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Condensing Boiler Running Continuously

jdbs3
jdbs3 Member Posts: 27
Have a Bosch Greenstar condensing boiler, model ZBR 21-3. Currently, the boiler high limit dial is set to Summer Mode, so the boiler heating zone pump and central heating are switched off. DHW remains active.

However, if with all thermostats set to OFF and Tekmar 360 (for radiant floor zones) set to WWSD, the boiler high limit dial is switched from Summer Mode, then:

- the boiler immediately turns on, the flame indicator goes on, the water heats up to as high as 199° dependent on the high limit dial setting, and the boiler circulator pump starts to run.
- the boiler will then continue to cycle through flame off, temperature drop, flame on, temperature rise, all the while with the boiler internal pump circulating water.

This is a huge unnecessary use of natural gas and electric for those days (or sometimes weeks) prior to being able to place the system in Summer Mode for the summer.

Is this the way condensing boilers are supposed to work when there is no call for heat from any thermostats?

Thanks!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    No. Check all your control settings and programming, and all the controls and aquastats.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • It sounds as though there is a continuous call for DHW.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hourTwo btu/ per sq ft for degree difference for a slab
  • jdbs3
    jdbs3 Member Posts: 27
    Thank you for your feedback. The DHW is indirect DHW; a separate tank.

    To better understand this, let me play devil's advocate.

    1. From the Bosch ZBR 21-3 manual - "In Summer Mode, the heating zone pump and consequently central heating are switched off. DHW generation remains active following the DHW program" This works as stated.

    2. In what I will refer to as Winter Mode (from mid Fall to mid Spring), the boiler high dial limit is set to 3 (radiant) or higher. In this mode, the boiler maintains a typical supply temperature for the high limit setting, so if thermostats are ON, but not calling for heat, the boiler will cycle as needed to maintain the supply temperature for when a thermostat calls for heat.

    I believe this is the way it should function. Correct?

    3. In what I will refer to as Between Season mode, I want to minimize the unnecessary use of natural gas and electric. All thermostats are turned to OFF. Dependent on the year, and how close we are to when Summer Mode would be used, we could go days or weeks with no need for heat (the house is very well insulated). But on those days when it is damp, we want to selectively turn on a thermostat, get heat in whatever time it takes for the heat to come up (radiant much slower, baseboard faster), and then turn the thermostat back to OFF. Typically this is only done once a day.

    This Between Season mode is where the boiler seems to be functioning exactly the same as Winter Mode.

    The boiler, Tekmar for radiant zones, SR 503 for baseboard zones have no intelligence other than the parameter setup and the wiring between them.

    From your replies, this Between Season mode is not functioning correctly. So should the boiler, Tekmar, etc. know the difference between this Between Season and Winter mode and act differently?

    I just do not want to ask my HVAC company to diagnose a problem if there is not a real problem and the system is functioning correctly in this Between Season mode.

    Thanks in advance for the additional feedback on my devil's advocate comments.





  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,991
    edited August 2022
    burner in the boiler should no operate unless there is a call for some kind of heat, space heat or DHW. The boiler will not maintain a minimum temperature if all thermostats are in the not calling for heat or off mode. There is a wiring or programing issue with your boiler. If the problem can not be located, there may be defective control within the boiler or the Tekmar or possibly a thermostat or DHW sensor/thermostat.
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    In my view, and I don't know the programming possibilities for that boiler, I'm not at all sure that there is, in fact, an available "Between season" mode. If there is, it may not be labelled as such.

    The Winter mode is probably operating exactly as it should -- and it would seem the Summer mode is, too. So I guess the question I would have is does the boiler control program even have a Between Season mode defined? And, if so, how does it differ from Summer mode? What, exactly, happens in Summer mode if one turns on a heating thermostat? How does that differ from what you want for Between Season?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GGross
    GGross Member Posts: 404
    Your boiler sounds like it is functioning on a "constant" supply temp method that is not being utilized correctly.

    There is a "European" method that dictates the boiler should maintain a set common supply temperature at all times during heating season, in other words whenever the boiler is not on warm weather shutdown. Viessmann boilers operated like this for a long time, they will absolutely fire with no call for heat. It would make sense that Bosch would function similarly as they are very tight competitors in Europe. IF your boiler is on warm weather shutdown, and it is still maintaining supply temp you have an issue. If the boiler is NOT on warm weather shutdown, and your home heating needs dictate that it should be, then you need a programming change of some kind.

