Taco ZVC406 Controller- Boiler Not Firing With Solid Yellow & Red Lights

Jetsetter99

I have a 15 zone radiant heat system with 3 older Taco ZVC406 controllers with metal covers (no C terminal for the thermostat connection only TT terminals). On one of the controllers, I noticed that when there is a call for heat on any of the 6 zones, the yellow light comes on, then the red but the boiler does not come on. The code on the Prestige Boiler starts with 0 which means no call for heat. I don't think the zone valves are a problem (they were all replaced last spring, before I bought the house,(Honeywell 4 wire)) and if I disconnect the thermostats and jump TT terminals I have the same problem, so not the thermostats. I am pretty sure the zone valves are opening. I moved one of the zones on the problem controller to an empty zone on one of the other controllers and it works as it should. Call for heat, yellow on, red on, boiler on. On the problem controller, If I turn up a zone on a different controller, the boiler fires up and I can feel that the piping on the zone on the problem controller begins to get hot and the room does heat as long as one of the other zones on the fully functioning controllers is calling for heat. I noticed that the 2 Ecobee Smart Thermostats on one of the controllers that functions fine has the end switch terminals 3&4 jumpered. Other zones on that same controller do not have jumpers and work as designed. Any ideas to troubleshoot the non-boiler firing controller would be much appreciated. Thanks, Ron

pedmec

Is the X-X terminals wired to the boiler or daisy chained to one of the other controls? I would look there and then check for a broken or disconnected wire.

Jetsetter99

Here is photo of the controllers. The problem child is the middle one with the "Bunk Room" zone where I moved zone 4 to the controller below. I am not sure what the X-X terminals are. Thanks.

Jetsetter99

If the X-X terminals are the Main End Switch connections, it looks like the are all tied together in the lowest controller. Here is the photo I forgot to attach.

20250127_102537.jpg

EdTheHeaterMan

The yellow circles are the X X terminals  and it appears that they are all connected.   You can pull on each wire individually to see if the wire is loose. If the wire is loose then you need a very small screwdriver to fasten the wire more securely. Like the screw drivers that you tighten the hinge on eyeglasses.  

Also check the blue wire nut connections to see if any wires are loose there. 

Click on the photo to zoom in.

EdTheHeaterMan

Follow up question/observation:

does the heater operate when zones from the other two ZVC-406 panels operate? Like maybe the Low Master Ba -1 zone or Ecobee -2 at the very top?

Jetsetter99

Yes all other zones on the other 2 controllers activate the boiler, so the end zone relay must be OK. I now suspect that one of the wires on the main end zone terminals (the ones you circled) or where they are all tied together in the lower controller, (with wire nuts that may be too small) may have a loose connection. I had a problem with both Ecobee thermostats losing power a couple of weeks ago and one of the wire nuts supplying power to the external transformer was not tight.

Any idea why terminals 3&4 on end switches on the Ecobee zones are jumpered?

I really appreciate your help. Thanks

EdTheHeaterMan

When you have a thermostat connected to a zone that does not have a zone valve connected to 1, 2, 3, & 4 then that thermostat will not operate anything unless you have 3 & 4 jumped. That is what the end switch on a zone valve does. It connects 3 & 4 when the valve opens, to signal the heater or circulator to operate. If there is no zone valve the connect 3 and 4, then you install a jumper. With a jumper in place whenever one of the Ecobee thermostats call for heat, the burners and or circulator will operate.

There must be something different about the Ecobee thermostat zones that do not require a zone valve to open but need the burner and pump to operate. Perhaps they operate a heating coil in a duct system somewhere.

Jetsetter99

I know the upper Ecobee actually operates 2 zone valves that are tied together. The lower Ecobee seems to operate only 1 zone valve. There is no duct system in the house. 2 of the zone valves are labeled Ecobee up and Ecobee down. I traced the wires on the upper Ecobee zone valve and found them tied to another zone valve on the upper floor. The Ecobees are using Fast-Stat common makers to power the thermostats. You can see them in the photos. I added photos taken earlier today here.

TACO Panel - 3.jpg

TACO Panel - 1.jpg

TACO Panel - 2.jpg

If this fix doesn't work, I'm thinking that the controller is not sending a signal from the main end switch (lower left) to the boiler relay. What should my meter read (24 VAC?) across the x-x terminals or am I just looking for continuity? Unfortunately I am not at the house now. I will be up there next weekend to try this troubleshooting fix. Thanks.

EdTheHeaterMan

Where do the wires that connect to the Ecobee zone thermostat location come from? Red and Green on zone 2 (yellow box) and the red and white (Yellow highlight) that connect to the Green and Blue under the wire nuts in the (yellow box) from zone 4. I don't believe that the thermostat is at the other end of those wires.

