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Fuel smart 3250-plus Hydrostat Operation



I'm not sure my hydrostat is working properly concerning the thermal targeting mode.

My system is as  follows:
Riello F5 Oil Burner
Buderus G115WS/4 boiler
Hydronic coil attached to rheem air handler in attic (with duct work for heat and a/c)
Honeywell T4 thermostat 24V
Taco SR503 relay with 3 taco 0015 circulators 
Buderus SU54 indirect

Zone 3 is the indirect heater 
Zone 1 is heating (hydronic coil)
Zone 2 is a future

Hydrostat settings are high limit 180, low limit off, eco mode 1

I have everything wired exactly as diagram 5 on page 5 of the 3250 plus manual
see above link 

Initially the system was not wired with the ZR connection to the hot of the indirect circulator and the system would consistently fire up to 145 degrees and the economy light would be active on the control. Therefore the indirect zone would never reach the high limit of 180 degrees 
However once the connection was made from ZR to the indirect circ hot terminal of the SR503 panel, the heat zone fires to the high limit of 180 on every call for heat and the green call for heat indicator light never shuts off. Even if there is not a call for heat and temp drops to 160 degrees the boiler will fire again up to 180. Even though the low limit is set to off. It is my understanding with low limit being off the boiler should not be maintaining temperature with no demand for heat or hot water. Am I wrong?
Also note that The zone/indirect switch is set to I.

Another question is why the green call for heat indicator light next to the temperature displayed on the Hydrostat no longer turns off once a call for heat is satisfied. None of my zones have an issue reaching set temperature. When there is a call for heat it is satisfied quickly from the pre purge setting on the Hydrostat. The boiler temp drops while the circulator is running when the zone is satisfied the boiler fires and returns the boiler to high limit of 180
The economy setting and light do not seem to engage on a call from the heat zone. 

When the indirect calls for heat the thermal targeting seems to engage set a high limit for 165 degrees and cycles as such 

After a few cycles the indirect zone does fire to the high limit of 180 

It is my understanding that the indirect zone should reach the high limit with how the system is wired and the heat zone should attempt to reach demand utilizing a lower temp as controlled by the thermal targeting feature. I even tried raising the economy mode and no change occured.

It seems like my system is working opposite of what is intended.

Does this operation seem normal?

I'm at a bit of a loss.

Please advise any suggestions 

Thank you 


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,590
    edited November 2022
    I never liked the Hydrostat controls. So if one were to give me trouble, I would replace it with a Honeywell L7224U. That was my personal preference.

    That said, I am wondering if your Taco SR-503 Zone 3 priority switch is set to ON or OFF? Should be ON

    Next, The ECO MODE can be set at 2 or 3 in your setup. I believe that the higher the number the more energy savings (lower operating limit temperature for heating zone 1 and 2) you will get. The idea is that with DHW the boiler goes to 180° to provide fastest recovery, and the other zones run at something less than 180°. Let me know if those changes make a difference.

    You will want to recheck your wiring to be sure the Taco SR-503 thermostat wires and the Priority Zone is correctly configured. The end switch X X terminals on the SR-503 are connected to the Hydrostat T T. The DHW pump and DHW temperature control are connected to Priority zone 3. The Hot from the Priority zone 3 is connected to ZR on the Hydrostat (Not the ZC)

    Taco has had several generations of the SR-503. Can you take a picture of the inside of your Taco SR-503 and post it. The one I have has the X X end switch terminals across the top row left side (unlike the diagram #5 you referred to). Other versions have more terminals and different configurations.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • BRBR
    BRBR Member Posts: 5
    Priority switch is set to ON 

    HVACNUT Member Posts: 5,752
    Is the jumper attached in location B? If so, remove it.
    If that's not it, what happens if you remove the wires from TT on the Hydrostat? Does the green light stay on? 
  • BRBR
    BRBR Member Posts: 5
    I’m not home at the moment but I have tried it with and without the jumper with no change in operation occurring.  Not even sure what the jumper does. 
  • BRBR
    BRBR Member Posts: 5

    HVACNUT Member Posts: 5,752
    Did you disconnect the wires from TT on the Hydrostat? Did the green light stay on?
    What's the 90 and 0 reading?
    Try shutting off the power for 5 minutes, then turn back on.
    Call Hydrostat. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,590
    edited November 2022
    Do you have the Hot and Common reversed on this control?

    On the SR503 control if you feed power to N and then use the common or neutral to feed H (this is reversed from the normal) then the zone 3 priority will send the power to the pump in the reverse. Priority 3 N will be the power side and Priority 3 H will be the common to power the pump.

    Now if you take that Priority 3 H (which is connected to the neutral) and connect it to the Hydrostat ZR you may have a problem. The Hydrostat ZR is looking for the Hot side of the circuit, and you are feeding it Neutral from the SR-503.

    Does anyone else see that?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • MikeAmann
    MikeAmann Member Posts: 988
    Good eye, Ed.
  • BRBR
    BRBR Member Posts: 5
    Thank you 
    you were the only one who caught this mistake 
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    BRBR said:

    Thank you 
    you were the only one who caught this mistake 

    He’s the only one that mentioned it.