Nest Thermostat and Taco SR502 C Wire Issue

Big_Tony90

I have a Taco SR502 - 2 zone Controller and I want to install a Neat Smart Thermostat that requires a C 24v Wire. When I wire the thermostat and use the Blue wire that is connected to the C terminal of the thermostat and the 24v Common bus on the Taco I get an error message that there is no power on the C wire.  Any help would be great.

neilc

is that blue? or green??
are you sure?

EdTheHeaterMan

That should work. Can you post a picture of the NEST wiring from the wall with the thermostat removed? Is the other Zone for Hot Water?

Jamie Hall

Bot surprised. The C wire is supposed to be the common return -- hence "C". The red wire (Rh or Rc) is the 24 volt power supply wire.

neilc

forget it,
that's not gonna work,
that 24v is alt power in, not out

EdTheHeaterMan

@Big_Tony90 I see the Yellow and the Green wire leaving the control. Where are they going? Is that Nest also operating an Air Conditioner? That could be the problem. If the Nest is also going to an AC system, the C must come from the cooling system transformer, not the heating system transformer.

neilc

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1351148998437/85259_PROD_FILE.pdf

EdTheHeaterMan

@neilc that won't help. That is the new book. It does not cover the control that @Big_Tony90 has. He has the older 2 pump zone control, There is no C for the thermostat at the top thermostat connection that are in the new controls in that book.

neilc

hang in here and Ed or somebody will come along and draw you how to do this,
you'll need another transformer, and a relay,
@EdTheHeaterMan

neilc

EdTheHeaterMan said:

@neilc that won't help. That is the new book. It does not cover the control that @Big_Tony90 has. He has the older 2 pump zone control, There is no C for the thermostat at the top thermostat connection that are in the new controls in that book.

I looked at that book hard, and at the picture above, and at that book, and at the picture above , , ,
No C nowhere in that picture , , ,
then I summoned you

EdTheHeaterMan

neilc said:

hang in here and Ed or somebody will come along and draw you how to do this,
you'll need another transformer, and a relay,
@EdTheHeaterMan

Like this?

neilc

@EdTheHeaterMan said:

neilc said:

hang in here and Ed or somebody will come along and draw you how to do this,
you'll need another transformer, and a relay,
@EdTheHeaterMan

Like this?

? ?
bad link ?

EdTheHeaterMan

neilc said:

@EdTheHeaterMan said:

neilc said:

hang in here and Ed or somebody will come along and draw you how to do this,
you'll need another transformer, and a relay,
@EdTheHeaterMan

Like this?

? ?
bad link ?

Patience is a Vertue... Fixed

neilc

@EdTheHeaterMan
so you could add a trans to the 24 input, if you needed extra amperage for other situations?
or use it as OP tries, for common,
now the nest base wiring question is relevant, and whether its a straight shot of wire between, and not spliced or broken elsewhere

I did see a note in taco lit saying to add a resistor between r w and c,
can't find that same link though

Big_Tony90

neilc said:

is that blue? or green?? are you sure?

Yes, that is a Blue wire.

Big_Tony90

EdTheHeaterMan said:

@Big_Tony90 I see the Yellow and the Green wire leaving the control. Where are they going? Is that Nest also operating an Air Conditioner? That could be the problem. If the Nest is also going to an AC system, the C must come from the cooling system transformer, not the heating system transformer.

Yes, The Nest is also operating an AC unit. So I need to get 24volt from the AC unit? I'm not questioning you just trying to understand. Why would it matter? I am willing to try anything at this point.

Big_Tony90

I drew in where I have the blue wire.

neilc

neilc said:

I did see a note in taco lit saying to add a resistor between r w and c,
can't find that same link though

I found it again,

Troubleshooting:
• Problem: Digital thermostats do not work correctly when connected
to a switching relay.
• Solution: Some thermostats are a “Power Stealing” type which means
they are powered by the switching relay with just 2 wires (R & W). A
resistor may be needed in order to have the thermostat work properly.
This resistor should be placed between the W & C (common) terminals of the switching relay. If the thermostat manufacturer does not
supply a resistor, a 1000 ohm ½ watt resistor has proven to work with
most models and is readily available at electronic supply outlets

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Taco-SR504-EXP-4-Install.pdf
pg2

Big_Tony90

So I tried one person's advice to get the 24v from the AC Common and the thermostat now shows power on the C but I am now getting an error on the Nest for the Rh power. I tested and the heating works when the Nest calls for Heat. DAMN Nest

EdTheHeaterMan

I believe your wiring is not as is should be. You may not need the RH wire by the way the air handler is wired. If you have a heat relay in the air handler like this:Try removing the wire from RH and see if the error message is resolved and both the heat and cool controls work.

If not, then you need to set up the wire like this.

Big_Tony90

I do not have Forced Hot Air so I do not have an air handler for heat. So I believe the first picture won't work for me.

