Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Wiring smart thermostat to two boilers

Hey guys,

I'm trying to connect a Honeywell T9 thermostat to control two parallel hydronic boilers both fitted with a L8148.

Can someone please tell me how to properly wire a dpdt switch i have here (r8222d) or a RIB2421B spdt switch that i also have here, without having to worry about burning out the transformers? For some reason I cant figure this one out probably out of fear of burning them out and making an expensive mistake.

Ill attach a simple diagram i drew to show how theyre wired so far. Mind you that this diagram doesnt include the vent damper molex connection that my aquastats have. sort of irrelevant to the question im asking so i just went for the first diagram that i saw.

Boiler 2 on the left, Boiler 1 on the right



Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,621
    Are you planning on running both boilers at the same time off the same stat? Or are you going to run 1 boiler or the other? Switch boilers manually or automatically?
    HomerJSmith
  • BMurrins
    BMurrins Member Posts: 35

    Are you planning on running both boilers at the same time off the same stat? Or are you going to run 1 boiler or the other? Switch boilers manually or automatically?

    same time off the same stat
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,621
    You can do it with the R8222D but you will need an additional 120-24 volt transformer.

    wire term 1 on the R8222d to TV on one aquastat
    wire term3 to T on the same aquastat

    wire term 4 to tv on the other aquastat
    wire term 6 to t on that aquastat

    mount new transformer and power line side 120 volt

    on the 24 volt side the common goes to the coil of the R8222D and to common on the thermostat.

    the 24 volt hot goes to R on the thermostat

    w from the thermostat goes to the coil of the R8222D

    There may be cleaner easier ways to do this but with what you have this will work


  • HVACNUT
    HVACNUT Member Posts: 4,757
    Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.
  • northlakes
    northlakes Member Posts: 36
    Out of curiosity and because I’m currently trying to figure out a solution for a two boiler system, Is your application as simple as you need both boilers to fire every time in order to satisfy demand at all times? 
  • BMurrins
    BMurrins Member Posts: 35
    hey guys thanks for the responses so far i really appreciate it.

    Out of curiosity and because I’m currently trying to figure out a solution for a two boiler system, Is your application as simple as you need both boilers to fire every time in order to satisfy demand at all times? 

    yeah this is for both boilers to fire simultaneously
  • Zman
    Zman Member Posts: 7,405
    I think HVACNUT has given you the solution to firing both boilers at the same time. The question of "why" is pretty valid. When you control boilers that way you often end up with one boiler actually firing more than the other (one aquastat being a little more sensitive) and both boilers short cycling. A simple two-boiler controller will auto-rotate the boilers run times and only fire both when the load requires it.https://www.supplyhouse.com/Tekmar-261-Boiler-Control-Two-Stage-Boiler-Setpoint-4153000-p
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUTHomerJSmith
  • BMurrins
    BMurrins Member Posts: 35
    it it’s not the most ideal setup that we have here but it’s a large apartment building being heated by 2 400k btu crowns. Horrible design but to modernize it, we’d be way over budget right now. Looking to get through the winter and redesign it come summer 2023
  • BMurrins
    BMurrins Member Posts: 35
    HVACNUT said:
    Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.
    This is brilliant. Thank you so much for taking the time to draw this
  • HVACNUT
    HVACNUT Member Posts: 4,757
    BMurrins said:
    HVACNUT said:
    Sorry its crude. You don't need an additional transformer. The one in the aquastat is 40 va.
    This is brilliant. Thank you so much for taking the time to draw this
    Yes, it's so brilliant my wife put it on the refrigerator with a gold star. Move over Honor Roll kids, Daddy's coming through.
    BMurrinsZman
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,621
    edited September 27
    The transformer in the L8148 is rates at .8 amps @ 24 volt. More like a 20va. I don't think it's a 40 va.

    The r8222d coil inrush is 20va sealed is about 10va