    Unfortunately I am not familiar with Bosch controls, I would however note that the temp your boiler is making is far too high for in floor. The tekmar control you posted may also use a 0-10v output that may over ride whatever boiler settings you have. Unfortunately this sounds like you may need to call an experienced Bosch installer to correctly set this up for your needs
  • jdbs3
    jdbs3 Member Posts: 27
    The Winter mode is probably operating exactly as it should -- and it would seem the Summer mode is, too. So I guess the question I would have is does the boiler control program even have a Between Season mode defined? And, if so, how does it differ from Summer mode? What, exactly, happens in Summer mode if one turns on a heating thermostat? How does that differ from what you want for Between Season?


    The boiler does not have a Between Season mode. The boiler high dial limit has '*' (for Summer mode as described above), then settings 1 - 6 dependent on application type (e.g. frost protection (1), radiant 3), etc.), and then 'max' for baseboard.

    In summer, I set the dial to '*', i.e. Summer Mode. The rest of the year, it is set to 3 (radiant), or possibly 5 (what it was set to by the previous owner).

    My Winter and Between Season modes are just labels I am using to describe how the boiler system is used when not in Summer mode.

    In my Winter mode, the first floor radiant heat thermostat is always on and programmed to keep the heat at ~68°. Ditto for 1 baseboard thermostat for 2 of the second floor bedrooms. The other baseboard thermostat is mostly set to OFF with doors closed to the other 2 bedrooms. And the radiant heat in the 2 second floor bathrooms is rarely used, so they are set to OFF.

    In my Between Season mode, again, the boiler high limit is still set to 3. But now I have set all thermostats
    to OFF.

    With this clarification,

    - Is the operation I described for my Winter mode (high limit always at 3) correct since EdTheHeaterMan said
    burner in the boiler should not operate unless there is a call for some kind of heat, space heat or DHW.


    - And for my Between Season mode (high limit still always at 3), but all thermostats set to OFF, should the system be acting differently?

    EdTheHeaterMan, does this additional information change your reply?

    What, exactly, happens in Summer mode if one turns on a heating thermostat? How does that differ from what you want for Between Season?


    Jamie Hall, good question - I'll check it now.
  • jdbs3
    jdbs3 Member Posts: 27
    What, exactly, happens in Summer mode if one turns on a heating thermostat? How does that differ from what you want for Between Season?/div>
    ,
    I just ran 2 separate tests:

    - Turned baseboard thermostat on: SR 503 controller zone light went on, boiler did not go on after ~7 minutes. Turned thermostat back off. SR 503 is a direct connect to boiler.
    -
    - Turned radiant zone thermostat on: ZVC 403 controller zone light went on, boiler did not go on after ~ 7 minutes. Turned thermostat off. ZVC 403 is a direct connect to Tekmar 360 which connects to boiler and Buderus mixing station.

    I expected that for Between Season, with all thermostats set to OFF, that the same thing would happen, i.e. the boiler would NOT go on.

    However, based on both your replies, possibly 1) this is my lack of knowledge of condensing boilers, and 2) As EdTheHeaterMan said
    burner in the boiler should not operate unless there is a call for some kind of heat, space heat or DHW.


    Let me explain.

    I had an oil boiler for 42 years. So I was used to the boiler constantly cycling (less in the summer) to maintain the boiler temperature.

    In what I call Winter mode (boiler high limit dial set to 3 or 5), I thought a condensing boiler did the same thing. That is, it would periodically come on to maintain the boiler. temperature for when a thermostat might call for heat.

    But, based on EdTheHeaterMan's comment, this evidently is not how gas condensing boilers work.

    So if the burner (flame) in the boiler should NOT got on unless there is a call for heat or DHW, and, likewise, the boiler circulator pump should NOT run unless a call for heat or the DHW call for heat, then it appears my Winter mode and Between Season mode are both operating the same, and both incorrectly.

