Taco SR 506 6 zone switching relay

nonobar · January 3

The Taco sr 506 contains 8 double pole double throw relays. 7 of them are 24v relays. The 8th relay is a 120 volt relay. Can anyone tell me what is the function of this DPDT relay? What do the two poles switch between?

dko · January 3

nonobar · January 4

The ZC and ZR are shorted on the SR Board. They are connected to nothing else. Does that mean that the 120 volt relay is not necessary?

nonobar · January 4

nonobar · January 4

Infrequently my heat goes out. (the boiler won't fire). I rap on the back of the Taco box and the heat comes on. I think I have a faulty relay. I've changed the 24 volt relay which seems to activate the XX on the end switch. That helped for a few months, but now the problem is back. I wonder if my problem may lie in the 120 volt relay (far left in the picture).

oreo123 · January 4

Had similar issue a few years ago.

in the pix no zone is calling - as there is no led lite on the right side.

1. turn a thermostat way up so its calling for heat.
2. with a voltmeter set on a.c. measure the upper left side to see if you have the 24 volts a.c. (If not then something is dragging it down or the stepdown transformer is kaput.)
3. with a voltmeter set on a.c. measure between x and x. If its zero volts then its calling for heat. If its ~ 23 volts its not calling for heat.
4. In my controller which was slightly different than yours there is a relay (I think it was K 1 on mine) that has contacts. If its 4 contacts then its one relay and 6 contacts two relays. When any zone calls for heat you should see that relay close.
5. If you can visually see the relay close I would tap on it slightly with a plastic screwdriver. IF it works I would suspect a cold solder joint on the connector. Turn off a.c. power to it and pull the relay to see if the contact in the plastic board connector is o.k. It should not wiggle which could be a cold solder joint.
6. its possible there is an issue with the contacts on the relay. Take an erasor and carefully brush the relay's contacts. Do not bend the contacts. Clear the eraser debris and try it.
7. My issue wound up being the connector on the board.

109A_5 · January 5

Hello @nonobar,

nonobar said:
The ZC and ZR are shorted on the SR Board. They are connected to nothing else. Does that mean that the 120 volt relay is not necessary?

Removing that relay you may loose circulator power. Depends how the relay logic is wired.

109A_5 · January 5

Hello @nonobar,

nonobar said:
Infrequently my heat goes out. (the boiler won't fire). I rap on the back of the Taco box and the heat comes on. I think I have a faulty relay. I've changed the 24 volt relay which seems to activate the XX on the end switch. That helped for a few months, but now the problem is back. I wonder if my problem may lie in the 120 volt relay (far left in the picture).

As @oreo123 said, you may have a failing solder joint where the relay or the relay socket is soldered to the circuit board. Or a failing solder joint at the terminal strip and the circuit board. Inspect with magnification. All zones fail when it acts up ?

nonobar · January 5

Yes, all zones fail. Aren't the solder connections on the back side of the board?

nonobar · January 5

Can you easily resolder a cold joint, or should I simply replace the board?

109A_5 · January 5

Hello @nonobar,

nonobar said:
Yes, all zones fail. Aren't the solder connections on the back side of the board?

Yes the solder joints are on the back side of the board.

nonobar said:
Can you easily resolder a cold joint, or should I simply replace the board?

Solder joints are easily re-soldered if you have the equipment and skills. The soldering equipment is way less expensive than a new controller. You could replace the controller if the confidence is high that it is the actual problem.

oreo123 · January 5

Do you have 24 volts a.c. ? There is a measuring point at left side top of board.

Do you see the relay closing? If yes then measure a.c. voltage across x and x. You do not need to remove the wires. If its zero volts the boiler should fire up. If its ` 23 volts and relay is closed then the problem is likely between the relay socket and one of the x terminals.

shut off power and remove relay. Look at the relay's socket carefully. I had 2 new controllers where neither one would pull in boiler. Wound up being that relay connector that was soldered onto the board.

How handy are you with a multi meter? Me, with the power off, I would ohm out the back of the socket to both x's. It should be a short. If its open there is a broken connection someplace.

