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Question about new Bosch Greenstar 151 combi floor model install

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


  • IronmanIronman Member Posts: 6,162
    What type of radiation do you have? I would only use the FW200 if you have radiant floors. It’s somewhat more complex than needed for most anything else.
    I may be wrong, but I think your installer is BSing you. The boiler has always had a return water sensor, but it needs the one of the communicating room controls for its indoor reset function to work.

    A 42kw boiler is pretty large. How big is your house?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • IronmanIronman Member Posts: 6,162
    I just ran this by my rep (who is extremely knowledgeable) and he confirmed  what I told you.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • shane_rowlesshane_rowles Member Posts: 10
    I have cast iron radiators, and my house is 1900 square feet. I feel like my installer is not telling me the whole truth, thats why I posted on here. I feel at the very least I should be set up with the bosch zone manager and bosch room controllers.
  • shane_rowlesshane_rowles Member Posts: 10
    Ironman are you then saying I should get the room controllers? Thanks
  • IronmanIronman Member Posts: 6,162
    Yes, if you want it to have indoor reset.

    Your installer may lack knowledge. I’m giving him the benefit of the doubt.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • shane_rowlesshane_rowles Member Posts: 10
    I live in Central Pa so it does get cold, I would say average winter temp is around 30, but I am getting the picture that my furnace is way oversized and I will not see the 95% efficiency due to that fact. Thanks for all the help
  • IronmanIronman Member Posts: 6,162
    I see that it's a combi, so you need 151k btus to get 3gpm domestic output.

    Do you have large iron pipes feeding the radiators? If so, then there's probably enough buffer to reduce or eliminate short cycling - except for any micro zones. If you have any, then you should combine it with another zone.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • fenkelfenkel Member Posts: 128
    We install alot of bosch boilers.. the fw200 works good!  It will save you some money... but if your boiler is set to run at 180 supply temp,your boiler will never be over 89% efficiency, in order to get 95% efficiency your supply water temp needs to be 130 or lower..
    Did your install do a heatloss calculation?
    How many bathrooms do you have?
    We install buffer tanks to reduce short cycling. The actual modulation on a 151 btu boiler is 4.4.. so your zones need to be close to 34k btus... if not you see alot of short cycles.
    I'd recommend you have a heat loss done on your home and then turn down the heating btu output... you can turn down the heat btu and leave the water heating btu at 100%..then you still get 3.6 gpm for  hot water capacity. 
  • shane_rowlesshane_rowles Member Posts: 10
    Ironman, yes I have large trunk lines in the basement probably 2.5 or 3 inch and that get reduced to 1 or 1.25 inch that feeds the radiators.

    fenkel, no there was no heatloss calculation done. I have one bathroom but I have a wife and three daughters LOL who unfortunately for me like to take long hot showers. How would I turn down the heating btu output? Would I just turn down the boiler high limit dial?

    Also I figured out my square footage of my radiators, I measured the height and the width and used a chart for this:
    Zone 1 - 357
    Zone 2 - 90.66
    Zone 3 - 76.5

    Zone 1 is my main house ie. kitchen, livingroom, dinning room, all bedrooms and bath.
    Zone 2 is only my enclosed porch/sunroom
    Zone 3 is the basement

    The reason I had it zoned was the sunroom get colder faster than the main house and it is used as an office, so when people are in there I wanted to be able to warm it up without heating the whole house and the installer suggested that I put the basement on its own zone as well. All the return lines in all zones are tied together and I do get some circulation in zone 2 and 3 when only 1 is calling but nothing is overheating so I don't feel that is a problem.

  • IronmanIronman Member Posts: 6,162
    Turning the dial will limit the water temp, but not the firing rate. I'd set it to 160*; you probably don't that much, but try it there.

    You have to go into the programming to limit the fire rate, but that won't change the lower end which will always be about 28k btus.

    An old converted gravity system has somewhat of a "natural" reset built into it because of its high mass. But getting the indoor reset functional will give you better comfort and efficiency.

    Since the boiler came with one CRC100, and Bosch's intention is for it to be used, and since the installer has not been truthful with you (for whatever reason), I'd expect him to put the right controls on. He spent just as much money on the Taco panel and Honeywell thermostats as he would have on the Bosch panel and room controls.

