Taco SR502 zone controller with Nest stat

Timco

My 506 has a common terminal for each stat terminal. The 502 uses the 24v take off for the common wire. Both send resistors with the unit. Under what circumstances is the resistor needed? The instructions just say they may be needed between the common and W terminals. Thanks.

STEVEusaPA

If you have a common 'C' for each zone you won’t need the resistor for the nest.

SteveSan

The Nest will need to be pre-charged and they suggest a 220ohm up to 5watt resistor. The 1000ohm .5watt resistors we send out with each board will not work with the Nest t-stat. Each "c" would need to go the com side of the green terminal plug.

Alan (California Radiant) Forbes

SteveSan said:

The Nest will need to be pre-charged and they suggest a 220ohm up to 5watt resistor. The 1000ohm .5watt resistors we send out with each board will not work with the Nest t-stat. Each "c" would need to go the com side of the green terminal plug.

Sorry, Steve: I don't know what you mean by the green terminal plug.

We just did a job in SF where there were only 2 wires to an existing thermostat and the owner wanted to replace it with a Nest. We charged the Nest with a USB cord and then installed it using the supplied resistor between the "C" and "W" terminals. This configuration charges the capacitor in the Nest thermostat when there is no call for heat.

SteveSan

Please see attach.

HomerJSmith

Maybe, I got this all wrong. I think that one can't charge a Nest thru the R and W connection on the base. The charging circuit is between the R and C connections, which requires 3 conductor wire. If one were to use a charging resistor it would have to be between the C and W connections on a two wire connected Nest base. That would trickle a small charge thru the W wire, thru a relay coil to the common connection on the transformer without activating the relay. I never tried this.

I believe that the C connection is a direct connection to the common of the transformer and where the third wire to the C connection on the Nest is made. The R connection is the hot lead from the transformer to the R connection on the Nest (24V). The W connection is to a relay coil from the W connection on the Nest to the transformer common.

ratio

The resistor does go between the R C [edit: faulty memory, didn't verify facts!] and the W terminals. It allows a trickle of current to flow when a heat call is not active, which is both a) enough (usually!!!) to keep a power-stealing thermostat operational and b) not enough to actually call for heat.

Three issues occur. One, the current may be insufficient to keep a thermostat charged, especially during long heat calls (no power available when the contact closes & shorts out the resistor). Two, WiFi radios use a LOT of power when the radio is transmitting. Three, some modern computerized control boards are sensitive enough to see the current that results from the resistor as a heat call, or, even more rare, don't notice that the call has gone away due to the trickle of current & think that the call is still active.

hot_rod

ratio said:

The resistor does go between the R and the W terminals. It allows a trickle of current to flow when a heat call is not active, which is both a) enough (usually!!!) to keep a power-stealing thermostat operational and b) not enough to actually call for heat.

Three issues occur. One, the current may be insufficient to keep a thermostat charged, especially during long heat calls (no power available when the contact closes & shorts out the resistor). Two, WiFi radios use a LOT of power when the radio is transmitting. Three, some modern computerized control boards are sensitive enough to see the current that results from the resistor as a heat call, or, even more rare, don't notice that the call has gone away due to the trickle of current & think that the call is still active.

excellent explanation!

There seems to be different versions of the Nest out there, maybe they have addressed the need for that "trickle" resistor?


A resistor is sometimes needed on relay boxes that have only two thermostat connections to dissipate stray voltage from power stealing thermostats.
We have measured some digital stats sending 15V or more back to the relay in the off position. This can either engage the relay or provide a small amount of voltage causing the relay to buzz or chatter. This is more of a "pull down" resistor function which basically turns that stay voltage into heat via the resistor.

The resistor is not required on Caleffi Zone Control relay boxes, the fix has been engineered into the board.

a PDF of some wiring schematics.

Alan (California Radiant) Forbes

ratio said:

The resistor does go between the R and the W terminals.

The "C" terminal has got to be involved, no? Otherwise, there will be no charging.

