NEST With Two Wires

Alan (California Radiant) Forbes

I've been using a resistor between the "C" and "W" terminals of the zone valve control with great success. The 220 ohm resistor that Taco recommends works, but the resistor gets very hot. I've gone back to the 1,000 ohm resistor that Taco used to supply. It provides less of a trickle current, but providing that the thermostat is off long enough, it will charge the NEST.

Clarification: The control pictured below is a pump control (Taco SR506) which does not have a large enough transformer to power any more than 2 or 3 power stealing thermostats. We added an external 75 VA transformer for the 6 NEST thermostats.

Jamie Hall

The trick, of course, is to get enough trickle current in the particular application to charge the silly thing enough so it doesn't go paws up when it's calling for heat. A neat balance. Also known, less kindly, as a kludge.

109A_5

Hello @Alan (California Radiant) Forbes,
I think I would kind of split the difference and try a 470 Ohm, 2 Watt. I would also put the resistors where the heat won't damage anything. Maybe put the resistors in first behind the wires above the edge of the circuit board.

One example, many other available;
470 Ohms ±5% 2W Through Hole Resistor Axial Flame Proof, Safety Metal Oxide Film
https://www.digikey.com/en/products/detail/yageo/RSF200JB-73-470R/18282

During a call there is probably 24 VAC across the resistor. So...

With a 220 Ohm, during a call the resistor has to dissipate 2.6 Watts so you would need a 3 Watt resistor or better yet a 5 Watt resistor.

With a 1000 Ohm, during a call the resistor has to dissipate 0.58 Watts so you would need a 1 Watt resistor. I would not recommend using a 1/2 Watt resistor, but it probably would work with the duty cycle. However you never know how long a call will be, so I like conservative values.

With a 470 Ohm, during a call the resistor has to dissipate 1.23 Watts so you would need a 2 Watt resistor. Hopefully a 470 Ohm resistor is enough to keep the NEST charged up during the idle time.

SteveSan

Morning Alan, Taco reached out to Nest a while ago about using resistors with their t-stats and were told our wouldn't work with theirs. Below is from one of their installation guides.

The heat call is detected by the controller board when the W wire is shorted to the R wire, which causes the
24V AC voltage to be detected. When the Nest thermostat is taking power, a small voltage is generated
between W and C (typically less than 8 volts) which can cause a false heat call in a small number of systems.
The addition of a 220 Ohm resistor at the furnace provides another current path to the thermostat, which
reduces the voltage input at the detect circuit to prevent false heat calls.

Our 1000ohm 1/2watt resistors will usually work with any other brand of t-stats.

John732

I’m Confused why would you want to reduce voltage, doesn’t the nest need 24v to charge, aren’t we reducing the amp(current) draw with resistor. ? 

109A_5

Hello @John732,
Actually reducing the Voltage @SteveSan is referring to gets more of the 24 VAC to the NEST.

Maybe think of it like this; With thermostats R wire is hot, C wire is common, neutral or low, the W wire is switched R wire. If a two wire NEST setup is calling for heat, R wire is connected to the W wire. The load, the heating equipment is between W wire and C wire.

The NEST charges during the idle time (no call for heat). With a thermostat switch open circuit the W wire is pulled low to the C wire (resistance of the heating equipment). However the NEST is drawing current through the load to charge so the voltage is raised on the W wire by the NEST charging current. The NEST helper resistor helps keep the W wire voltage low or closer to the C wire, so no false calls for heat and more charge current for the NEST. The lower the value of the helper resistor the lower the W wire voltage will be. The side effect is the lower the NEST helper resistance the more power is dissipated by the resistor during a call for heat, thermostat switch closed, since 24 VAC is now across the NEST helper resistor.

Teemok

Resister logic aside, the wire color reversal on zone 5 is irksome.
Enough charge current and time vs the length of heat calls is a balance. Turn it all off for some months or have a very extended power outage and it might put the system into a dysfunction loop. Wouldn't that create a condition where the nest is not charged enough to keep it's switch closed long enough for the zones heat call to satisfy. Heat call drops, nest starts to recharge, temperature drops into nest restarts and closes it's switch again only to drop out again.

EdTheHeaterMan

Teemok said:

Resister logic aside, the wire color reversal on zone 5 is irksome.
Enough charge current and time vs the length of heat calls is a balance. Turn it all off for some months or have a very extended power outage and it might put the system into a dysfunction loop. Wouldn't that create a condition where the nest is not charged enough to keep it's switch closed long enough for the zones heat call to satisfy. Heat call drops, nest starts to recharge, temperature drops into nest restarts and closes it's switch again only to drop out again.