    Again, IDK if it was just ignorance, stubbornness, or just plain dishonesty on his part, but you shouldn't have to live with it.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • fenkelfenkel Member Posts: 128
    Id do a heatloss on your house first...slant fin has a website program for heatloss calculation,  id spend the time to do it first... its simplest heatloss program to use...
    Heres how i do a heatloss and set up a boiler..
    First do a heat loss calculation, 
    Then i use the btus from that calculation  and divide that number by the net ibr btus for the boiler im installing..
    Let say your house have a heatloss calculation of 80 btus. 
    The net ibr for your boiler is 131
    So 80/131 equals 61%.
    So i would reduce my heating output to 61%.. i sometimes take another 5% off and start there..  then id use 56%..
    Ive read that some installers will cut heat output down to 25%... i wouldnt do that...
    Your goal is to reduce your water temp to increase efficiency at the same  time reduce the short cycling...reducing  the amount of short cycling will  increase the longevity of your boiler..
    Since you have have the taco controller for your zones, id install the fw200.. this will lower you water temps when the outdoor temp raises, which then will improve the boiler efficiency. 
  • shane_rowlesshane_rowles Member Posts: 10
    edited November 2020
    fenkel, I did a heat loss calculation on my house using slant fin and came up with 63902 BTU/HR loss. I did use a 99% design temp of 10F. I am sure it might be a bit lower or higher but I figured this would give me a fairly accurate estimate.

    I am not sure which number to use for my boiler net ibr, the max input is 151,600BTU/HR and min is 36,000BTU/HR. Also Max output is 137,500BTU/HR and min is 31,700BTU/HR

    If I go off my max input 63902/151,600 = 42.15%
    If I go off my max output 63902/137,500 = 46.47%

    I checked my max heat output setting and it was set at 100% so for now I adjusted to 55% and kept DHW at 100%.

  • shane_rowlesshane_rowles Member Posts: 10
    Ironman, I believe they used the taco and Honeywell equipment because they did plan on using the FW200. I am not sure if they are getting bad info about the outdoor reset or underbid the job and didn't want to spend the extra 130 dollars on it. At this point I have no idea what to think. I am going to install the outdoor reset since everything is already set up for it and that is the path of least resistance. You and fenkel have showed me a lot about this and I do appreciate the time you two took to help me with my issue.
  • fenkelfenkel Member Posts: 128
    Alot of installers dont install the outdoor resets..
    Because of the following
    1. The have to program it. 
    2. It takes time to setup properly 
    3. Could cause call backs.
    4.takes more time to install it..
    If you have to purchase an fw200..check out ebay..the sell cheap..
    If you have time..experiment  with the settings...just write down what you change and the valves...
  • fenkelfenkel Member Posts: 128
    Shane the net ibr on that boiler is 117btus not 137btu..
    So. 64/117 is what you set it at is what i would have used...

  • shane_rowlesshane_rowles Member Posts: 10
    I already have alot of time in it trying to figure out how it is supposed to work so taking time to program an outdoor reset wont be that big of a deal. I am glad to hear that I have the output setting correct. Thanks for all of the help
  • fenkelfenkel Member Posts: 128
    There is one more item you might want to try:
    Reduce your supply temp for heating the home..
    Are you setting the heating supply to max on left controller?
     Try and turn it back at little...
    Once you install the fw200, you can.set the heating supply temp more accurately.
    This can greatly increae your efficiency..
    Question ,what do you have for heat omiters? Baseboards, radiators or in floor?
    If you have baseboard,  the slant fin heatloss calculation will suggest at 180 supply temp how many feet of baseboard are needed to heat the area to design temp.. or 590 btus per foot of baseboard.  If you have more baseboard than the required amount the heat loss calculations determined, you can lower your heating supply temp..

  • shane_rowlesshane_rowles Member Posts: 10

    I have cast iron radiators, and I have the left controller set to 5 which is around 160F.
  • fenkelfenkel Member Posts: 128
    Good...youll be about 88 to 90% efficiency on coldest days..once you install the fw200, it will get even more efficient on warmer then try and lower the supply temp down a little more, maybe around 156.. fyi: for better efficiency, youll want the system to have return temps under 130 degrees..
  • GWGW Member Posts: 3,906
    edited November 2020
    yea there’s some glitches is what your being told. Large mass always runs more efficiently with outdoor reset and even more so with the FW200 mounted in the living space (full constant circulation) 

    Install it in the largest and or warmest zone of your home

    Also- I’m not a combi boiler fan- the boiler is always oversized unless your house is yuge (5000 sq ft). To bake cold water and make it hot instantly requires a yuge amount of BTU. Hence an oversized boiler 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • shane_rowlesshane_rowles Member Posts: 10
    Hi guys, so I got the FW200 installed, its on the front of the boiler because the one I got didnt come with the wall hardware and I was curious about the baseline and design temps for the radiators. I used the default settings and was not sure if they should be adjusted. Thanks

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