It's like connecting your hair dryer up to line voltage. Without a neutral wire, it won't turn on............unless you're in the bathtub : (

nicholas bonham-carter

Alan, you must have seen that hairdryer in the tub situation on a recent murder mystery!—NBC

STEVEusaPA

Now I'm getting confused. Back to the OP, and my response.

Am I incorrect in stating if the relay has a R W C for each thermostat (zone), connecting the Nest R W C won't work without a resistor?
Is that true of all t'stats requiring a common or is this Nest specific?

HomerJSmith

Alan, I fail to see what the resistor in your drawing is doing. You'll have to explain that to me. A 5- Watt resistor in my experience is a ceramic resistor that gets very very hot from the current going thru it. I don't see that current flow in the Nest or thru a 18 ga wire. A .5-Watt resistor which is 1/2 Watt seems more probable. Perhaps some one missed the period in front of the 5.

hot_rod, as I recall, the Nest is not a current robbing thermostat as the term use to mean. A current robbing thermostat wouldn't operate at all unless wired properly. The Nest does operate with two wires, just not charging. I think the Nest has a charging capacitor which is a special capacitor that act like a battery in it. It wouldn't need replacing like a battery does, added note.

Honeywell came out with one (MagicStat) in the '80's that required a relay to function properly. I still have their technical sheet on it.

Steve, As I stated, I believe the C connection on the Taco is connected to the common on the transformer, the same as the W connection on the Taco only the W connection goes thru a relay coil first. So, the R connection from the transformer on the Taco goes to the R connection on the Nest thru the Nest relay switch to the W connection on the Taco to a relay coil on the Taco back to the common on the transformer on the Taco. That's a circuit.

The charging circuit for the Nest is the following: The R connection on the Taco is connected to the Taco transformer and it goes to the R connection on the Taco to the Nest R terminal, then, thru a charging circuit to the C terminal back to the C connection on the Taco to the common of the Transformer. That's the charging circuit.

Alan (California Radiant) Forbes

@HomerJSmith Yes, ½ watt.

STEVEusaPA said:

Now I'm getting confused. Back to the OP, and my response.

Am I incorrect in stating if the relay has a R W C for each thermostat (zone), connecting the Nest R W C won't work without a resistor?

@Timco was asking when you would need to use the resistors that Taco supplies with their zone calve controls.

You don't need a resistor if you have three wires. If you only have two wires, let's say to an older thermostat and you can't get a third wire to it, you can use the "W" wire to charge the Nest. That's when you use the resistor connected to the "C" terminal as shown in the last diagram I posted.

You also asked, "Is that true of all t'stats requiring a common or is this Nest specific?" I'm not sure about other power-stealing thermostats.

Side note; probably common knowledge: If you have a non-power stealing thermostat, you can connect the "C" terminal to power and you won't need batteries to keep your thermostat going.

Alan (California Radiant) Forbes

Hot Rod said, "The resistor is not required on Caleffi Zone Control relay boxes, the fix has been engineered into the board."

If you remove the resistor, you won't be able to keep the Nest charged.

To protect the relay, wouldn't the resistor be placed in series with the "W" terminal?

hot_rod

HomerJSmith said:

Alan, I fail to see what the resistor in your drawing is doing. You'll have to explain that to me. A 5- Watt resistor in my experience is a ceramic resistor that gets very very hot from the current going thru it. I don't see that current flow in the Nest or thru a 18 ga wire. A .5-Watt resistor which is 1/2 Watt seems more probable. Perhaps some one missed the period in front of the 5.

hot_rod, as I recall, the Nest is not a current robbing thermostat as the term use to mean. A current robbing thermostat wouldn't operate at all unless wired properly. The Nest does operate with two wires, just not charging. I think the Nest has a charging capacitor which is a special capacitor that act like a battery in it. It wouldn't need replacing like a battery does, added note.

Honeywell came out with one (MagicStat) in the '80's that required a relay to function properly. I still have their technical sheet on it.

