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Taco ZR403 vs. ZR404

Hi, Forgive me for being long winded.  

I have a boiler with an internal control that can run a system pump and a DHW pump. It has selectable priority so that power to the system pump is dropped when there is a DHW call if there is a call for heat (TT is closed) at the same time. This is basic stuff right? Same on Slant Fin / Buderus etc.

On my system I have two pumps and they are connected to a TACO SR503 Relay control. Using a Buderus concept diagram not specific to the SR503, I removed both jumpers and ran the System pump hot to the ZC terminal to get priority. This works (Pumps are off during a DHW call) but it seems the ZC terminal is now powering the circulators rather than the line in. ie, the relays are still closed but ZC is dead and the pumps do not run. This is consistent with the TACO literature. In fact there is a big fat WARNING about it.

I looked further into it and discovered another wiring diagram on the Buderus site which is specific to the SR503 and it wires the System pump hot to the Line in on the SR503. And HOT to the ZC terminal. This drops the whole SR control out when there is a DHW call and would require a lot of rewiring to get the end switches to fire the boiler doing it this way.

My question is, The SR404, SR403-EXP etc. appear to be more sophisticated devices than the SR503 in that the ZC terminal powers the relays through an extra 120VAC relay rather than the pumps and that when ZC power is dropped the relays will turn the circs off and ZC does not provide power to the pumps only the relays and the pumps are powered by the line in. Is that a true statement? Would the SR404 or SR403-EXP be a better control to use in this situation? OR Is there an easy way to fix this using the SR503? My system pump circuit on my boiler has limited capacity (5A) and Id rather not power two pumps from it. Also, there is no visible indication that priority is working on an SR503 wired this way, all the lights are still on.


  • Could you not,

    just wire your 503 with a direct input(from panel) power source, wire your heating circs the std. way,, then (leaving priority-jumpers in place),, wire the DWH (low-voltage circuit aquastat) in series with it?

    Just curious.  :-)

  • ChasMan
    ChasMan Member Posts: 459
    No Aquastat

    I don't have an aquastat. I would have to mimick it with another relay. Im trying to avoid the snapping as much as possible. All I have is DHW Power and System Pump Power.  
  • ChasMan
    ChasMan Member Posts: 459
    What About If??

    I put a small  relay with a 120 v coil and a NO contact on the DHW Circulator power line. Then use that contact to make / break the thermostat line on the priority zone? The downside I see with that is that the boiler will think their is a call for heat since the TACO box will close the end switch on what is now a DHW call.

  • Even more curious now,,,

    What make boiler is it?

    I have trouble understanding just how the boiler knows your DWH tank needs to be satisfied without an aquastat?
  • ChasMan
    ChasMan Member Posts: 459
    Slant Fin

    This is a Slant Fin EC13A. I ordered it with the optional Reset control which includes logic for running a system pump and a DHW pump. It knows the DHW temperature via a thermocouple that plugs into the board. The board has no connectors for traditional aquastat stuff like RC.
  • I`m not impressed with that technology,,,

    if you`re out of HW on a Friday night, try finding that T/C kicking around!

    I would be inclined to use a regular 503 & DWH immersion aquastat to suit my needs.
  • ChasMan
    ChasMan Member Posts: 459

    No doubt it could be better executed but it does have a manual override for if / when it goes haywire.

    I had another thought. Im just going to try and short the H and ZC terminals with a single pole relay with 120 v coil powered off of the boilers system pump. It will solve the main issue of relying on the internal control's relay to drive two pumps. ZC isolation relay tech is my name fore it. Hard to size a contact protection resistor with that variable load. The ideal way would be to have two relays, one for each cirrcuit. Im leaning towards making my own.
  • ChasMan
    ChasMan Member Posts: 459

    The little relay worked out well. I now have the isolation of ZC using this relay and power for the pumps no longer flows out of the control board. I spent a bit of time with the control and found a nice point on the board which can provide 5vdc high on a DHW call. Perhaps I should have used a PLC instead of a relay panel.
  • CMadatMe
    CMadatMe Member Posts: 3,086

    Need to use the SR Exp verision and add the PC600 card. This will give u extra end switch to run a domestic pump for priorty.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • ChasMan
    ChasMan Member Posts: 459
    I was...

    I was headed that way then I figured what the heck. I just put the relay in. I already spent the money on this control and it works fine. My boiler allready has post purge logic built in.
This discussion has been closed.