Irksome. Is that a technical term?
The software inside the NEST does not work like the Analogue description you just offered. As a matter of fact, there are no actual switch contacts in the Nest thermostats. just resistors that change form more resistance to less resistance simulating open and closed contacts. Not exactly sure what they are called but I know that they are not switch contacts. So when the software finds an anomaly like insufficient power to recharge the batteries, you get an error code that sometimes results in a no heat situation. Not the best scenario for a vacation home in the mountains, and the App just says NEST off line.

EBEBRATT-Ed

@EdTheHeaterMan Triacs maybe?

I don't see the big deal. If someone wants the nest bad enough$50 worth of parts transformer and Rib relay will be a permanent fix. i dislike the Band-Aid approach. I like to walk away from something knowing it will work for a long time.

pecmsg

The triacs on the equipment computers don't like the power steeling. Sometimes they work sometimes they dont!
https://youtu.be/uLso0ZBqOi4?t=34

Teemok

"Irksome kludge". Preferred pet name. Beats little bugger. @EdTheHeaterMan Thanks for that clarification. Regardless of the tech, It's acting as a switch. R and W that is. When it is acting closed there's no charging capacity so the dysfunction loop would exist? I guess R and C might have a reason to high speed switch to vary VA but that is moot when there no voltage present.

Alan (California Radiant) Forbes

Resister logic aside, the wire color reversal on zone 5 is irksome.

I saw that, too. Decided to leave it alone to remind me of the original installer who worked for me back in the 90's. He smoked lots of pot; on the job, and my customers complained. But his workmanship was top notch, so it was hard to let him go.

Most of the guys I trained struggled with hydronics. I hired them as plumbers because that was the majority of the work we did back then and then slowly got into hydronics because that was my motivation. Just a few of them loved hydronics and this guy was one of them; didn't want to do plumbing anymore, but had this bad habit. He left and went to work for a competitor which was when he did this job in Pacific Heights in SF.

So, I left the wires alone.

109A_5

If anyone is interested, I believe the NEST uses a circuit basically like this to switch the R and W wires. N-Channel Mosfet Transistor. Very low trigger current compared to a coil of a relay or a TRIAC.
The NEST version is probably more complex since they may use the transistors to multiplex commands to the Nest Power Connector when used.

Teemok

Lost hair trying to train stoners while working in high end homes. The stress, by association, cured me of any desire to use cannabis. I've known lots of stoners that claim they work better when high. Maybe it's true for some kinds of work. It's mostly more like they feel like they work better and as a bonus they forget mistakes rapidly.

Jamie Hall

I had assumed that it was a Triac in there, @109A_5 , but you are right -- it could be a Mosfet. Either one, the fundamental problem remains: the thing charges when the call for heat is off, and not when it is on. That's just dandy for things like forced air -- which is what it was designed for and for which it works well -- but it works poorly, if at all, when faced with longer run times unless it has an independent power connection through R and C.

EdTheHeaterMan

Really?



I can't believe that @pecmsg has agreed with something relating to the NEST Hockey Puck. I never thought I would see the day when he said something even remotely positive about it. Is the world coming to an end? Has Hell frozen over? What other strangeness can we expect in this year 2024?

pecmsg

EdTheHeaterMan said:

Really? I can't believe that @pecmsg has agreed with something relating to the NEST Hockey Puck. I never thought I would see the day when he said something even remotely positive about it. Is the world coming to an end? Has Hell frozen over? What other strangeness can we expect in this year 2024?

They Still
S
U
C
K
!

TwoTones

Disappointing because the Nest is so terrible, especially in the two wire configuration….

John732

they way i have my boiler and have done others is with rib relay and external transformer. but they also have a kit made by FAST STAT( common maker) and you can make it work that way thus avoiding all problems and no need resisters require.... just follow instructions for boiler diagram. basically it has part you put behind stat and a relay at boiler if you set up right it closes relay which close dry contact on boiler (t & t) with zone board i havent done but im sure theres a way . might be more pricey but no worry about heat generated at resistor. just a suggestion of another way. as someone else suggested on this thread also.

MikeAmann

Assume that you have an electrical outlet in your hallway close to the thermostat.