Steve, As I stated, I believe the C connection on the Taco is connected to the common on the transformer, the same as the W connection on the Taco only the W connection goes thru a relay coil first. So, the R connection from the transformer on the Taco goes to the R connection on the Nest thru the Nest relay switch to the W connection on the Taco to a relay coil on the Taco back to the common on the transformer on the Taco. That's a circuit.

The charging circuit for the Nest is the following: The R connection on the Taco is connected to the Taco transformer and it goes to the R connection on the Taco to the Nest R terminal, then, thru a charging circuit to the C terminal back to the C connection on the Taco to the common of the Transformer. That's the charging circuit.

Yeah, I'm not sure the true definition of a power stealing stat, either. Some require power to operate, some to recharge the capacitor or battery.

Tech support tried dozens of different models and brands of stats with the Caleffi relays. Some worked fine, others needed the resistor, like the round Honeywell digitals. Having a dedicated common (3 wires) seems to make all of them work.

Some thermostats have a resistor in the box that may or may not be required depending on what they are wired too. There seems to be an array of different control logics used in digital stats, and it is a moving target.

ratio

Originally, a thermostat was a mechanical switch, it 'flipped' via temperature & the heat turns on, until the switch flips back. Only two wires were needed, the heat call & the power to energize it. Then came digital stats & that mechanical switch became a relay that had to be powered by electronics to 'flip' & turn on the heat. At that point, a constant power source was needed. Batteries were one solution, but some smart people realized that you can allow a little bit of current to flow without energizing the heat—enough to power the little bit of electronics & store up a little power to carry over during the heat call, when the thermostat basically bypasses its power-stealing circuit with the heat call relay. This was all based on the load (furnace, zone controller, etc.) being a simple relay, with continuity to the common (to allow current to flow) and a (proportionally) high power requirement (to keep the current flow from turning on the heat). The furnace/zone control/etc. boards were updated with electronics too, though, with new low-power relays that would trip (or try to) due to the low current flow inherent in power stealing, or µcontroller input terminals that (more or less) didn't provide a usable path to common. This is where the resistor comes in to the picture. It allows some current to flow all the time, without allowing the controller to see a real heating call.

Note that most of this is for an installation without a 'C' (common) wire to the thermostat. If a C wire is present at the thermostat, power-stealing isn't necessary (although the thermostat may only operate that way). As @hot_rod noted, sensitive inputs may still need a resistor.

NB: This is a simplification. I am not an electrical engineer. HTH, HAND.

HomerJSmith

I just want to know (inquiring minds want to know) how you can charge a Nest with just 2 wires as Alan's diagram shows.

hot_rod

Also as more and more components move to digital controls and circuits, the typical “vending machine” switches found in many zone valves do not work so well. A contact type switch needs a minimum current to seal and make a tight electrical connection. It called a sealing, wetting or a fritt current.
Reed switches like used in Z-one zone valves are designed to handle low, milliamp current that digital controls present to a switch.

HVACNUT

Sorry about the drawing but you can see that when the circuit is open (R and W at the thermostat), W is temporarily part of the Common, as it goes through the unpowered relay coil.

Alan (California Radiant) Forbes

HVACNUT said:

Sorry about the drawing but you can see that when the circuit is open (R and W at the thermostat), W is temporarily part of the Common, as it goes through the unpowered relay coil.

Don't apologize; I like it!

@HomerJSmith I'm far from good at electrical, but when the Nest is not calling for heat, the "W" wire is carrying a voltage trickle via the resistor and "C" terminal back to charge the thermostat's capacitor. Why it doesn't short when the Nest calls for heat ("C" and "R" making contact) is beyond me. Also beyond me is how the capacitor in the Nest can be charged when its "C" terminal is empty.

HomerJSmith

I'm not an electrical engineer, either, but I did receive the Highest Honor in my Electronics Course at Millford Elementary School, 5th grade class.