Being this is somewhat intermittent I would tap gently on the relay. And wiggle it gently up, down, left, right to see if it works.

If you decide to resolder it you need a solder sucker, solder, and a soldering iron. Wear glasses as solder can splatter.

The wiring for the circ pumps. The far right zone I think that the black and white wires are swapped. I do not think that will make a difference with the current problem.

Here are my questions: do you see the relay close, do you have 24 volts, assuming its calling for heat what is the a.c. voltage between x and x terminals (wires can remain on)?

nonobar · January 5

I am good with a multimeter. I will check out the connections, but without seeing the back of the board, I'm not sure which of the six relay terminals go to the X-X. I have not measured the 24 volts, but I'm sure it's there because all six leds are lit and all the circulators are running. I don't have the dexterity or the space to see if the relay is closing. Tapping on the relay does start the furnace. I am still not sure what is the function of the two DPDT switches on the 120 volt relay. I can't believe that TACO does not share a proper schematic diagram of the board.

nonobar · January 5

For that matter, I wonder what the function of the double poles on the 2nd leftmost relay is (other than providing a short to the X-X). In other words, what does the second double throw switch do?

hot_rod · January 5

If you do remove the board and flip it over you will see all the solder traces, so you can see what relay terminal wires to what.

oreo123 · January 5

leds are prob running on less than 5 volts. without seeing entire wiring schematic its more difficult to guess.

In this order: verify that you have 24 volts a.c. (that could be powering certain relays). In your pix there is a test point in the upper left side;

measure ac volts between x and x;

Assuming at least one zone is calling for heat - power on and off and see if the questionable relay is closing. You should be able to compare it to the relay for whatever zone is calling.

Joe Mattiello · January 5

120 volt relay is for ZC/ZR terminals which are 120 volts. these terminal are normally used in conjunction with ZC/ZR terminals located on a triple aquastat used on a tankless boiler application. If it's a cold start boiler using TT input terminals on boiler aquastat, the jumper stays in place to jump 120 volts from ZR (120V out), terminal to ZC (120V input). hopefully this helps

nonobar · January 5

Joe Mattiello said:
120 volt relay is for ZC/ZR terminals which are 120 volts. these terminal are normally used in conjunction with ZC/ZR terminals located on a triple aquastat used on a tankless boiler application. If it's a cold start boiler using TT input terminals on boiler aquastat, the jumper stays in place to jump 120 volts from ZR (120V out), terminal to ZC (120V input). hopefully this helps

what would happen if I yanked the 120 volt relay out of the board?

109A_5 · January 5

Hello @nonobar,

nonobar said:
what would happen if I yanked the 120 volt relay out of the board?

Try it, just don't zap your self in doing so. I doubt you will damage anything as long as you don't leave the test unattended. My bet is you will loose circulator power or control.
As far as the DPDT maybe they are switching the Hot and the Neutral or paralleled Hot contacts for redundancy for the circulators. Maybe the second set of contacts just controls the indication of the circulator relay is active if there is two indicators for each zone, Call and circulator activation. If you really want to know, on a warm day or off season reverse engineer it, not much there, then you will know for sure.
Almost no manufactures supply schematics any more (unless it is contractual or the like). If "Right to Repair" becomes the law of the land you may see more schematics. Some things are considered 'Not User Serviceable' and the HVAC industry just changes the box when they think they are defective. So why would a schematic be needed ?

With the infrequent nature of the intermittent or failed normal operation and the cure by taping it, I am betting on a bad solder joint.

oreo123 said:
leds are prob running on less than 5 volts. without seeing entire wiring schematic its more difficult to guess.

I doubt there is 5 VDC even on that board. I don't see any logic chips, regulators, filter capacitors, etc. The LEDs probably just run off of current limited rectified 24 VAC, I do see extra diodes and resistors.

To me the input 120 VAC Hot and Neutral are connected properly. looks like the colors are intentionally fliped by design

Taco SR 506 6 zone switching relay

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