Question on my new Bosch Greenstar 151 combi install

Discussion in 'Boiler Forum' started by shane rowles, Nov 23, 2020.

  1. shane rowles

    shane rowles New Member

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    Nov 23, 2020
    Location:
    Pennsylvania
    I am not a plumber so I paid to have my Bosch Greenstar 151 Combi floor model installed. I converted from a single zone coal boiler to a three zone gas boiler. The original plan was to add the Bosch Greenstar FW200 Weather Compensation Boiler Control and outdoor sensor to make my new install as efficient as possible.
    Since the FW200 control is not compatible with the CZM100 Zone control, and the CRC thermostats they used a Taco zone control and Honeywell on/off thermostats.

    The problem is the outdoor reset never got installed and when I asked about it I was told that the boiler that was installed was a new model and the only thing the outdoor reset did was shut the boiler off when the outdoor temp went above 70F. They claim that there is a sensor on the return water inside of the boiler and that is how it decides on how to modulate for the most efficient water temp. I don't know enough to know if this is true or not and I feel like I now just have a smart boiler using simple on/off controls and I am not getting the most efficiency out of my boiler.

    Should I still push to get the FW200 and outdoor sensor or push to convert the Taco zone controller and Honeywell thermostats over to the Bosch CZM100 zone controller and the CRC200 and CRC100 room controllers? Any and all help would be greatly appreciated. Thanks

    Boiler name is Bosch Greenstar 151 combi floor model
    Boiler model is KWB 42-3 A23 S3123
    model number is 7 733 600 084
     
  2. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    Before changing any of the setup, first, measure up how much radiation and type you have on EACH zone, broken down by zone.

    The minimum firing rate on that thing is 36,000 BTU/hr-in, which is already nearly the whole house heat load of my house @ +5F outdoors, and likely to be way oversized for any given zone to operate without short cycling at condensing temperatures if it's low-mass fin-tube. Even with high-volume cast iron rads it could still short-cycle at condensing temperatures, depending on the amount of radiation on the smallest zone(s). With typical fin-tube baseboard would need over 150' of baseboard per zone to emit the full minimum-fire output at 95% efficiency to avoid cycling on zone calls.

    A discussion of these issues along with the napkin-math analysis lives here.
     
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  4. shane rowles

    shane rowles New Member

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    Nov 23, 2020
    Location:
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    Thank you for taking the time to write a response Dana. I read the article you suggested and I quite didn't understand how to figure out if my boiler will work correctly. Here is the info I have and figured out.

    I live in central PA and the average winter temp is around 30F
    I have cast iron radiators in my entire house which is 1900 square feet including my basement
    Zone 1 - has 357 square feet of radiator
    Zone 2 - has 90.6 square feet of radiator
    Zone 3 - has 76.5 square feet of radiator

    I do understand zone 2 and 3 are very short. Zone 2 is an enclosed porch or sun room that always gets a lot colder the the rest of the house and zone 3 is the basement. They connected all the return lines for all zones and I have noticed that there is some circulation in zone 2 and 3 when zone 1 is calling for heat. I hope they did that to help on the short cycling but at this point I have no idea. What are your thoughts? Thanks
     
  5. Dana

    Dana In the trades

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    01609

    Cast iron is good! (In your case it's even NECESSARY!) The thermal mass of the water is more than that of the cast iron (a gallon of water has the same heat capacity as ~75lbs of cast iron). And you'll need the thermal mass to avoid short cycling at condensing temps.

    The 357' EDR of zone 1 only emits ~18000 BTU/hr at condensing but it's at least a bit more than half the min-fire output of the boiler.

    The 77' EDR of radiator on zone 3 would only emit about ~4000 BTU/hr at condensing temps, the 91' EDR zone 2 only emits ~4500 BTU/hr, so when serving just one of those zones it's going to cycle quite a bit, so you'll have to monitor that and tweak the programming parameters if it's short cycling on those zones. (Burns shorter than 3 minutes, with more than 10 burns per hours takes a real toll on efficiency & longevity.) But if the outdoor reset curve gets tuned well there will usually be overlaps with calls from the other zones, increasing the amount of available radiation. In most cases you should be able to set up the parameters to tune out any short-cycling.

    Even with all three zones calling for heat there isn't quite enough radiator to emit the full ~34K output of the boiler at temps low enough to deliver 95% efficiency, but as long as the main zone #1 is calling for heat it won't short cycle. Once the outdoor sensor is installed you can empirically find the low-temp limits at which just the sunroom zone calling for heat would become a short-cycling problem. If it chronically runs colder than the other spaces it will be calling for heat constantly, or at least at a much higher duty cycle than the other zones, so it's going to be the zone to watch while setting it all up.

    Circulation of water in the "off" zones when the main zone is calling for heat it means there are no check valves on the zones. If it's not overheating those zones it's fine to just leave it alone.

    A typical 1900 foot 2x4/R13 type of house with clear glass double-panes or storm windows, no foundation insulation and at least R20 in the attic would typically come in around 35000- 40,000 BTU/hr @ 0F. (In central PA your local 99% outside design temp is probably in single digits F above or below that mark.) If the house is tighter than average and the foundation gets insulated to the current IRC code minimums (= R15 continuous insulation) it could easily be in the 30,000 BTU/hr @ 0F range.

    With a heating history on the place it's easy enough to figure that out using the boiler as a measuring instrument to determine the approximate heat requirements, and from that the water temp that your rads would need to deliver it. This would give you a pretty good starting point for dialing in the reset curve.

    I haven't looked at a Bosch manual recently enough to remember what parameters are tweakable or how to program the curve, but you should make it your bedtime reading material for a week or so until you really have it down cold.
     
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  6. shane rowles

    shane rowles New Member

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    Thank you so much for your input and I appreciate the time you took to explain this too me.
     
  7. fitter30

    fitter30 Well-Known Member

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    Retired service tech
    Location:
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    .
    Are you using c or f in your calculations? 357x50= 17850 357x122= 43554. 50*c=122f
     
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  8. shane rowles

    shane rowles New Member

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    Location:
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    I am fairly certain that Dana used F for the calculations
     
  9. shane rowles

    shane rowles New Member

    Joined:
    Nov 23, 2020
    Location:
    Pennsylvania
    Dana,

    I did a heat loss calculation using slant fin and I came up with 63902 BTU/HR loss on my house at 10F. My house is brick with horse hair plaster and lath and single pain windows with storm windows. I do plan on replacing the windows just haven't got there yet since I have 25 of them. I have the outdoor rest ordered and will install it this weekend.
     
  10. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    The SlantFin app is a fairly crude I=B=R method tool, which is going to overstate reality by more than a Manual-J would. Your real design load is probably closer to 50K, and could be even less if you're assumptions about infiltration losses are high.

    But assuming the 64K is correct and the load/radiator ratios are the same in all zones (which they aren't), 64,000/357'= ~180BTU/hr per square foot EDR, which takes an average water temp of about 185F @ 10F outdoors.

    If the design load is really 50K, it's 50,000/357'= 140 BTU/hr per square foot EDR, which takes an AWT of about 165F@ 10F outdoors.

    See the nomograph on page 2 of this document.

    Run a fuel-use load calculation (which is really a measurement) for sanity checking any Manual-J or I=B=R load calculations.
     
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