Nobody answered my question about charging a Nest with two wires, so I will go out on a limb, here, and say you can't. However, I did suggest way, previously, that I never tried.

HVACNUT, nice drawing, but there's not any difference from Alan's diagram, just greater detail. C is connected to the common on the transformer and W is connected, there, too, and C is connected to W thru a resistor.

Electricity is a lot like plumbing in some ways. In order to have flow in a piping circuit, there has to be a pressure differential. I learned that in my 4th grade Plumbing Class at Millford's which was a mistake. I should have done something important like chasing girls instead of learning about Plumbing, oh well. HVACNUT, your drawing show no pressure differential, so how does current flow. It doesn't because it's not a circuit. An electrical circuit implies a pressure differential when it is working , just as flow of water in a hydronic circuit require a differential.

HVACNUT says, "... when the Nest is not calling for heat, the "W" wire is carrying a voltage trickle via the resistor and "C" terminal back to charge the thermostat's capacitor." I'll bite, and how does that happen? I'll save you the suspense, it doesn't (my opinion, yet to be disproved). Don't you think that the current would trickle back to the thermostat thru the relay coil on the Taco board? Hmmm.

ratio

The pressure differential is there, it's just hidden. We'll find it.

First thing: there is a path from W to the xfrmr common—through the contactor! It's clearly tied in to the W on one end, and C on the other, and it must have continuity to work, ergo there is a path. (Yes, there are other things going on, but one step at a time!)

Any current that enters the W terminal will eventually get to the common terminal of the transformer. You can prove this by probing for voltage between R & W at the thermostat. You will see ≈24 VAC there. Your meter is actually completing the circuit at that point, however it's impedance (resistance) is high enough that the contactor never notices the minuscule amount of current flowing.

The power stealing thermostat do exactly this, but allow a middling amount of current to flow, enough to charge a small capacitor or battery but hopefully not enough to activate the contactor.

The resistor that must occasionally be added from W to C is to give the current a way around a sensitive relay that might try to activate, or a µcontroller input that can't let much more than a tiny current flow.

HVACNUT

> @HomerJSmith said:
>
> HVACNUT says, "... when the Nest is not calling for heat, the "W" wire is carrying a voltage trickle via the resistor and "C" terminal back to charge the thermostat's capacitor." I'll bite, and how does that happen? I'll save you the suspense, it doesn't (my opinion, yet to be disproved). Don't you think that the current would trickle back to the thermostat thru the relay coil on the Taco board? Hmmm.


FAKE NEWS!
You'll never find that quote from me because I never wrote it. Please don't do that.

HomerJSmith

Sorry, HVACNUT, I know you're too savvy to say that. I should have read that post more carefully. My troublesome dyslexia.

I don't mean to be discourteous to anyone, but am aware that sometime I come off curt. We all have our failings. What appears to be curt on my part is information seeking by me. Better information helps me do a better job.

FAKE NEWS! That's funny.

Alan (California Radiant) Forbes

Thanks @ratio! Between you and my electrical engineer son, the path through the relay coil was made crystal clear.

Those electrons are very crafty.

HomerJSmith

Has anyone had any success in charging a Nest type thermostat with only two wires?

ratio, I understand what you're saying. The question boils down to whether the R & W contacts on the Nest is a simple switch in a relay like any analog thermostat or whether that switch has a hidden circuit that charges a charging capacitor. If so, why would one need a C contact on the Nest board?

ratio

The Nest must have a hidden circuit to allow any kind for power stealing. However, the power 'stolen; is perforce minimal. I've heard of people who had issues with the Nest failing to remain charged during the coldest part of winter, when heat calls are long and frequent—the opposite of what's best for power stealing. (This would be exacerbated by outdoor reset, the goal of which is long runtimes.) Also, the WiFI radio is a huge power hog, likely using up hours of normal operation every time it keys up.

