Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

Taco SR502 zone controller with Nest stat

TimcoTimco Member Posts: 2,969
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.
Just a guy running some pipes.

Comments

  • STEVEusaPASTEVEusaPA Member Posts: 4,142
    edited June 6
    If you have a common 'C' for each zone you won’t need the resistor for the nest.
    steve
    Timco
  • SteveSanSteveSan Member Posts: 58
    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.
  • 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.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
    Grallert
  • SteveSanSteveSan Member Posts: 58
    Please see attach.
    Alan (California Radiant) Forbes
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 6

    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.


  • ratioratio Member Posts: 2,440
    edited June 6
    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_rodhot_rod Member Posts: 13,315
    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.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,333
    edited June 6
    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 : (




    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • nicholas bonham-carternicholas bonham-carter Member Posts: 8,133
    Alan, you must have seen that hairdryer in the tub situation on a recent murder mystery!—NBC
  • STEVEusaPASTEVEusaPA Member Posts: 4,142
    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?
    steve
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 6
    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.
    STEVEusaPA
  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,333
    edited June 6
    @HomerJSmith Yes, ½ watt.

    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.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
    STEVEusaPA
  • 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?
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • hot_rodhot_rod Member Posts: 13,315

    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.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    STEVEusaPA
  • ratioratio Member Posts: 2,440
    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.

  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 6
    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_rodhot_rod Member Posts: 13,315
    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.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • HVACNUTHVACNUT Member Posts: 3,357
    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) ForbesAlan (California Radiant) Forbes Member Posts: 2,333
    edited June 7
    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.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
    ratio
  • HomerJSmithHomerJSmith Member Posts: 923
    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.

  • ratioratio Member Posts: 2,440
    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.

    Alan (California Radiant) Forbes
  • HVACNUTHVACNUT Member Posts: 3,357
    > @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.
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 8
    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) ForbesHVACNUTSuperTech
  • Thanks @ratio! Between you and my electrical engineer son, the path through the relay coil was made crystal clear.

    Those electrons are very crafty.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
    HVACNUT
  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 8
    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?
  • ratioratio Member Posts: 2,440
    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.

  • HomerJSmithHomerJSmith Member Posts: 923
    edited June 8
    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.
  • ratioratio Member Posts: 2,440
    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!

    SuperTech
  • HomerJSmithHomerJSmith Member Posts: 923
    Privacy concerns??? Ya, they're probably sending all your doings back to Google which is why I never use Google.
Sign In or Register to comment.

Welcome

It looks like you're new here. If you want to get involved, click one of these buttons!