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Modulating boiler question...

Crispy Member Posts: 17
edited January 2020 in Gas Heating
I have a new IBC DC 20-125 "Mod Con" on natural gas. I'm living in my circa 1890 construction project without very good insulation in Northern NY. I have the thermostats jumped to run constantly as the house never reaches a set point. Downstairs, I have 600 sq. ft. (footprint of house) of radiant floor heat using Uponor plates and insulated with R-19. The radiant is mixed to 125F and returning at 108F. Upstairs, I have one small cast iron radiator in a bathroom running at boiler temperature which is presently 156F. I do not have the outdoor "Reset" sensor installed. I'm a little hot right now to be condensing but when I lower the boiler temp or watch it come up from being down, I can clearly see the condensing range when water is pouring out of the drain.
My question is about the modulating part. Will my boiler modulate when running like this and how can I tell? There doesn't appear to be a display for this infomation. Thanks in advance for any comments on these mod-cons and the "reset" control, C.


  • GroundUp
    GroundUp Member Posts: 1,573
    It'll modulate when the boiler water reaches setpoint. So if it's set at 156, when the display is reading 156, it's probably modulating. Kinda sounds like you need more emitter and/or higher water temps to meet the load
  • Crispy
    Crispy Member Posts: 17
    edited January 2020
    When I turn up the boiler temp to say 170F, it easily hits that temp and begins to cycle. At 156F set point, it reaches that temp, displays a "5" which is "Space Heating", runs constantly and does not cycle. I was hoping under this condition, it was running at, let's just say, 70,000 btus. Is that possible? C.
    Edit: Upstairs, I will be adding 14 feet of 9" Base Ray cast iron baseboard that was in the house and two more cast iron radiators.
  • unclejohn
    unclejohn Member Posts: 1,830
    As your boiler approaches your set point 150. The burners starts turning down. Some how some where there is a screen that you will see at what rate the boiler os firing. Do install the Outdoor reset ASAP.
    Sent from Boca Del Toro Panama
  • Ironman
    Ironman Member Posts: 7,119
    You really need to add more radiation upstairs so the boiler can operate @ 140* or less all the time. At that SWT, it will always be condensing and operating at a higher efficiency.
    IDK if that boiler is equipped to provide 2 SWT zones, but if not, there's a way to run the cast iron rads off of the indirect connections so that the floor could be supplied with much cooler water than 160*+ and the boiler would be at 95% + efficiency instead of 88% that it's at when running 160*.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Crispy
    Crispy Member Posts: 17
    Thanks all. Ironman, I haven't heard that one yet. So do you dedicate the domestic tankless circuit in your boiler to the cast iron (in my case) loads? Would I be giving up both of my domestic options as I wouldn't want to operate a higher temp indirect zone? Also, is there usually a way to get that side to call from a thermostat other than flow? Thanks!
  • mattmia2
    mattmia2 Member Posts: 7,736
    Some boilers have a way to do a 0-10v input that will supply water temp as a percentage of that voltage so you can say set the minimum supply temp to the 125 degrees you need for your radiant and set the max to what the high temp emitters need and call for that with 10v. You can do that with certain external controls or with relays and resistor voltage dividers.
  • Ironman
    Ironman Member Posts: 7,119
    If that's a combi boiler, then you may not be able to do what I recommend. I was referring to heating only boiler that has provision for an indirect.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 19,277
    Below 130F return is where you start seeing efficiencies surpass standard 87% equipment. The lower the return the better, and the lower the modulation the higher the efficiency as you have a lot of HX surface exposed to a low firing rate, ideal condensing conditions.

    How low can you go?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Crispy
    Crispy Member Posts: 17
    Thanks guys. I may have the provision to dial a zone special for indirect. I’ll have to look into that. My DC is more simple than some of the other models now in the line but I get where you’re going.
  • Tomizai
    Tomizai Member Posts: 11
    At the risk of asking a silly question, is the heating side of the boiler piped in a primary/secondary method? If it is piped direct and not primary/secondary then you're ability to get heat out is going to vary dramatically and performance will suffer.

    Assuming it is piped primary/secondary, the DC boiler doesn't really give any information about the modulation or firing rate of the boiler. That said, you can remove the door and listen to the blower. As it speeds up you're getting more heat. As it slows down, you're getting less heat. Also on the top left of the boiler, there is a mirror that allows you to view the flame thru the sight glass.

    As to the outdoor reset, installing it allows the boiler to change its target water temperature. This will allow the boiler to modulate down earlier, saving you money, on those 25 - 45 degree days.
  • kcopp
    kcopp Member Posts: 4,188
    Why isn't the outdoor sensor installed?
    I also think you need to add some radiation to the upstairs.
    You can set the hot water output down into the condensing range (125F) and this will help efficiency as the coil is separate from the heating side.