Taco ZVC405 (replacing a ZVC 404) wont work with existing 2 Nest Thermostats 3 wire config

BCApollo

Baseboard Hydronic system Peerless Boiler and DHW, Taco ZVC405 controlling 3 taco 4-wire zone sentry valves with the DHW on priority. Heat demand called with 2 Nest 2nd gen (non learning) and 1Honeywell analog. Everything was working using the prior ZVC-404. The 404 recently stopped responding (yellow) to any call including a jumper on zone 1 so I replaced the controller with a ZVC405 as 404s are backordered for weeks. Installed wire exactly same config moving priority DHW to zone 5 and turned on the priority switch. Both fully charged Nest thermostats and a new spare that I had do not work together or individually but all zones will work with the analog thermostats. I came here and read every post & sticky on Taco controllers and Nests then tested the two transformers RC wires and they all read 27v. Taco sis now using 40VA transformers (I was hoping it was that easy). Reading the note regarding the 1k ohm .5 Watt resistor didnt seem to apply as I have a 3 wire RWC not a "Power stealing" 2 wire RW configuration but I tried it anyway and got an "install stable power source" warning on both of the Nests tested 1 at a time. I even tried the 220ohm 5 watt configuration with the same result. I called Taco and they only care if there is 24v at RC if yes then blame Nest. The system worked with the 404. The call ended with the tech saying if I think its bad take it back get another one.
I went back to all analog for now as its starting to get cold again in Southern NY. This system has been rock solid since I installed it in 2005 other than the pesky 1st generation taco zone valves which are all upgraded to the current zone sentry (3rd gen) and have been very stable. Wires all ring good end to end. Nests were running good and stable no issues since Dec 2020. Are the new Taco units just producing inconsistent voltage? If I have to pull this I would consider dumping Taco for a more stable system. Any ideas? BTW I really like the comradery here.. you experts are not only helpful but, at times, pretty damn amusing too. Thanks in advance.

EdTheHeaterMan

I am wondering if the C on the old 404 is in the same location as the new version.
Do you have the metal cover 404 and the plastic cover 405?
@SteveSan is a Taco tech support guy and will have the answer. Can you take a picture of the wiring inside the box

hot_rod

If you need more Va, the Caleffi relays 6 zone come with two circuit breaker protected 40Va transformers.
3&4 valve configurations can snap in an additional transformer also.

Transformers are in parallel, so 80 Va across the connections, for power hungry zone valves, or multiple actuators on one output.

Screw terminals for all the wires, no wire nuts required.

EdTheHeaterMan

If you had the old metal cover ZVC 404 and you needed to connect a C wire for a Nest thermostat, you needed to bet that C wire from the bottom connectors to the zone valves. The newer ZVC 404 and 405, with the plastic cover, have C connection at the top thermostat location. So when you say "Installed wire exactly same config moving priority DHW to zone 5 and turned on the priority switch." That may be your problem.

Try wiring the
thermostat R to the Taco 405 R at the top.
thermostat W to the Taco 405 W at the top
thermostat C to the Taco 405 C at the top
Since the wiring has been updated from tho old to the new, the R and W on the Taco may be reversed, in reference to to old control that had only 2 terminals labeled T T. On the old control one of the T terminals was R and the other T terminal was W. And it is important to get it correct. It will not work if you get that reversed.
With the new control the labels are now R, W and C so not to make a mistake.

BCApollo

EdTheHeaterMan: Both controllers are plastic case. Should have added that I verified all pin IDs locations including RWC before connecting anything. I was stating that all wires were successfully migrated to new box. 
Both controllers are exactly the same, save an additional zone and a secondary transformer. I know what you're saying as I saw the differences when I successfully swapped out my first metal 404 in 2011. 

HotRod: I appreciate the info on the Caleffi. The Taco 405 has two 40va transformers as well. I don't think my problem is current as it fails to work with just a single zone during my debugging. My 404 worked with a single 40va transformer 

SteveSan

BCApollo Please give me a call at your earliest convenience, sent you a PM. What if you jump R-W, does that open the valve? If there is an internal battery on the Nest, they would need to be pre-charge before connecting to the control. For reference is the wiring diagram when using the "older metal cover" needing a com connection.

jcmathis44

SteveSan said:

BCApollo Please give me a call at your earliest convenience, sent you a PM. What if you jump R-W, does that open the valve? If there is an internal battery on the Nest, they would need to be pre-charge before connecting to the control. For reference is the wiring diagram when using the "older metal cover" needing a com connection.

@SteveSan with the diagram you listed above for the metal cover 405, where would I pull the com from for the Nest Power Connector? Would I tie into one of the Transformer leads or pull from somewhere else?

SteveSan

The Com would connect to the #1 terminal at the zone valve connection on the ZVC control as shown in the diagram. So if wiring the t-stat for zone 1 you would bring it down to the #1 zone valve terminal on zone 1, then continue to do the same for the other zones.

Teemok

Either power from the ZVC's R is being send back down from the thermostats on W or not. If you disconnect just the W wire and read the voltage between it (the wire from the thermostat) and the ZVC's C terminal you should see 24-27v. I have seen bad switches in a stat close and I'll get voltage but not enough current to trigger the control. Nests charging could be a thing. The Ecobee has a long start up wait period before it will close it's switch. It tells you it's calibrating. Does the nest indicate it is calling for heat?

jcmathis44

jcmathis44 said:

SteveSan said:

BCApollo Please give me a call at your earliest convenience, sent you a PM. What if you jump R-W, does that open the valve? If there is an internal battery on the Nest, they would need to be pre-charge before connecting to the control. For reference is the wiring diagram when using the "older metal cover" needing a com connection.