My guess is that in the heating season, the power stealing can mostly keep up, as long as we don't experience longer than normal runtimes (ODR, below design conditions, deep setback, correctly sized equip, etc.) A pre-charge will likely be sufficient to ride through the environmental causes; but if you have ODR in particular, a C wire is a near certain requirement.

HomerJSmith

ratio, you're right. The Nest does have a hidden charging circuit according to their literature and should charge with the normal two wire connection, although there could be problems that would require the C wire. It appears to me that the charge is thru the Taco relay as one of the symptoms is chattering of the relay and repeated start of the boiler. The resistor on the Taco between the C & W terminals as shown on HVACNUT's drawing does provide a path back to the common on the transformer, however, there is still a path back thru the relay coil, but there is impedance in the coil. I suspect that the resistor take some of the current which keep the relay from chattering, although I not sure how that works. If the resistor on the Taco wasn't there there would be a direct fault (short) of the transformer 24V and common. That would disable the transformer's primary winding.

Evidently the Nest has a battery (batteries wear out, even iron based batteries) rather than a charging capacitor. (Their reference to a battery.)

Proper and problem free performance would require 3 conductors to the Nest.

In an infloor hydronic sys that I re-did because the original design was faulty, I installed 7 analog thermostats, later, a new owner had another contractor install 7 Nest thermostats. The fully charged thermostats ran for 2 days before crapping out. All these Nest's operated on separate manifold 40VA transformers with RIB relays to power the thermo actuators and Taco zone valves.

I had to rewire and use the C conductor to get them to keep a charge.

Their literature: " Nest thermostats use very little power, less than 1 kWh/month, while a 40W light bulb that's used 4 hours per day consumes about 5 kWh/month."

There were others who previously posted on this site with problems with a two wire connection to the Nest.

ratio

The current is shared in proportion to the impedance of the path. You can Google "Ohm's Law" for more information, but that rabbit hole doesn't come to an end…

I am not a fan of the Nest thermostats. They are often a pain in the you-know-what to install and get working, & they imply profound privacy concerns. Their only claim to fame is a novel user interface. IMHO their target audience is hipsters & millennials looking to signal the right values to other hipsters & millennials.

& get off my lawn, durn kids!

HomerJSmith

Privacy concerns??? Ya, they're probably sending all your doings back to Google which is why I never use Google.

FDNYRETIRED

I am not a plumber or know very much yet about all this stuff yet. I am researching about setting up 2 Nest E's or the Gen 3 on my NCB 240-E with a Taco SR 502-4. also using the ODS on the Navien. But I only have a 2 wire Tstat to my zone1. On top of that it is the old hard copper wire. I do have 24VAC to a Honeywell dial rectangular mechanical T stat on zone 1. That my old POS Quietside 120-A was hooked up on. This controls Convection radiators on 3 floors. Then I have Zone 2 on a Honeywell Rectangular Tstat with slider mercury operated switch. That operates a 12' Slant fin radiator in a 240 sqft room. But that one at least has the thin wire and a c-wire that I can use.

So first I need to run new wire to the tstat upstairs. Unless the heavy guage hard wire will fit into the connectors on the Nest. I use to have a Weil-McLain hot water oil burner, that was first converted to gas. Then after Hurricane sandy put 6' of water through my house. National Grid had a rebate on the Quietside 120A. That was nothing but a problem since day one it was put in. So if I run the Zone 2 with c wire I guess I have to use the resistor. Between W1 and com? But if I first try to fit the old wires from zone 1 into the tstat. to see if I can keep the battery charged, will that resistor cause a problem? My neighbor has a 2 wire system for over 2 yrs. On a Gen 2 Nest and has not run into problems.

Also I have researched and find the some people are saying that the ODS will not work right on a 2 zone system. Unless your primary zone is sending power. Yet some say they have no problem. Navien says it can't be done. They also say running a Nest on a ODS screws things up. Because you are counteracting what the sensor outside hooked to your Navien does. Related to how the Nest Gen 3 works.