    Because in both these modes with no thermostats calling for heat, ditto DHW, the burner (flame goes on), temperature rises, circulator pump runs, flame goes out, pump continues to run, temperature falls, flame goes on, ...

    And as EdTheHeaterMan stated:
    There is a wiring or programing issue with your boiler. If the problem can not be located, there may be defective control within the boiler or the Tekmar or possibly a thermostat or DHW sensor/thermostat.


    So now I think it is not my Between Season mode that is running incorrect, it is both Winter and Between Season modes that are operating the same, but both operating incorrectly.

    Is this correct?

  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    It would seem to me that since there is no Between Season mode available, perhaps it is a bit much for the boiler controls to behave as such? If the boiler reacts properly, if perhaps a bit slowly, to a thermostat in Summer mode I'd just leave it at that -- reprogramming a boiler's preprogrammed controls is not for the faint of heart.

    That boiler will run periodically to maintain the boiler water temperature at what is needed in heating mode. That temperature is controlled by the dial. In DHW only mode, the asterisk, it should only run when needed for that load. It is also a modulating boiler, so while the flame is on, it should be on at varying levels from low to high depending on the heating load (DHW will always be high). If the heating load is too small for the flame to get down to, the boiler will cycle on and off to maintain temperature -- with all your zones off, but still in heating mode, this may be part of what you are seeing.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jdbs3
    jdbs3 Member Posts: 27
    Thank you again for all the great feedback. I am getting closer to the answer.

    GGross

    Unfortunately this sounds like you may need to call an experienced Bosch installer to correctly set this up for your needs


    and jamie Hall

    Summer mode ... perhaps it is a bit much for the boiler controls to behave as such?... a boiler's preprogrammed controls is not for the faint of heart.


    I agree with both comments. I am not trying to get the boiler to behave in any specific way, rather trying to understand how it works in each circumstance. The more I understand the system components, the more I can make sure it is working properly and optimally. That is how I got to here.

    I also have no interest in touching the boiler or the other system components, other than what is available to the end user, e.g. boiler high limit dial for Summer Mode, DHW temperature, possibly Frost Protection if we go away in the Winter. And there are a few Tekmar 360 parameters that I might change, e.g . Outdoor Design which is set at 6°, that I might change it to 10°. This is gas, and the system is much more complex than my old oil boiler in my last house.

    Currently, I have no plans to change anything. I have the HVAC person returning on September 12.

    However, based on what EdTheHeaterMan stated:

    burner in the boiler should not operate unless there is a call for some kind of heat, space heat or DHW. The boiler will not maintain a minimum temperature if all thermostats are in the not calling for heat or off mode. There is a wiring or programing issue with your boiler. If the problem can not be located, there may be defective control within the boiler or the Tekmar or possibly a thermostat or DHW sensor/thermostat.


    and what jamie Hall stated.

    In DHW only mode, the asterisk, it should only run when needed for that load.


    I now think that 1) both my Winter and Between Season modes (my labels, not an actual boiler mode) are acting the same, and 2) my lack of knowledge for condensing boilers led me to believe that only this Between Season mode was not behaving correctly.

    It appears there is no difference in how the boiler is working in my Winter and Between Season modes. It is functioning incorrectly in both.

    So I am taking EdTheHeaterMan's comment as stated as to how a condensing boiler should work; that there is a wiring or programming problem with the boiler, or a defective control within the boiler, or Tekmar, etc.

    Is my thinking all correct? Or did I miss something someone replied with?




  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    I have a suspicion that the winter vs. summer modes may differ only in what set of controls are actually doing the controlling.

    Modulating/condensing boilers are a completely different critter than the oil boilers to which you are accustomed! In yours, how is the domestic hot water produced? I presume, since the model you quote is a heating only model number, you have an indirect hot water tank?

    In that case, what may be happening is that when the DHW aquatat calls for heat, and the heating side is off, the boiler should fire and the hot water should go to the water tank and back. The burner should fire, probably on and off, to supply heat to the DHW until the DHW aquastat is satisfied -- then it should turn off. In heating mode, the boiler should fire, ideally continuously, but only enough to maintain the desired water temperature in the heating circuits. In that mode, when DHW calls, that call should take priority and the boiler may fire to a higher temperature to supply the DHW.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    GGross