@SteveSan with the diagram you listed above for the metal cover 405, where would I pull the com from for the Nest Power Connector? Would I tie into one of the Transformer leads or pull from somewhere else?

I did what you suggested with connecting C to terminal 1 on zone one but I am still getting no power message on nest. Any suggestions on what to check?

Teemok

Those are zone valve terminals. There's no C there. Upper left is where the transformer normally feeds the board. Often yellow and red. If you measure voltage between any R thermostat terminal and each of the transformer wires, one wire will have no voltage, that's not C. The wire that has voltage is C.

Edit: Thanks for the correction ED. I always put the red (R) wire in the first T and the white (W) in the second T. Then the above is true. I used to have the correct 24v wire color for C in memory but It gone. I'd have to test it out like you.
C is the wire at the 24v transformer connector that has 24-26v between it and R (first T with red wire in it) The other 24V transformer wire (not C) will have no voltage between it and R(first T)

Alan (California Radiant) Forbes

I'm pretty sure @SteveSan is correct, i.e. that Terminal #1 is a good place to land for a "C" connection.

@jcmathis44 Do you get 24 volts at the thermostat? Is the thermostat fully charged?

Teemok

I stand corrected. I've not checked it. Sorry @steveSan

I have NEVER found a thermostats C wire hooked to a #1 zone valve terminal. I'll test it next old board I find. I'm betting it depends on which of the TT's the red (R) wire is in.

HomerJSmith

My 2 cents worth which is exactly worth 2 cents. Interesting comments, close but no cigar.
On a 24 Vac transformer, the transformer has 110Vac and 24Vac leads. For current to flow, there must be a load between the 24Vac leads. That load is the charging circuit in the Nest and the relay on the ZVC405. Let's call one 24V lead R and the other C. Trans R goes to Rh on the Nest. W1on the Nest goes to Trans C. When the switch in the Nest closes power goes to the relay in the 405, closing the relay points sending power to the Zone Valve motor, opening the ZV. Rh on the Nest has power, but no current flow, and goes to the charging circuit in the Nest, charging the battery, and from the charging circuit to Trans C then current flows in that circuit.
Without a charged battery, a charging circuit, or current flow thru W1 from Trans R, the Nest is an expensive door stop. What to do? First, and I said First, make sure you have power at the 405. Ya, I know what you're thinking, "Of course, there's power at the 405, Homer. I just didn't fall off the turnip truck." Secondly, check all the wiring connections to make sure you have a circuit from the Trans R to the R on the 405 to the Rh on the Nest (red wire). The Trans C that goes to the relay on the 405 to the W connection on the 405 to the W1 on the Nest (white wire), that would be a circuit. The Trans C is directly connected to the C on the 405. The green wire connects the 405 C terminal to the C on the Nest, that's a charging circuit. Barring a defective Nest or 405, it will work.
You can check you're wiring with a cheap digital VOM that you can buy at Home Depot or Harbor Freight for peanuts. (actually you want to buy it with money and not peanuts) Between R & C on the 405, you should get about 26Vac, between W & C you should get 0V, between R & W you would get some voltage up to 26Vac.
This will all be covered in my next book.

EdTheHeaterMan

@jcmathis44 , did I read this correctly? are your Taco ZVC controls the plastic cover models? You may not need to follow the metal cover instructions. Can you take a picture of the control a little further back so I can see ALL the wires? top and bottom, thermostat and zone valve, and any other wires to the boiler too.

HomerJSmith

You should be able to pre-charge the Nest battery with a jumper to any 24Vac transformer and a jumper wires. To do it with the 405, jump between the R connection to the Rh on the nest and jump between the C connection to the C on the Nest.
Use your VOM to check the continuity on the Fuses on the 405. Remove the fuses and use the continuity or ohm setting. Continuity will beep and Ohms will be close to 0 ohms.

EdTheHeaterMan

HomerJSmith said:

You should be able to pre-charge the Nest battery with a jumper to any 24Vac transformer and a jumper wires. To do it with the 405, jump between the R connection to the Rh on the nest and jump between the C connection to the C on the Nest.

The Nest has a USB charging port like you would find on an android phone

Teemok

@HomerJSmith That's what I tried to say.


Edit: @EdTheHeaterMan I get it, the thermostat connections on the board are marked T T and not R W so my instructions made no sense.

SteveSan

On the older, metal cover ZVC’s when adding a common wire you must make sure theR from the t-stat goes to the right T connection and W from the t-stat goes to the left T connection. The com would share the same terminal with the #1 zone valve wire for that zone. If you measure voltage between the right T and the #1 zone valve, you should get 24v.

jcmathis44

Thank you all for the replies. I will try again tomorrow and let you know the results.

SteveSan

@jcmathis44 If you can be in front of the control tomorrow, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000 Just ask for Technical Services.

jcmathis44

@SteveSan thank you very much. The issue was the R from thermostat was on left terminal so I switched that and all is good.

SteveSan

Glad to hear all is good. Happy Thanksgiving to you and your family.

jcmathis44

Happy Thanksgiving to all of you.