from my fuse panel and the remaining cables come to my plumbing system. You need to find out your situation (or get an experienced plumbing person to advise you on how these things are normally linked). Maybe you are missing power?

In my schedule, the wires to the right come out of my wall and connected to my original thermostat. The wires to the left come out of my circuit and connect to my new thermostat. Maybe there are two or three more wires from your fuse panel required for this to work (just guess here)? Sorry I couldn't help but I just started like you without knowledge of how this works.

As for the smaller relay card, PJmax is right. I considered losing the ability to control 1 fan speed (eg medium) so I could use the much smaller card, but in the end I got big. But the smaller board is a really nice size. If I had to do it again, I'm not sure if it's worth keeping 3 fan speeds. 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 card as this manufacturer does not make a 5 relay board, which would have been perfect . Each relay is just a controllable on / off switch.

If you plug in a Nest E that only has the ability to control 1 or 2 fan speeds, you definitely get the 4 relay card.

BTW, when screwing down the relay card terminals on your wires, be careful and make sure it is tight. These relay card screw terminals do not provide much feedback so I found it difficult to say if the connection was good and that it was tight enough. Pull hard on the wires to make sure.

Stevce Member

Thanks Dan for the detailed answer! Much appreciated!

While I do not have a degree in physics, I am technically savvy after working with electrical and electronics as a semi-advanced amateur all my life having built circuit boards in my teens and connected in the basement all the way to the box. Just a tad rusty but like cycling on theory as I get started.

I take my reliable multimeter to the wires to see how volts flow and I should go with the smaller 4 relay card because I can live with 2C / 2H (with + high) on my Venstar Colortouch T7850 [19659036] . The most important thing I really needed was a way to convert 120V line flow to 24VAC so as not to burn anything out and looks like there are many out there who have taken this transformer / relay path with great success.

It is not safe for the first timer, but lots of measures twice (or ten) cut theory once.

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

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

I have 6 wires coming out of the wall shown in entry # 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 count which one I have)

 working on a thermostat

vandacca Thread Starter

Hi Stevce, [19659006] FroM the picture you gave, it looks like you have 8 threads coming out of your wall! This is probably enough to work with the schedule I provided. I couldn't read the labels under the wires, but I suspect the black thread corresponds to my L1 and the white thread corresponds to my L2 or N. And I guess the green thread is COM.

my situation, I only used the wires from the wall / base plate. The wires connected to your thermostat may have different colors on the wires coming out of your walls, and so I recommend that you focus only the wires / colors coming from the wall, otherwise it may confuse things during communication.

Astuff posted some useful tips in this thread. Set your Multi-Meter (MM) to measure the current at the highest AC gain range, then push the MM 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 can briefly do the same with HEAT and COOL, but if the fans are not running, it is difficult to see if the heater / cooler is on and you probably do not want to heat / cool it for too long without the fans running. You can 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 hot / cold air from the ventilation openings.

Only perform this test when you are fairly confident in the wiring. Good luck and read all posts from Astuff. He really helped me to understand how all this works.

Stevce Member

Thanks Dan.

One clarification that I should have said is that the Venstar backplate photo with the 8 threads above is a stock photo I posted from the internet for illustration purposes . My bath.

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

The diagram from Honeywell in entry # 37 above shows how it is probably connected – one of the 3 tags on the left that I need to work out. Sorry for my miscommunication.

vandacca Thread Starter

Well, in that case you need to find out what is going on with your implementation. Maybe additional power cables are needed, or the former installer did something non-standard. Do you have a back plate in your installation as the stock photo? If so, it can help to see how it is connected. Good luck.

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