Expanding Taco Relays

chernomorez

So, I have a 4-zone / 4-circulator system connected to Taco SR-504-4. Now, I'm adding another zone with it's own circulator. This zone would also need to have a common wire available for the thermostat What are my options for the relay:

• Discard SR-504-4 and get SR-506 or SR506-EXP (most expensive, but easiest to wire)
  
• Buy another SR-501/502 and wire it in parallel to the SR-504 (much cheaper, but is it even possible, or I must have the -EXP's)
  
• Same as above, but wire as master-slave.
  
• Any other options?
  

HVACNUT

What's connected between the SR504 and the aquastat? The XX dry contacts or ZR,ZC?
If using XX, an SR501 will be fine. Unless your thinking of adding another zone down the road. 

Ironman

Like this:

chernomorez

Thanks to all who replied. Ending up buying an open box sr506 on Ebay and selling my older sr504.