Connecting a C-wire, need advice

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Old 10-31-20, 03:39 PM
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Connecting a C-wire, need advice

I have heat only 2 zone force hot water system. L8148E Aqua stat controller. 40 VA external transformer. On the bottom of transformer, it has letters R (Load), and C. Does the C mean common??

I uploaded the diagram but I'll try my best to describe it as well.

Stat-A "Rh" --> Trans "R"
Stat-B "Rh" --> Trans "R"
Stat-A "W1" --> TacoA "#1"
Stat-B "W1" --> TacoB "#1"
Stat-A "C" --> not connected
Stat-B "C" --> not connected

L8148E "T" --> TacoA "#2" --> TacoB "#2" --> Trans "C"
L8148E "Tv" --> TacoA "#3" --> TacoB "#3"
L8148E "w" and "z" are connected from factory with jumper

Should I connect Stat-A "C" & Stat-B "C" to Trans "C" or L8148E "z"



I use "A" refer to thermostat #1, and "B" to thermostat #2 in the description
 
  #2  
Old 10-31-20, 04:30 PM
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Welcome to the forums.

Your diagram is shown as correct and you would only find the C connection on terminal 2 of the zone valve. You can send that C to your thermostat.
 
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Old 11-01-20, 05:29 AM
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Hi, you can also pickup the C from the other terminal on the transformer, if thats easier.
Geo🇺🇸
 
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Old 11-01-20, 07:04 AM
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Yes, “C” means common.

Often one 24 VAC transformer lead “C” powers several units on a system. The other transformer lead goes to a switch, thermostat, aquastate or whatever that when contacts close, connects and activates unit.

Think of it as a three part loop.... transformer, switch and unit when connected starts/activates circuit

In heating systems there are many of these circles/circuits working independently but fed from same C common.
 

Last edited by doughess; 11-01-20 at 08:41 AM.
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Old 11-01-20, 08:42 AM
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I connected Stat "C" to transformer "C". It is easier and neater because I don't have to splice since power to T-Stats come from Transformer "R". Thank you all for your advise. I connected it this morning and everything is fine.

Back Story:
Have 2 Nests less than 1 year old. 1 died, 2nd dropped WiFi quite a few time sand looks like it's about to die. The Voltage is very stable as soon as I connected the Stat "C" wire to the Trans "C".

Have a nice day and be safe.
 
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Old 11-01-20, 09:47 AM
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The nests don't have a stellar track record.
 
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Old 11-01-20, 10:10 AM
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Double check the load on that transformer. I don't recall what the zone valves use but two valves and two smart stats but there is a chance that could be a bit much for 40VA.

Check out the load for your model zone valves and your stats. If your Taco valves are 0.9amp draw, then 24V*.9amp = 21.6. Two valves and two stats would be a lot on 40VA.
 
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Old 11-02-20, 08:47 AM
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I tried measuring it with a current meter. I can't seem to get a good reading. I found numbers are all over the place. After a while, I realized I was measuring the current near the transformer and the current probe gets a reading when it come near the transformer (I'm a noob). I tried measuring the current near the zone value, I can't get a reading even it's calling for heat. The Nest is not supposed to draw much power. The transformer is rated to handle 2 zone valves.

Fact check:
  • System was in place for 16 years using 2 wires (thermostats used were older style that used AA batteries)
  • Replaced old thermostats with Nests less than 1 year ago using 2 wire solution and they seem fine until one unit die and other one showing symptoms. (My guess is that batteries are not holding as much charge when it was brand new)
  • Just added "C" wires recently. So loading differences would be whatever power required from 2 thermostats. I have no idea how much they draw to charge the batteries. I looked into Tech info. The "lin" is 200 mA (each). But I don't think (hope not) it's drawing that much continuously. My current probe is not good enough to measure that.
Any thoughts?
 
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Old 11-02-20, 09:01 AM
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edit: I guess to clarify. in my original post, by "double check the load on the transformer", I met computationally. sorry for the confusion.


Good move adding the c wires. Advertising smart stats working without them is borderline criminal imo. They might work out the gate, or they might not. They might work for a year, or they might not. To operate properly and to not interfere with hvac equipment, they need dedicate power via C.

I am not sure what model zone valves you have but the taco three wire I have seen are 0.9A. If you have those, than the two valves will require a max of 21.6 VA. If the thermostats are 0.2mA (its beyond frustrating that Google/Nest don't make this value readily available) then 0.2mA means each stat will need 4.8VA.

That totals to 52.8VA. Now, those values are at peak draw for each device, which obviously isn't happening all the time. However, if you get a cold spell and both stats and valves are going off together you could maybe start to have an issue. Last thing you want is your transformer to fail in the middle of a holiday or the night.

I believe an overloaded transformer will start to overheat until it fails. I am not sure about measuring current/voltage fluctuations or instabilities as an indicator.

Ive learned all this through rewiring my own HVAC system. I added two ecobees, HFAC-201 controllers to a zone that included two taco three wire valves. To play it safe, I upgraded the transformer that powered them to 60VA. The pros around here with more experience might know that 52.8VA isn't a worry. They can (and should ;-) ) chime in to check my logic here. Transformers are pretty cheap so I would probably just upgrade it; or at least keep a spare one handy.
 
 

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