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Bosch Greenstar 151 Combi - Not heating properly

redstar Member Posts: 9
edited March 2019 in THE MAIN WALL
I'm having issues with a unit installed 3 years ago. In essence, it doesn't heat the house up to 70F on many days. It seems like its always a step behind - meaning that when its say 55-60F outside the pipes are slightly warm, but not warm enough. On colder days the pipes get hotter, but not quite enough. In either case, the house takes a long time to increase temps if I turn the thermostat down during the night.

I've attached a picture of the control panel. I recently changed the dial on bottom left from 6 to 7 but that didn't seem to have any effect. The big green number varies between around 90-130 at most while outside temps down to around 35F.

The outside temp sensor is very close to the exhaust which could be part of the problem, but typically the sensor is only about 5F more than the actual outside temperature.

Any help would be very much appreciated!


  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited November 2018
    You should not be setting back your thermostats with the type of boiler you have. It's made to be efficient using low temp water not bang on bang off with 180 degree water like your old boiler. If you feel you must use setback, maybe only drop it 1 deg or so at night and have the recovery start at like 4am so you're back up to temp at 7am.

    You can also change the Outdoor Reset Curve to give you hotter circulating water depending on outdoor temps, but you probably really only need to stop using thermostat setback.

    FWIW- my boiler circulates 122F water when it's 35F outdoors.
  • redstar
    redstar Member Posts: 9
    Thanks for the reply. I generally don't use setback, just thought I'd mention it as an example. Right now my thermostat has been set on 70 for the past severa days, and this morning the actual temp in house had dipped to 64.

    I will look into changing the outdoor reset curve. Is it something you'd be able to give simple instructions to on here? I want to make sure I don't mess anything up - I'm not familiar at all with these units!
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited November 2018
    Did this condition exist since install or is "being a step behind" something new to this heating system?

    Do you have radiators or slab heat?

    If radiators, did you add additional radiators when installing the new boiler?

    FYI- the outdoor temp sensor is supposed to be installed on the north side of the house whenever possible to shade it from the sun.
  • redstar
    redstar Member Posts: 9
    edited March 2019
    We have radiators, no new ones were installed.
    Understand about the north side, the area its in now is fairly well shaded though, and it happens at night as well as during the day. Do you think the temp meter being close to the exhaust is a major issue?
    Would a first step be to change the location of the temp sensor and change the outdoor reset by a step or two?
  • kcopp
    kcopp Member Posts: 4,188
    That would be a start.
    The program can be adjusted.
    Why not have the original installer make a visit....
  • DZoro
    DZoro Member Posts: 1,048
    Your reset curve is probably a little shy of water temp. Moving the sensor to north side will help, but may have to increase the water a bit also. The install manual should walk you through that process. If the boiler hasn't been tuned and cleaned this year, your tech can easily do the reset curve. Save some money if you can move the sensor yourself before he gets there.
  • redstar
    redstar Member Posts: 9
    Also, if I simply turn the smaller white dial it changes the heat setting, I now have it turned to 6 out of 8 bars. This was originally set at 3 out of 8 bars. Not really 100% sure if its having any effect.
  • redstar
    redstar Member Posts: 9
    Ok, more questions. I've called a few places and they either can't come out for another several weeks or they don't seem familiar with these settings.

    From my understanding, I need to change the "Base Line" and "Design Temp". Base Line is the water supply temp at the outdoor temperature the unit goes on, correct?
    What is the "Design Temp"?

    Currently factory settings seem to be:
    Base Temp: 78F
    Design Temp: 168F
    Heating OFF: 68F
  • redstar
    redstar Member Posts: 9
    Another thing to confirm - the manual refers to this as the "heating curve". Is this the same as the Outdoor Reset Curve or am I looking in the wrong place?
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited November 2018
    I'm looking at your install manual from here:

    On page 54 of that manual it describes the "Design Temp" and shows the default Outdoor Reset Curve below. Right now it looks like your boiler will supply 124F water at 34F outdoor temp (see red lines on graph).