Heat Sensor: In instances when the sun is directly affecting the temperature inside your home, the Nest 3 will detect this and immediately choose cooling options to lower the heat. (I do not see that much of a problem unless it is constantly setting new positions)

Enhanced Sensors: The Nest 3rd generation is more sensitive than the Nest 2nd generation, learns and adapts quickly.

Automatic Adjustments: In order to conserve energy, your Nest 3 will assess the outdoor weather. If sudden changes are detected, Nest will adjust the schedule and create a better heating and cooling setting. ( This might cause a problem)

Time-to-Temperature: Your Nest 3 learns the time it takes to cool down or heat up your home, which will show on the screen. (No problem there)

So trying to dial a reset curve in with a Nest might mess things up? Because the ODS is reading outside temp. While the Nest is doing the same thing. But I think the nest is doing it through weather sites pulling info off the net. While your ODS is real time. If both do not read the same outside temp you run into problems. I have no idea if this is BS or not. I am still playing with my reset curve and how to set it up, Trying to keep the boiler modulating and condensing onto the exchanger.

On a side not related to privacy and hacking. This is from 2014 but if you follow security and hacking here is something to think about. Also Google yesterday has discontinued their Nest Alarm System. It also turned out there was a microphone that was never turned on inside the unit, but they never disclosed that info. They say it was an oversite, but it had to be user actuated anyway. That is not the reason they discontinued the system, They are partnering with ADT soon.

https://blackhat.com/docs/us-14/materials/us-14-Jin-Smart-Nest-Thermostat-A-Smart-Spy-In-Your-Home.pdf

Also if you never seen the inside of the Nest and a teardown
https://learn.sparkfun.com/tutorials/nest-thermostat-teardown-/all or this from 2017 which shows better details. https://medium.com/@justlv/nest-thermostat-e-teardown-and-on-making-beautiful-devices-for-the-home-ae6ada01bb26

So this is actually my first post here and I am still trying to learn Reset curves. I turned my ODS on and set the B setting to 1 which is for finned tube baseboard. But my inlet and return water temp was to high. It started cycling and I actually put it back to not using the ODS. Then I tried again the next day and put B to 5 for High Mass Radiant. Kept everything to default. Except F I turned from 100% to 80% and it was modulating. Outlet water temp was about 128*F. But I am not dealing with really cold weather here yet. If anyone knows or can direct me to setting my reset curve I would appreciate it. I am using the search on the site. To see if I can find some threads on it. I have not tried a custom curve, because I do not feel comfortable doing that yet. I do not understand the supply absolute Max, and Min temp set points yet. For supply and return values.

HVACNUT

Any info in the thermostat literature about using a resistor if there's no Common. Taco now provides them with their SR zone panels. You can always splice thermostat wire to the "old copper" to make the thermostat connection. 

SteveSan

If they are using Nest t-stats, Nest requires a 220ohm up to 5watt resistor. Our 1000ohm .5watt resistors don't work with Nest t-stats. Make sure the Nest has been pre-charged before wiring to our controls.

FDNYRETIRED

HVACNUT said:

Any info in the thermostat literature about using a resistor if there's no Common. Taco now provides them with their SR zone panels. You can always splice thermostat wire to the "old copper" to make the thermostat connection. 

Your right and I thought about doing that first to see if the 2 wire will charge the battery. I looked how it was tied into my switch box. That's what the plumber did. Spliced it into the old wires. That would be the way to go. Then I also realize That I have a outlet. and I an get something like this https://amazon.com/Transformer-AGPTEK-Compatible-Thermostat-Honeywell/dp/B07HRK7CWX/ref=sr_1_32?dchild=1&gclid=Cj0KCQjwuL_8BRCXARIsAGiC51DYUlNvte_vABf2ZOy1QZeyM2kBUZV2CbBcgTixwBCUU0IxMe8ZSqgaAtAZEALw_wcB&hvadid=238290493361&hvdev=c&hvlocphy=9004461&hvnetw=g&hvqmt=e&hvrand=11076861572838180247&hvtargid=kwd-446905492247&hydadcr=24656_9648981&keywords=common+wire+adapter&qid=1603292175&sr=8-32&tag=googhydr-20

Thanks

FDNYRETIRED

SteveSan said:

If they are using Nest t-stats, Nest requires a 220ohm up to 5watt resistor. Our 1000ohm .5watt resistors don't work with Nest t-stats. Make sure the Nest has been pre-charged before wiring to our controls.

