Navien NCB-240 & common wire

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chialex

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Hello experts,

I have a new Navien NCB-240 water heater and I want to install a new Sensi wifi thermostat. That thermostat requires a common wire that would supply 24VAC. On the Navien I found a terminal marked CN25, it has three connectors and in the wiring diagram this terminal is marked as "AC24V, External AC Output". I connected an unused wire the CN25 terminal and the other end to the C connector on the thermostat. I get no voltage on the thermostat through that wire (measured between R and C wires). The other connections I made on the thermostat are R and W, which are connected to the T/T terminal on the Navien. The thermostat works fine with battery backup power, but without the common wire it drops the wifi connection from time to time.

It looks like Navien does have a connection point for the common wire, but I get no current on that connection. Any advice? Could it be that there's a jumper or a switch on the Navien that would supply power to the common wire connection?

I appreciate any insight into this.

Thank you, Alex.
 
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Jadnashua

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If you have a wiring diagram (often there's one on the inside of the case if you don't have the manual). Find the 24vac transformer, and follow the common wire. Make sure you are on the secondary side, not the primary, of the transformer.
 

Widgit Maker

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http://www.manualslib.com/manual/790769/Navien-Ncb-180.html?page=19#manual

Page 19 , paragraph 3.1.2 (instruction for installing a low water control) shows that CN25 is a low voltage ac supply (see note). Common is on the left and hot is on the right. Center terminal is not identified. To have voltage to thermostat you must have a complete circuit. In other words you must connect both common and hot to the ac voltage input terminals of your thermostat.
 

Zl700

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With a volt meter determine which T of "TT" to ground is 24 VAC on CN10, that will be your "R". That makes the other one "W"
The best place to secure a "C" is the ground screw terminal at the rear of the cabinet or the frame of the circuit board. Look for a green wire and attach your thermostat C wire under that lug. This is the ground or common for both 120V and 24V control circuits.

DONT USE CN 25, it is a constant power source for powering a 24V low water cut off. This will destroy your thermostat and likely fry the circuit board. Thermostats get connected to "T & T" which is CN10.
 
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Jadnashua

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DONT USE CN 25, it is a constant power source for powering a 24V low water cut off. This will destroy your thermostat and likely fry the circuit board. Thermostats get connected to "T & T" which is CN10.
A thermostat that requires constant power DOES need both the common and 'hot' terminals from the 24vac transformer! Normally, it may only require the 'hot' side, which, when the thermostat is calling for heat, would apply it to the control, which already has the return side on it to allow the circuit to be completed.

Many of the newest WiFi enabled thermostats work best (if at all) only when they have constant 24vac applied. Now, whether that terminal is the best place to find it, can't say.
 

Zl700

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I think your confusing constant power terminals with the thermostat connection terminals that would have a floating power. Your thinking there's a separate power connection which there is not on stats.

As I stated, the hot side of T & T connection is the R and it closes across to W for heat activation on just about all 24 v appliances. But note that's not a common to sustain power stealing operation.

A closed circuit T & T is required to activate heat. Hooking his stat to constant power will only fry his stat. Power stealing t-stats like the illuminated wifi stats often require the common wire to sustain operation and programming once it is satisfied and opens its electro-mechanical contacts.

Still not following, look up the nest Tstat wiring. Again I state, CN25 per Navien is a point of source to check circuit board low voltage and power a external 24v LWCO if used.

Can we hold you financially accountable if OP follows bad advice?

Or was that all a neutral response because you like to see your words in type?
 

Jadnashua

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It appears that if the thermostat does not require more than 0.3A to operate, you could use that terminal for power. Your best advice is to call the company and ask their recommendation.
 

Widgit Maker

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I am new to this site.

"Or was that all a neutral response because you like to see your words in type?"

Is that the way differences of opinion are resolved on this site?
 

Jadnashua

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The notes on the wiring diagram say it can source up to 8w/0.3A. If the thermostat doesn't need more than that (or the combination of anything attached to them), it should work. They do not show the actual 24vac transformer. The manufacturer is often the best source of info when it is not perfectly clear. If he can find the actual 24vac transformer, find the two leads on the secondary side, and use the common from there.
 

Zl700

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Thanks Jim, you really cant stop can you?
Back the original request, The OP already has it wired right and just needs a C to keep it from dropping out when its in idle mode.

My Navien manufacturer given advice about determining which T on the NCB is R on the stat and grabbing a location for a common wire "C" still stands.
 

