Taco SR506 Switching Relay: thermostat Calls for Heat but zone not activated

There is a relay missing on zone 4. What zone number are you having the problem with?

When the problem presents, does the Red LED for the problem zone light up?

Hello heatingrabbit,

Well not much left, is anything wiggle or tap tap sensitive ?

You say in the picture Zone 1 is calling, I don't see the Zone 1 LED on, just the power LED. Is there 24 VAC across the non-calling T-T terminals ? Maybe the Zone 6 priority circuit, maybe the right end relay(s) NC contact has gone open locking out all the other zones. Did you swap around the relays on the right side too ?

What are you talking about with voltage at ZR/ZC? It's jumped in the Taco panel. Is something connected to ZR/ZC in the aquastat?

Can you jump TT at Zone 4 and check for 24 volts where the relay coil contact would plug? If there's 24 volts there, the relays might be closing but there's no line voltage there to go across. Check the 120 volt coil relay with the strap on it on the left.

" I also measure the voltage of the relay coil terminal when zone 1 is called. The coil of ZC ZR got high voltage (~14V) to activate but all other relay coil including zone 1 got low voltage (~0.8V). "

The 14 volts measured without even the load of the relay coil in the circuit may mean a poor connection, a relay contact that is tired, worn or oxidized or an oxidized switch contact from never being used. The 7 relays are 24 VAC coils. There is not much on that circuit board, it it was a poor solder joint you probably could physically agitate it by flexing things.

I would use the ohmmeter function to test the NC contacts of the 120 VAC relay, then verify the coil is 120 VAC and if you can safely apply 120 VAC to the coil and test the NO contacts.

That means that the ZC ZR relay is clicking. do you hear it clicking or see the clapper inside the plastic moving when you send the call for heat?

You can also try the 120 VAC coil to see if that clicks when it is powered on. I have put space connectors on the end of an lamp cord and placed them on the relay coil then plugged the other end of lamp cord into a wall socket to see if the 120 VAC coil relay is working.

The coil in both relays are working. but the relay contacts may not be connecting. you can try to simulate the contact action by removing the relay and connecting the terminal in the socket with a jumper wire.

In order to get heat for now, you can just connect the the space heat circulator pumps to run constant. Then when ever a zone calls for heat the X X will operate the burner. you may over heat your home but at least the pipes won't freeze. When you want DHW to charge up the tank, you can run the pumps for the water heater(s)

I did some research and I found a picture of the back side of the circuit board of a similar unit to yours. This allowed me to better understand how that board actually works.

An internet description for some foundation;

" When there is a demand for heat from the thermostat, ZR becomes hot, sending 120 volts to the triple aquastat. When the boiler temperature rises above the minimum temperature setting (typically 140°), ZC becomes hot, sending 120 volts to the SR board and powering the relay for the circulators. "

I think if you pull out the Zone 6 relay you will find there is an extra contact in the socket used for providing the low side path for Zone 1 through Zone 5 relay coils when Zone 6 is inactive. When the priority switch is set to Off the switch is actually closed and provides an alternate low side path for Zone 1 through Zone 5 relay coils. When the priority switch is set to On the switch is actually open and removes the alternate low side path for Zone 1 through Zone 5 relay coils, so when the Zone 6 relay is energized the low side path is open disabling Zone 1 through Zone 5 relay coils (the priority).

The relay second from the left does two things during any T-T call, an isolated X-X contact closure and also puts 120 VAC on the ZR terminal. The left most relay's purpose, when de-energized, is to disable all Zone circulators if the boiler temperature is too low. Since there is no triple aquastat in your system, there is just a jumper from ZR to ZC which energizes the 120VAC coil of the left most relay with any T-T call.

I think you will find the NO (Normally Open) contacts of that left most relay are bad or there is a failed solder joint on the circuit board under that relay. If these NO relay contacts don't close there is no low side connection back to the 24 VAC to energize any of the Zone relay coils and their associated LED status indicators. Maybe you can inspect the relay contacts through the clear relay cover.

As a test see picture, don't damage the socket contacts. Shut the AC power off first !!!

" Does this behavior suggest the relays are fine? "

No, intermittent circuit connections are often agitated by vibration and/or heat. It could be the relay contacts or the solder joints and in a very rare occasion a defect or crack in the copper foil of the circuit board.

The relay is probably the issue, but solder joints fail too.

To bypass the relay. Turn off the power.

Remove the relay and insert a very thin wire (the thickness of the relay contact pin) into the socket OR in this case since I believe they are only using the NO contacts, if you can't find a thin enough wire to insert into the socket, carefully, securely rap a short piece of thermostat wire around the two relay pins up near the base of the relay (as illustrated below) and there should be enough of the pin left to make contact with the socket when the relay is re-inserted. The added wire should only touch the desires pins, touching the Coil pins would be bad since there is 120 VAC there !!!

Bottom view of the relay

The solder joints can be inspected rather easily. Turn off the power. Disconnect the thermostat wires, remove the nuts on the left side and gently squeeze the barb on the plastic stand-offs and lift the board just beyond the barb, when all the stand-off barbs are released the board can be raised off all the stand-offs and folded out with the AC wires attached. Or disconnect the AC wires too if that is easier for you. Inspect for cracked solder joints where the relay socket is soldered to the board.

I'd at least change the left relay if you don't want to fool around with jumpers and/or further inspection. With a new unit you can't do much, or any, easy swap-tronics everything is soldered to the board and they added more solid state electronics, capacitors fail. Personally I rather have the older unit, and I can repair electronics but swapping socketed relays is so much easier.

LY2N-AC110/120

or

LY2-AC110/120

Looking at the data sheet the relay with the 'N', LY2N-AC110/120 has an indicator that the coil is energized, in this case probably an LED, with some relays it just appears as a mechanical color change.

I'm a little confused with;

" Created a jumped relay like this.

Will try this on next time the system fails. "

That left most relay is unique with the 120 VAC coil did you put the wire on it then take it back off ? Did you use a 24 Volt relay as a form or template to make the jumper ?

The only thing I don't like about the way you did it is the twisted wire, since it makes the wire twice as tall, reducing the effective pin length, the relay may not seat into the socket correctly, however it may be fine that way. If it rocks, keep it square or parallel to the socket or slightly justify it towards the coil pins a bit since I believe the the NC pins are not used. The goal is to make a valid test and not to introduce other things that can also be intermittent.

OK, I see at least that guarantees some heat if you can't be there to babysit it and you can get some sleep too.

I think I would just replace that 120 relay. It cycles on every new T-T call unless there is presently an active call. so it probably cycles the most (compared to the others) and just may be tired.

The funny thing is, for your system it appears that relay does not do much except waste a bit of electricity and possibly cause grief.