By not knowing what is at the other end of those wires, there is no way to tell what is going on there. I triec to trace them in the older photo that has all 3 Taco ZVC boxes in one picture, but they get covered by other wires and there is no way to see where they come from.

But what ever is connected to top box zone 2 and 4 does not operate a zone valve… I can guarantee that. perhaps a zone valve end switch operates the zone 2 thermostat at the top ZVC Box.

To be more clear. how many thermostats do you have in your home. since you have 3 ZVC 406 control boxes and only 4 of the thermostat locations are not used, that would mean that you have 14 thermostats. But I think that you may not have that many thermostats. since you have one thermostat operate 2 zone valves and also go to the Ecobee zone. That would be one thermostat to account for 3 thermostat positions.

Does that make sense? It is ashame that the person that did all that wiring didn't leave a written diagram for the future service guys and gals. But with just the few square feet of space that are in your photos, there is no way to know what thermostat goes where.

Jetsetter99

Because of the Fast- Stats, there is power from an external transformer that is fed thru the Fast-Stat that then uses the original T T wires, red and white, to feed a sender that creates a C wire to power the thermostat and a new R and W wire at the thermostat. The sender is suspended inside the wall behind the thermostat. I added 2 more smart wifi thermostats using Fast-Stat C makers (Bunk Room and Lower Master) as well as a 3rd smart thermostat for the Master where I had an unused wire so I was able to get power from the controller by hooking up the 3rd (blue) wire to T1 where the zone valve connects and reversing the red/white wires on the T T terminals. Taco Support told me how to do this and it works. You can see the C wire connection on the lower controller, Zone 4. It is the brown wire that is actually connected to the blue wire in the middle controller.

There are 14 operable thermostats in the house and 15 zone valves. There is 1 thermostat that is inoperable (no power to it) but it's zone valve is wired into the upper Ecobee. I have verified that by tracing the wires from the zone valve to where it connects with the Upper Ecobee zone valve outside the controller. That is probably why the Ecobees are jumpered, so when there is a call for heat, the boiler comes on. The Ecobees must be directly wired to the yellow wires on the zone valves, and the red wires (end switch) from those zone valves probably are not connected to anything. The lower Ecobee must be connected the same way, but operates only 1 zone valve. No idea why they did it this way, but they work. I will look at all of this next week with a new perspective and let you know what happens. I really appreciate you taking your time to look at this. Thanks again. Ron

109A_5

Hello Jetsetter99,

This should be super easy to troubleshoot. Sticking to the original post. The thermostats call for heat, the ZVC406 acknowledges that call (Yellow LED). The ZVC406 then energizes the zone valve, the zone valve end switch closes and the ZVC406 acknowledges that (Red LED).

Then the Main End Switch (relay contacts) should close activating the boiler. However by your description it does not. So jumper the screw terminals (Orange annotation in image below) the boiler should fire up, if so that defines the ZVC406 is defective. Probably the relay contacts or a failing solder joint on the circuit board. If the boiler does not fire there is a wiring issue since the other ZVC406 work as expected.

One other thing you can do is move the wires from the Orange annotation in image below to the Green annotation terminals in image below, they have the same function and look unused in your system.

Another curiosity for me if the NORMAL / RESET switch has oxidized and even though it looks like it is in the correct position electrically it may not be.

EdTheHeaterMan

@109A_5 has an interesting point. The control that is not bringing on the boiler on does have an "Extra End Switch" that may be used as long as the switches for priority and ODR are not turned on. The photo indicates that the DHW priority is OFF and the reset switch is set to NORMAL, so that extra end switch should work for you.

Good Call 109A_5

Jetsetter99

I called Taco earlier this morning because I had the same thought about the Extra End Switch. They verified that it is exactly the same as the Main End Switch. Both are dry switches and when activated they said there should be continuity across the terminals. I will definitely try troubleshooting the end switch as you suggest. Hopefully just a bad wire connection or bad relay on the main end switch. I will post the results next weekend when I go to the house. Thanks for everyone's help. 🤞🤞🤞

Jetsetter99

I went to the house today and did some testing and found that the white wire from the end switch of the problem controller was barely into the wire nut where all 3 controller's end switches were connected together (below the bottom controller). That white wire was only stripped about 1/4", and I suspect that this has been an issue forever, as I doubt there was any reason to have rewired this connection since the system was originally installed in 2007. I suspect the zones on this controller never worked properly. Anyway, I made a solid connection and all of the zones function as designed. Tstat calls for heat, boiler comes on.

Again thanks so much for your help. I really appreciate it.

Ron