    You need four numbers to set your ODR Curve:
    1)Min outdoor temp
    2)Max outdoor temp
    3)Min supply water temp (that is the water going to your radiators)
    4)Max supply water temp.

    Looks like the default curve is set as follows:
    1)Min outdoor temp: -4F
    2)Max outdoor temp: 68F
    3)Min supply water temp: 78F
    4)Max supply water temp: 176F

    If you put these in excel you get 124F Supply Water at 34F outdoors:

    Since your house and radiators were designed to use 180F supply water, the current supply water temps are too low for your home's actual heatloss. So, you can add radiation, swap out existing radiators for "low temp" radiators, do both, or raise your supply water temps using your outdoor reset curve. In your case, if you change the Min outdoor temp from -4F to +14F look at the new Supply Water Temp at 34F outdoor temp, it's 140F vs.124F on the old ODR curve:

    With that in mind, you will need to make adjustments to get the best comfort in the house, you may still need a higher Min outdoor temp setting and/or you can adjust the Minimum Supply Water temp up from 78F to maybe 100F and see how that works.
    I would put all the numbers in an excel graph before dialing them into your boiler. There are other factors to consider and you don't want to put your boiler in a position where it short cycles itself to death because it can't modulate down low enough to supply 78deg water so it keeps starting and stopping as to not overshoot 78F water.

    Just as a reference, here is my ODR curve that keeps my house warm enough and prevents my mod-con boiler from short cycling on warmer days... it's just intended as a reference, no two homes are the same. FWIW- I did have to add additional radiators in rooms where I could, and in rooms where I couldn't add more, I replaced the existing radiators with special "low temp-high output" radiators designed to work with low temp boilers. The first winter without the additional and low-temp radiators I needed higher SWT's to keep the house warm.

  • redstar
    redstar Member Posts: 9
    NY Rob: Thank you so much for putting that together. I truly appreciate it. Incredibly helpful, and I'm sure you have other things you'd rather be doing - so THANKS!

    I think I'll start slow and change things just a bit to see if I can get it where it needs to be without risking other problems. Does this look OK?

    1)Min outdoor temp: -4F change to 0F
    2)Max outdoor temp: 68F keep same
    3)Min supply water temp: 78F change to 85F
    4)Max supply water temp: 176F change to 181F

    Look OK to start? I'll get to the excel this weekend when I can so I can get a better idea.
  • redstar
    redstar Member Posts: 9
    Couldn't locate the Min outdoor temp setting, so increased the Base Temp and Design Temp by 6F each. The supply temp increased as expected and after a few hours house temp was as set at 70F. Hopefully that does it, but if needed small adjustments should be no problem.

    Can't thank you enough for your help. When you have no idea what you're doing these things can take a while to figure out.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited November 2018
    Glad it worked out for you, and I give you credit for asking for help and diving in to it.

    So, what are your current settings?
    1)Min outdoor temp:
    2)Max outdoor temp:
    3)Min supply water temp:
    4)Max supply water temp:

    The next step is to protect your boiler from damaging "short cycling".
    You would need to measure the actual lengths of the radiator elements (just the fin-tube part not the cover) and add them up for each zone. We can then calculate the amount of BTU's each zone could put out and compare that to the minimum BTU output of your boiler. If a zone will output 6,000 BTU's with 85F supply water and your boiler can only modulate down to 8,000 BTU's it will short cycle. We address that by raising the minimum supply temp a bit so your zone will put out 8,000 BTU's matching the boilers minimum fire rate.
    It sounds worse than it is, it's really simple math that can extend the life of your boiler considerably.
    I'll be more than happy to help you with the calculations if you'd like to work on it.
  • redstar
    redstar Member Posts: 9
    I only changed two settings, the:
    Base Line: 78 to 84F
    Design Temp: 168 to 174F

    I couldn't find the setting for Min Outdoor Temp, I assume its at -4F.
    I'll measure the radiators next week. Thanks again for offering to help.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Okay, so here's your original ODR Curve on the left with the new ODR Curve next to it on the right...