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First, C should be hooked up to the terminal marked "common" on CN25. If that doesn't fix it try swapping the R and W wires going to the T/T connections. Usually one of the T/T is connected to R so you had a 50/50 chance of getting it right.
 

Jadnashua

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Thanks Jim, you really cant stop can you?
Back the original request, The OP already has it wired right and just needs a C to keep it from dropping out when its in idle mode.

My Navien manufacturer given advice about determining which T on the NCB is R on the stat and grabbing a location for a common wire "C" still stands.
Not sure what your problem is either, but the 24vac transformer has two wires on the secondary and you need both of them to power the thermostat. The R is already at the thermostat. Common is the 'other' side of the secondary, which is what completes the circuit required for the thermostat to have power. Some will work without the common, and a dumb one will (the Nest Series 3 will work without a common, but works 'better' with one). You should be able to find the transformer, and trace the other side, or, call the manufacturer. If you were really worried about overloading that set of contacts with the thermostat, install an inline fuse. As long as you don't mistake the primary for the secondary, you should be fine.
 

chialex

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I would like to thank everyone for your suggestions. I haven't decided what to do yet, but this discussion helped me to understand my options. Both, the thermostat manufacturer and the heater manufacturer suggested using third-party 24VAC conversion kits like Venstar or Lock State. I think both of these kits contain a transformer and a relay.
 

Jadnashua

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A properly installed isolation relay totally isolates the power between the two systems...the control wires are only used in the device they come from, and the thermostat's power only controls the coil of the relay...they never 'cross'. Most devices have enough power available from their internal low-voltage supply (usually a transformer) to support powering a thermostat, but it does require access to both sides of the 24vac transformer for it to work whereas, you only need one side at the thermostat to switch the unit on. All the thermostat is is a switch, connecting power to the device to activate it. Just like a light switch, the load completes its circuit after you apply power to it. In the case of a smart thermostat...it needs power to operate. Some use batteries for this, some require power...they don't care where it comes from, but if you can't get it from one source, you must isolate them from each other, and that's what the relay does.
 

yourefree

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Hi. I am have this same situation. I am installing a Tekmar 519, replacing a standard battery powered room temp. I would like to wire in from the Navien CN29 24Vac terminal but called them and they said it is only for the low water pump. The power is what I need though. Chialex what did you end up doing? Should I just go and buy a transformer and wire that in? I have never done a transformer but looks simple enough.
 

Kurt in Seattle

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Same exact issue as Chialex here. Issue is that the Navien NCB-240 doesn't have the standard thermostat wiring connection labels. It has A/H (2); T/T (2); LWCO (2) AC24V output (3). I have wires into both slots at T/T. and then a jumper between the two LWCO slots. Installing a Nest eThermostat that isn't getting enough power. Would a C (common) wire go into the wire panel described above; or to the grounding point?
 

Kurt in Seattle

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Here's a photo of the thermostat wiring panel at the Navien.
 

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fonseka

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Here's a photo of the thermostat wiring panel at the Navien.
Did you find where to hook up for C wire?? I connected a blue cable to C (on thermostat) and grounded the other end on the navien chasis. I got the 24 Common to show on the Nest E but R is not receiving enough power. Not sure what is going on as multimeter shows 26 v current.
 

fonseka

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I now noticed the Navien (180-E) is reading 22-24 volts when using ground (common connected to chassis) to the white wire on Navien board(witch is connect to W on Nest base). When I switch the wires to provide power to R on the Nest base, the volt meter now measures 22-24 volts on other wire. Not sure if the Navien is switching the power between red and white for safety reasons. Anyone found a solution to this?
 

Jadnashua

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In a typical thermostat, the power comes in on the red wire and none of the other control wires are a common. The thermostat connects that R wire to various other places to activate things. IOW, all of those other wires are OUTPUTS, not INPUTS. WHen the thermostat calls for heat, the internal switch connects that red wire to the white wire...then, that white wire goes into the heating appliance (typically to a relay coil), goes through that coil and on the other side of the coil is the C, or common. That completes the circuit so the relay can energize.

So, while you may measure voltage on the W wire, depending on where and how you're measuring and your reference point, assuming the heating unit is wired up properly you cannot get power to your thermostat by connecting the W wire as common - when it's not calling for heat, the power is running through the relay coil before it gets there. WHen it is calling for heat, the R and W wires are connected together by the contacts in the thermostat (i.e., they're the same wire then), and there would be no current going between the two, only along them, since they're tied together.

Grounding points when you don't understand what's going on can fry things fairly quickly. In most of those systems, the 24vac system does NOT have one side grounded at all.
 
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