Can't get Nest to recognize C wire

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Old 12-06-20, 05:47 PM
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Can't get Nest to recognize C wire

I'm trying to wire up two Nest thermostats to a Taco SR502 relay. These are the Nest thermostats with 6 wire connections, not 10 like the learning Thermostat. I have the C wire connected to the "24V/COM" block on the Taco, but it says the C wire isn't connected. The Nests seem to work, but they aren't getting enough juice to keep continuous power, and the wifi keeps disconnecting.

I've attached an image of the way one of my zones appears to be wired, and the voltage measurements I'm getting between C and R, and C and W. C appears to indeed be supplying 26v+, but when measuring C to the R or W block on the Taco as opposed to C to R or W wires where they pigtail into the zone valve seem to be different voltages.

My first hypothesis was that the Taxo's transformer wasn't supplying enough power for everything, so I bought another external 24v/40a transformer for the Nests. I haven't hooked it up, because I want to make sure that I know what I'm doing first, and wasn't sure if this was going to be essentially mixing two circuits together.

 

Last edited by PJmax; 12-06-20 at 09:26 PM. Reason: imported diagram
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Old 12-06-20, 08:51 PM
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The 24v/com block has two connectors so make sure you are using the correct one. The one side marked 24v is R and the one marked COM is C. Also, sometimes the R and W get swapped so verify colors/wires end to end at all of the splices. For sanity's sake most use white for W and red for R but it is wrong a lot of the time.

Also: How are the wires connected at the zone valve? Does R stop or is it spliced through?

This doesn't make sense as the relay normally has separate connections to the thermostat than the zone valve. Guess is that your drawing doesn't match what is there. Can you post pictures?
 
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Old 12-06-20, 09:43 PM
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There are a million variations of the Taco controllers.
The common 502 has a single common terminal.
That Taco should be able to run two nests E's.

Your diagram doesn't look right. The thermostats connect where I have the red and white dots and the zone valve connects separately down at the bottom. If this is not your version..... you'll need to post a picture of it.
How-to-insert-pictures.

 
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Old 12-07-20, 07:28 AM
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I drew my diagram as accurately as I could. It was hard to follow the wires as they are spliced to the zone valves and all zip-tied together, and of course they're all brown. The thermostat is spliced into the R/W wires on the side where they connect into the Taco. The Wires I am holding in the picture are my R and W wires, along with the C wire which come from the stat (old braided wire) and go directly to the zone valve along that brown-sheathed cable out the top left of the relay box. They appear to be spliced in, and come back from the valve to the R/W block on the relay, at the top. The relay wires at the bottom of the relay case (green/white) go to the circulators. I have one circulator for my hot water tank, and another for my two heating zones.



 
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Old 12-07-20, 09:36 PM
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See what I mean..... same model number....and another variation.

That looks like a 24vac and common connection. Is it labeled C/com ?
If it is a true common connection you should measure 24vac from thermostat R to that terminal.
 
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Old 12-08-20, 07:11 AM
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It's on COM. And I do measure more than 24v from COM to R terminal, but if I measure from COM to the R wire where it splices to the zone valve, it's basically half the voltage. Is that normal?
 
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Old 12-09-20, 07:43 AM
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Does anyone have an idea? Will an external transformer work, or am I getting voltage drop from the weird way this thing is wired together?
 
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Old 12-09-20, 11:04 AM
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My hands are temporarily ties as we wait for the picture server to be reset.

Does your nest recognize the C now ?
There are three wires from the controller to the stat. R, W and COM. This wiring should not go to a zone valve. The zone valve wiring should be connected to the bottom of the controller.
 
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Old 12-09-20, 03:35 PM
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I did a little test just now. I disconnected the zone valve from where it spliced into the R and W wires, so I just had the stat connected to C/W/R, with the zone valve disconnected completely. That did the trick, and the Nest recognized all three wires and had increased voltage going in. So I guess that confirms that somewhere in that splice in with the zone valve, it's dropping voltage.

I'm hesitant to change all the wiring around since this is how my HVAC contractor installed my new heating system. Unless there is some workaround to get extra voltage to the Nests, then I'll have to have them come in and take a look.
 
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Old 12-09-20, 03:48 PM
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You don't need a work around. You need a properly wired system.
I modified a copy of your diagram. This is how your wiring is supposed to look.
 
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Old 12-09-20, 03:53 PM
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I'm assuming they did it this way, because both my zones are actually on "zone 1" on the board, and the thermostats are actuating the zone valves. Zone 2 on my board is for the hot water tank circulator. I'm guessing that's why it couldn't be wired up the traditional way. The other Nest I have, in what is actually Zone 2, is wired up the same way.
 
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Old 12-09-20, 04:01 PM
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You can't put two zone valves directly on the nest...... it'll FRY.
 
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Old 12-09-20, 04:16 PM
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No what I meant, was that somehow, the Zone 1 and Zone 2 on the board are not controlling my actual heat zones. The thermostats are somehow wired in so that they actuate each valve as they call for heat, but they are on one circulator which is activated as Zone 1 on my board.
 
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Old 12-17-20, 10:29 AM
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It sounds like you actually have a similar setup as me. The previous owners of my house put an extension in and, to add a new heating zone, they took an original heat zone and split it with two zone valves (all about 50 ft away from the boiler). I installed two ecobees for these zones. They have an independent transformer that powers them and allows them to open their respective zone valves. Both of my Taco zone valves have end switches that will send a signal to the boiler once the valve is opened by the thermostat.

So, taking a step back. Taco makes two main types of boards for HVAC control. Zone valve (ZV) boards and switching relays. If your system is all valves with one circ, the best option is to use a ZV board. One downside is if you have multiple circulators in the mix (ZV boards typically can control 1 or 2 circs only, with the second being for DHW). With these boards, a stat is usually wired directly to them (R-W-C). The stat sends a signal along R-W to the board signifying a call for head. The board then sends a signal to the valve (which is wired directly to the board) to open, to the circ to turn on, and to notify boiler there is a heat call.

The alternative boards, Switching Relays, are really designed to handle zones with independent circulators and no zone valves. Here, R-W-C comes in from the thermostat, signals heat, and the board turns on that zones circulator.

In my case, I have four heating zones using three circulators. Zones 3 and 4 share a circ and are the zone valves in the crawl space 50ft away. I wired it such that zones 3 and 4 going to "zone 3" on the switching relay board. This was easiest because they share a circ and i had limited wires avaiable.

You have a switching relay, so I am guessing that end switches from your valves go back to zone 1 on your board to fire the circ/boiler etc.

What model zone valves do you have? With that, we can determine the proper wiring.

Can you also confirm if there are any other transformers involved?

Are the valves powered off of the Taco SR's transformer?
 
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Old 12-17-20, 02:07 PM
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Got it. So you need your two thermostats to each run their own zone valve and both zone valves tell Zone 1 on the controller to start the pump. Definitely do-able. As mentioned.... what model zone valves. Most are typically three wire.

Technically you should be using a three stat controller because you will be taxing the transformer in the existing control.
 
 

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