EdTheHeaterMan

You do not have forced warm air for heat. I know. You have forced hot water for heat. There are 2 ways to make that happen. the first diagram is the 5 wires from the thermostat way. The cooling relay is in the outdoor unit, the fan relay is in the air handler the heat relay in the air handler has a set of contacts that operate the hot water zone for heating. Follow the brown wire from the heat relay to the SR502 control. That is the only thing that the white wire from the thermostat is operating. The hot water heat.

The heat relay in the air handler is only there to turn on a set of contacts to operate the zone relay. There is no function in the air handler that the heat relay operates. All it does is to turn on the switch to the zone control. W from the thermostat is to operate the heating. The thermostat does not care if the heat is operated by a furnace or a boiler or a gas burner or an oil burner. It does not care if it is a steam boiler or a water boiler. It don't care if the radiators are cast iron or inexpensive copper tube baseboards. All the W wire knows is to turn on when it is cold and turn off when it is hot.

In your case, all the W wire does is to turn on a relay. After that, the relay can operate anything you want.

So if you find out where the W from the thermostat going, you can decide it you are using the top diagram or the bottom diagram. My guess is the top and all you need to do is remove the RH wire from the Nest and you will have no more error N72.


If you are not following the technical explanation, then just remove the RH wire and see what happens. The Nest is smarter than you think. Just because YOU want to use the RH does not mean the you need to use the RH. The Nest is telling you that you don't need it. And It knows what it needs and what is does not need. It is pretty smart.

EdTheHeaterMan

This might make my diagram more like your actual wiring. I don't know where the wires are physically going

This is the same diagram with the thermostat wires going to the Taco Zone Control relay first and yellow and green wires going to the air handler second.
The point is that you have a heat relay somewhere in your system. That heat relay eliminates the need for the RH wire.
1. The R from the air handler connects to the RC on the NEST
2. The G from the air handler connects to the G on the NEST
3. The Y from the air handler connects to the Y on the NEST
4. The C from the air handler connects to the C on the NEST
5. The 24VAC coil from the heat relay connects to the W on the NEST
6. The other 24 VAC coil on the heat relay connects to the C on the NEST or the air handler
7. The relay contact common connects to the Taco Zone thermostat terminal and the Normally Closed contact on the heat relay connects to the Taco Zone thermostat terminal. (T T or R W)

If you can get that done, then you will be in good shape.

I believe that all that is already done. all you need to do is remove the RH wire from the NEST Thermostat.

pecmsg

Why are these discussions always about the N E S T ?

EdTheHeaterMan

pecmsg said:

Why are these discussions always about the N E S T ?

Because the NEST is so smart it doesn't like anyone who can't keep up with their intelligence. Like when Sheldon talks to Penny.

Jamie Hall

pecmsg said:

Why are these discussions always about the N E S T ?

First, they are commonly sold in big box stores ("simple replacement! Save energy! Save money!") for people to install without detailed directions as to how they need to be powered (just label the old wires and hook them up).

Second, for reasons best known to Silicon Valley, the Nests seem to be somewhat fussier as to where they get their power (of which they use a lot) than some other makes. Various kludges (to be charitable) have been proposed to make them behave (such as the resistor trick), but the underlying problem is that there are so many differrent ways that the equipment they are to control is hooked up (even, often, to two wire thermostats not caring which wire is "hot" and which wire is ground) and so much variety in that other equipment that there is no simple way to describe how to do it.

It would be nice if they came with -- or at least offered as an option -- a simple little box which contained a power supply and a relay, and which had two output terminals on it to correspond to the two terminals of a two wire conventional thermostat and a connecting cable to the Nest all neatly labeled for the Nest's terminals. Dream on...

EdTheHeaterMan

pecmsg said:

Why are these discussions always about the N E S T ?

I remember one customer has a Weil McLain Ultra boiler with Unico air handlers
4 Zones with 4 NEST
3 outdoor units
3 Air handlers
3 Honeywell Zone valves
1 Taco ZVC 403
1 Honeywell zone damper control for the first floor and finished basement.

He has some computer programer from his company install some type of building control system to operate everything from the internet. After 4 years they were unable to make it work properly. He had the Nest doorbell. And he asked me about the Nest thermostat. Since I was a Nest Pro at the time, I thought this would be a good fit.

On the second and third floor there was no problem. I added a heat relay to the Unico air handler and that turned on the blower and also the zone valve for that air handler.

When I could not make the 2 Nest thermostats work with all the minushia of the Honeywell zone dampers and the Taco ZVC without getting error messages I came up with a control box with 6 RIB relays and 2 transformers. Basement Nest was powered by one of the transformers, the Y controlled a cooling RIB, the W controlled a heating RIB and the g controlled the Blower RIB. The other transformer and relays did the same for the first floor Nest. Did that take up a day of wiring? YES but at the end of the day it worked the way it was supposed to.

One of the problems with the Nest is that there are no hard contacts that connect R to W, Y, or G. There is some sort of resistor that change to simulate the closing of contacts and opening of contacts. If the Nest designers started with actual switch contacts, there may have been less problems with the device.