8-wire 120v Honeywell conversion to 24v NEST Learning

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  #1  
Old 01-07-19, 07:47 PM
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Question 8-wire 120v Honeywell conversion to 24v NEST Learning

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 1 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

Fan
-----
- Com: Purple wire
- Fan I: Red wire
- Fan II: Blue wire
- Fan III: Brown wire
(see attached photo)



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 3 fan speeds with the Nest thermostat? For example, do I need an additional fan controller module?

Thanks,
 

Last edited by vandacca; 01-07-19 at 07:50 PM. Reason: Fixed attached image.
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  #2  
Old 01-07-19, 10:17 PM
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It is a bit more involved. You actually would need 5 relays. One for heat, one for cooling, and one for each fan speed.

Only need one transformer to drive it all, though.
 
  #3  
Old 01-08-19, 08:58 AM
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Thanks @Astuff. Any advice how to wire it all up? I assume I can cut a corner and only use 1 or 2 fan speeds? Wouldn't I need some kind of controller to translate 1 fan signal into 3?
 
  #4  
Old 01-08-19, 11:29 AM
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Yes, you skip fan speeds but shouldn't need to.
Use a multi relay board like https://bravocontrols.com/shop/mecha...ng-and-4-gang/ 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.
 
  #5  
Old 01-08-19, 03:06 PM
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Thanks again @Astuff. My system is definitely 120V. I'm not concerned with space, as I can always add a 2nd box and plaster around it.

A few follow-up questions. I assume that my "Com" purple wire is a common ground for the 3 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 3 Fan relay switches on that multi-relay board. For example, I assume that the "Com" line will be used as one of the 2 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.
 
  #6  
Old 01-08-19, 04:45 PM
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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: https://support-assets.nest.com/imag...ller-Guide.pdf
 
  #7  
Old 01-10-19, 04:22 PM
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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?
 
  #8  
Old 01-10-19, 07:55 PM
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That relay board comes with either 4 or 8 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.
 
  #9  
Old 01-10-19, 08:25 PM
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@ A......Your link in post 6 isn't working.
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.
 
  #10  
Old 01-10-19, 09:10 PM
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Something strange with Nest's site or else I have a typo somewhere I can't find as it seems to work some times and not others. Try changing https to http . If you Google "nest pro installer guide" it points to the same place for the pdf.

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: https://nest.com/support/article/Lea...s#how-it-works and https://nest.com/support/article/How...our-thermostat
 
  #11  
Old 01-10-19, 09:48 PM
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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.
 
  #12  
Old 01-10-19, 10:54 PM
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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)
 
  #13  
Old 01-12-19, 12:38 PM
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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.
 
  #14  
Old 01-12-19, 09:02 PM
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@Astuff, does your last post mean that I may also need a 33ZCRLYBRD board as well? Or if I go with the 8 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.


I can't express enough my gratitude for your assistance.
--Dan
 
  #15  
Old 01-12-19, 09:41 PM
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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.
 
  #16  
Old 01-13-19, 07:43 AM
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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 3 or 4 ways, similar how they have phased power into most homes.
 
  #17  
Old 01-13-19, 10:18 AM
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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.
 
  #18  
Old 01-13-19, 07:53 PM
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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.
 
  #19  
Old 01-13-19, 08:48 PM
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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.
 
  #20  
Old 01-16-19, 07:09 PM
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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?
 
  #21  
Old 01-16-19, 09:30 PM
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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.
 
  #22  
Old 01-17-19, 09:15 AM
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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?
 
  #23  
Old 01-17-19, 10:41 AM
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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.
 
  #24  
Old 01-17-19, 03:07 PM
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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.
 
  #25  
Old 01-19-19, 08:38 AM
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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.
 
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