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How to wire up a Como OT thermostat to Vitodens 100 with secondary circulator

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satwar
satwar Member Posts: 24
edited May 2017 in THE MAIN WALL
The installation manual shows a 2 wire connection directly between the thermostat and the boiler. The Taco SR501 switching relay provides power to the secondary circulator with a standard 2 wire thermostat, but it can not be wired the same with the Como OT because there must be a continuous connection between the thermostat and boiler. There would seem to be two control power sources (boiler and SR501) which probably don't like being in the same circuit. It would seem that I need a way of getting the control power from the boiler into the switching relay control circuit without using the existing SR501 control power source.

Anyone seen this implemented with a pre-built relay box, or is it better to just build a custom box ?
Robert O'Brien

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  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    I'm not entirely following. Como OT?
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
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    Single heating zone? If so, wire secondary circulator to primary pump terminal. A heat call from Como will power both. The 501 and existing thermostat become useless
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
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    If you want to retain existing thermostat, power the 501 from Vitodens pump terminal.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • newagedawn
    newagedawn Member Posts: 586
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    ive used the 501 to control both pumps, even called taco to find out if the relay could handle the extra load and they said ya as long as the amp capacity is within the 501 range, i used a grundfos on primary and and taco sr3450 on secondary and the relay worked fine all winter
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • satwar
    satwar Member Posts: 24
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    Quote: Single heating zone? If so, wire secondary circulator to primary pump terminal. A heat call from Como will power both. The 501 and existing thermostat become useless

    Good answer, thank you Robert.
  • satwar
    satwar Member Posts: 24
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    I apologize that I didn't frame the question correctly. The boiler is a Vitodens 100 with Combiplus DHW kit. In this arrangement the primary circulator operates with both a DHW call and space heating call, and a 3-way valve decides whether the heat will be applied to the DHW circuit or space heating circuit. The secondary circulator (space heating circuit) only operates during a space heating call.

    The current thermostat wiring runs to a Taco SR501 (Switch Relay) which electrically isolates the thermostat from the boiler but passes the call for space heating using a switch relay (SR501). The Como OT thermostat, is required to be directly wired to the boiler,
    so that it may communicate with the boiler using OT protocol. Although the SR501 appropriately applies power to the secondary circulator when the call for heat is given, it is not able to provide a continuous electrical connection between the thermostat and boiler.

    At it's core my problem must be a somewhat common problem when using a modulating thermostat to control a modulating boiler.
  • Jamie Hall
    Jamie Hall Member Posts: 23,526
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    As I read the literature for the thing, you've got a problem. The Como OT is not a simple on/off thermostat. Rather, it sends (and receives) a digital signal for information to (and from) the boiler. Presumably that Vitodens is equipped to respond appropriately.

    That said -- you're out of luck. That thermostat must, as it says, be connected directly and continuously to the boiler controls. They work together and talk to each other all the time. The Vitodens boiler together with the selections on the thermostat control whether or not the primary pump operates.

    I might add -- pay attention to the wiring instructions. That wire in between the two has to be either the Como wire, or has to conform: that is, in most installations, a shielded twisted pair with no splices, with the shield grounded to a true ground at one end and one end only. You might get really lucky and be able to use normal two wire
    wiring, but in a modern house I would expect there to be too much EMI for that to work reliably. Remember -- you are carrying a digital signal!

    Now... you might get away with operating your secondary circulator off a relay powered by whatever signal is controlling that three way valve; that is, if the valve is switched for heating, the secondary circulator would run.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Solid_Fuel_Man
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
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  • satwar
    satwar Member Posts: 24
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    Yes Robert, but the wiring diagram doesn't show the secondary circulator, which becomes a problem when you can't use the thermostat wire and switch relay (SR501) to power the secondary circulator on a space heating call.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
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    The Como will run the boiler off the curve, no need for the existing thermostat to interface with the boiler at all. You can use an ECM circulator and leave it powered 24/7 with no conventional thermostat at all. Or use the existing thermostat as a "backstop"
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Jamie Hall
    Jamie Hall Member Posts: 23,526
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    To look at it another way -- the Como OT must be connected directly to, and be the only control for, the Vitodens and its pump and valves and burner. No options. However, you can put whatever other controls you want on other pumps or valves, and can control those from the Vitodens pump and valves with suitable relays (not directly) if you like, so long as the relays are sensitive (take very little power to operate).

    Now... unless you are rather careful and thoughtful about how those other controls operate and what they actually do, you can wind up with them fighting the OT and defeating it...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • satwar
    satwar Member Posts: 24
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    Thanks guys, for taking your time to work on my query.

    There doesn't seem to be a perfect solution and I don't have the expertise to properly access the risk. I'll just wait and see how much improvement I can get in energy efficiency from attempting to manually tune the heating curve.