TACO SR 506

nonobar

When a thermostat calls for heat, does the relay in the TACO unit simply provide a short circuit at the XX terminals?

Ironman

The “XX” terminals are a set of dry contacts that close when a zone(s) calls for heat.

Typically, they should connect to “TT” on the boiler.

HVACNUT

And that's an older panel with the cube relays. Any reason why it's a question years after it was installed, or just curious? Because now I'm curious. 

Alan (California Radiant) Forbes

It also closes the circuit to the associated pump, turning it on.

HomerJSmith

The answer is yes. That thermostat wire that is connected to it goes to the TT connection in the boiler aquastat turning on the boiler.

nonobar

I have asked this question because I suspect that the relay which controls the XX terminals may be faulty. At random times, my burner may not fire even though the circulators are running. It happens infrequently. I can always start the system again by simply turning off the AC power, waiting a minute, then turning the power back on. I wonder if one of those relays is sticking at random times.

Alan (California Radiant) Forbes

Yes, I've had that happen as well. Some of the newer controls give you additional end switches, but this one doesn't.

I usually replace the control, but maybe there's a repair. Someone from Taco is bound to make an appearance here and let us know. @SteveSan or @Joe Mattiello

nonobar

Wouldn't the repair be to simply replace the faulty relay? Those OMRON LY2 relays are less than $20. Has anyone seen a schematic of this board? I can't figure out why there are 8 relays on the board. I only have 5 zones plus Hot Water.

What is the purpose of the metal clip on the leftmost relay?

psb75

Is the one with the metal clip a spare relay?

SteveSan

Is the one with the metal clip a spare relay?

The first one with the metal clip is a 120v for zc/zr. Second one is for the end switch ( 24v ) and 3-8 are for the zones starting with zone 1.

You can switch out one of the relays from 3-8 for the end switch ( 2nd ) to see if the relay is starting to go.

If you are still having issues with the end switch it might be just the age of the board and would need to be replaced with a newer SR506-4.

If you have any questions, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000

nonobar

Good question! I don't know the answer. I do know the leftmost relay is 120/110 VAC. The others are 24 VAC

nonobar

If the relay which controls the XX terminals is stuck open, will there be a voltage reading across the XX terminals?

Alan (California Radiant) Forbes

XX on the Taco board is a dry contact, but if the boiler is on, you might get a reading off the transformer in the boiler even though it's only one leg............I think. Never tried.

HomerJSmith

I have asked this question because I suspect that the relay which controls the XX terminals may be faulty. At random times, my burner may not fire even though the circulators are running.

I have some questions about that statement. Is it the wiring between the SR and the boiler aquastat, the relay in the aquastat, something in the boiler wiring, Is the boiler at high limit, etc. You could jump the XX terminal when the problem happens and see if the boiler fires. You could jump the TT connections in the boiler aquastat and see if it fires. You could check the continuity on the XX terminals with a continuity meter when this happens. Disconnect the current wires to the boiler when you do a continuity test. Touch the present at the XX termials wires to the boiler together and see if the boiler fires. You need to diagnosis this further.
Personally, I don't think it's the relay in the SR.

nonobar

Is one of these the boiler aquastat??? It's a Buderus GB142

HVACNUT

nonobar said:

Is one of these the boiler aquastat??? It's a Buderus GB142

Well, yes and no. You're boiler has temperature sensors rather than a conventional aquastat. 
However XX from the Taco panel should connect to position 3 (WA-Green) on the connection board.

If a circulator is running and XX is closed (working) then there should be 0 volts a XX. If XX is open (not working), it will read either 10vac or 24vac. Not sure which because Buderus uses a multi volt transformer. 

EdTheHeaterMan

nonobar said:

When a thermostat calls for heat, does the relay in the TACO unit simply provide a short circuit at the XX terminals?

I think I know what you are asking, BUT you are using the incorrect terminology.

A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit.

Your question should be "When a thermostat calls for heat, does the relay in the TACO unit simply provide a completed circuit or a set of dry open contacts to close?"

@109A_5 is the self proclaimed expert on short circuits. I'm surprised he has not weighed in on this post.

Film at 11

Duckbutter

nonobar said:

I have asked this question because I suspect that the relay which controls the XX terminals may be faulty. At random times, my burner may not fire even though the circulators are running. It happens infrequently. I can always start the system again by simply turning off the AC power, waiting a minute, then turning the power back on. I wonder if one of those relays is sticking at random times.

If your boiler is above low limit, it won't fire when a TT is calling for heat until it falls below that temp.

nonobar

Duckbutter said:

If your boiler is above low limit, it won't fire when a TT is calling for heat until it falls below that temp.

What would make the boiler go above "low limit"?

EdTheHeaterMan

I believe that @Duckbutter means High limit. The boiler you have does not have a low limit. It does modulate based on outdoor the return water temperature, and perhaps the outdoor temperature if you have one connected.

But to answer your query about why it might be above a temperature where the burner will not run... If zone DHW just finished a call for heat and zone 2 just started a call for heat, the boiler may do a self check to determine the water temperature at the supply and the return, and finds there is a high enough temperature in the system to wait for the burner to fire until the water temperature drops to a point where more heat is needed. It also depends on wether the DHW Circulator is wired to the boiler control or the zone control.

Duckbutter

Ed, I did mean "low", just hadn't read past the initial description, hadn't realized this is a modulating Buderus and assumed it was a standard boiler.

You perfectly illustrate my point though, if the water is already hot, it won't run while the circ's on.

nonobar

Duckbutter said:

Ed, I did mean "low", just hadn't read past the initial description, hadn't realized this is a modulating Buderus and assumed it was a standard boiler.

You perfectly illustrate my point though, if the water is already hot, it won't run while the circ's on.

The boiler fails to fire infrequently, and when it does, the cast iron baseboards are stone cold.