Would that mean a resistor on each zone w1 wire to com? If I cant get away with using the 2 wire and a plug in transformer on the one with my 2 wire system to zone 1. Zone 2 I have the right Tstat wire. So would that only have to be the one resistor to zone two W1to com?

SteveSan

Each t-stat would require their own resistor between the w-c. Most Nest do require a com connected so you can try using the 220ohm up to 5watt resistor at first but you might need to run a com if that doesn't work.

FDNYRETIRED

Thanks

FDNYRETIRED

I finally found Nest E t-stats T 4000ES. You have no idea what I went through. Every big box store I tried only had the $249.00 Nest E. Best Buy does't even sell them any more. I finally found them for $169.00 at Bad bath and beyond. My luck come Black Friday they will be $100.00. If you pre order them from the Google store They are I think $122.00. But I was going to do this the day I got them.

So this is what I find the connector plate on a Nest E model
T 4000 RS only has 6 connections. So if you have a HAVAC system for AC and Heat or fan. I think they will not work but I am not sure. Me I only have heat and a 2 wire system. So no big deal. But the kick in the A$$. If you have a electrical box. You got to spend $15.00 and order a metal plate

http:https://store.google.com/us/product/plate_for_thermostat_e//

I tried to fit the plastic back plate and the holes do not line up. Either the vertical ones or the horizontal ones. Plus they are so close to the box, that if I wanted to just use the self tapping sheetrock screws supplied will not work. I am so pissed off, because they could mention that with the advertisement before you buy them. This way you would be prepared to get what you need. I can't find the plate in a Home Depot, Lowes, Ace or an Electrical outlet store in my area.

Now I had a Navien Certified tech here. He made sure my ODS was set up and I know what I need to do. We started talking about the Nest and Taco switching boxes. He looked at my Taco SR502-4. He said before I run new wire to get a common. That I should try it with the 2 wires first. He said that 90% of the time it works without a common and a ODS. Also that I do not need to put a resistor between the W1 and Com. If the battery drains then he said to run a common wire from the Taco switch com block to the Nest C connector. He advised not running a resistor from W1 to com with the 2 wire system. Though it is possible to do it. But with 2 nest T-sats it is better to have their own common wire, and not to do a resistor from each W1 wire to com.
He said these Nests are a PITA to set up if they don't want to play nice with your system. Worse ar HAVAC systems. Boiler units you have a better chance of not having problems.

SteveSan

Pre-charge the Nest before wiring to the Taco SR control. Run a common wire with to the com side of the green terminal plug on the left side of the SR control. Any questions please feel free to give Taco Tech Support a call @ 401-942-8000 Mon-Fri 8am-5pm EST.

FDNYRETIRED

Thank you that's what the tech also said  to  run a com wire to the left side of the SR control. He pointed to the block that says [EDIT TO SAY 24vac] It has a block with 2 screws on it. Each com wire to each hole, then tighten down the screws.
But he said to try it first just 2 wires.  Because I have to run wire from one floor to another. 
He said he would come back once I got the mounting metal back plate for the Nest,  and if I run the wire to the second floor. He would hook it to the switch box. I don't want to void my warranty if I did it myself. 
Since it's a 2 zone my other zone already has thermostat 5 wire running to it. So I have a common to use for that one. 
Also yes I did charge the Nest's thrift the USB with a charger

FDNYRETIRED

Just to update I hooked up the Nest E stats. Figure I take my chances without the C wire. So far so good the Batteries have been holding a charge 3.90 volts. It has been almost 3 weeks.