Would a plug-in wall transformer of the 24v ac 40 va variety work for charging a "NEST" type thermostat? In other words, a NEST "battery eliminator". Give it it's own power supply and then the thermostat could function with the original 2 wire R & W configuration.
I assume that the only thing to watch out for would be to match up the commons or neutrals for proper polarity. Or would that even matter?
If this covers the charging problem, then a RIB would not be needed either, correct? And NO "C" wire.

https://www.walmart.com/ip/24VAC-40-VA-Plug-Transformer-Auto-resetting-PTC-fuse-Compatible-All-Versions-Doorbell-Nest-Ecobee-Sensi-Honeywell-Thermostat-PTZ-camera-UL-Certified/3013004268?wmlspartner=wlpa&selectedSellerId=101463420

109A_5

Hello @MikeAmann,
It may work if done properly and left that way.
The issue I see is the phasing between the NEST transformer and the transformer in the heating equipment. As far as phasing is concerned with the NEST calling for heat, only the R wire is really the only wire joining NEST with the heating equipment, since the W wire is connected to the R wire. And each transformer has its own load.
The issue I see is when the NEST is not calling for heat. If the transformers are out of phase, since the R wire connects the NEST transformer with the heating equipment's transformer there could be 48 Volts between the C wire of the NEST and the W wire of the NEST. The NEST may not like that.
If phased correctly there would be 0 Volts between the C wire of the NEST and the W wire of the NEST. But can you guarantee the phasing between transformers will not be inadvertently changed if someone else works on the system ?
For best practice I would use a relay, since it provides 100 % isolation between the two transformers, then the phasing is not an issue.

109A_5

With the wiring diagram above, keep in mind (only two thermostat wires), since the relay in this case only needs two input wires (coil) and two output wires (N/O contacts), the relay can be located with the heating equipment. So the NEST charging transformer is with the NEST and the relay is with the heating equipment. So you don't have to hide a relay somewhere at the thermostat's location.

MikeAmann

@109A_5
Thank you so much for your excellent reply.
You covered everything and with an easy to understand explanation.
The diagrams are icing on the cake.
Perfect. This solution (I think) is ideal because it avoids the whole "I don't have a C wire" problem completely.

SgtMaj

If you want set back temp just go back to this hundred year old Minneapolis.
All problems fixed.

PGB1

One suggestion for a resistor is to use ceramic resistors.

They dissipate heat very efficiently & help keep the terminations cool. But, like 1095_a mentioned, keep them away from damageable items. The body gets a bit warm.

Not to over-simplify, but how difficult is it to run new wire to the Nest so a Common is present? Will this solve the problems?

Perhaps tug gently on the thermostat wire to find out if it was stapled inside the wall. If it wasn't, tie the new cable to the old and pull the new in while the old is pulled out. If it is stapled, low and medium flex flex fish sticks work very well in walls to send new wire up without using the old wire. Klein calls theirs Glow Rods. Cheaper versions exist.

Paul
Disclaimer: The sparrow in the tree outside knows more about Smart Thermostats than I ever will know. So, be sure to ask the pros here before listening to me. (My recent post about ecobee versus a 40va transformer will attest to that!)

Alan (California Radiant) Forbes

Not to over-simplify, but how difficult is it to run new wire to the Nest so a Common is present? Will this solve the problems?

The pump control is in the basement mechanical room. All of the rooms in the basement are finished - bedrooms and bathrooms. There are three floors above the basement and it would be impossible to run new thermostat wire to the thermostats without removing a good quantity of wall coverings.

I know the contractor who did the original installation and he did quality work, but running 2 conductor wire is asking for trouble, even 25 years ago. I've always wanted options in case something goes wrong or you wanted a thermostat that showed the outdoor temperature............or, you wanted a c-wire.

PGB1

Here's a really 'way outside of the box' work-around to using resistors that you may find acceptable:

Install an ecobee thermostat in the boiler room (where you can run the Common wire easily).
Place one of the remote 'Smart Sensors' in whatever room you wish. They are wireless & simply get plunked down somewhere.

It will use the temperature in that room to turn the heating/cooling on and off. (Set point)
You can install more than one sensor per thermostat. They can be set to "follow" you around, using whatever room you are in as the thermostat set point.

Manual adjustments to the thermostat (temperature +/-, fan on-off, schedules, heat-cool-off-auto, etc) are done with the phone or computer or tablet.


This is just an idea to help you get out of using resistors.
Paul

Teemok

@PGB1 I've run in to a problem with what you suggested above. It seems when you select a wifi remote sensor as the only sensor and it drops off wifi for any reason the internal wired sensor in the ecobee takes over by default and the zone runs wild. Maybe I've missed a setting to prevent this? Wifi is great but I hate to relying on it when failure mode means cooking the space.

PGB1

Your point about Wi-Fi is a really good one, Teemok!
I never thought about the sensor cutting through Wi-Fi to talk to the thermostat. I assumed they spoke directly. I guess ecobee technical support would know if there's a work-around.

My ecobee3 Lite (with newest firmware) works fine without Wi-Fi- but it has no remote sensors.

Thanks for posting about the lack of communication without Wi-Fi!
Paul

HomerJSmith

I wouldn't use thin-film resistors. I use carbon resistors, if you can find them. I think they hold up to heat better.