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Gas Condensing Boiler - Coming from an oil boiler world

jdbs3
jdbs3 Member Posts: 27
Had an oil boiler for 40+ years; knew how they worked and had sufficient knowledge to make modifications (install new circulator pump, expansion tank, back flow protector, etc.) as well as drain the system and purge it post modifications. One thing I never did was clean/tune them.

Now in a gas condensing world. Have a Bosch Greenstar Residential Gas Condensing Boiler, model ZBR 21-3. The boiler is 1 year old, and was just serviced by a local heating company. Given this is gas, I have NO plans to ever make modifications to this system, but do want to be knowledgable about how it works.

I have googled how they work, read through the Installation and Service Instructions, talked with technical support at Bosch once; basically learned a lot.

I found the following in one of the threads on this forum: "The boiler will modulate and attempt to run as long as possible on as low of a fire as it can. This is the most efficient approach, but Americans have a hard time grasping it because we're used to bang, bang technology - 100% on; 100% off. The lower the RWT (return water temperature), the more efficient the boiler is operating."

Okay, that helped. But it also raised some questions:

- If none of the zones are calling for heat, does this mean the system will still ALWAYS be running/circulating water through the boiler?

- I then manually turned off all thermostats. And I also set the ECO button for the DHW. What I expected to happen was that the boiler would now go off, and only turn on when there was a call for DHW. But the system is still running and sounds like there is water circulating in the mechanicals room.

- I left all thermostats turned off last evening and rechecked this a.m after 12+ hours. No change, the system is still running (a low hum) and it sounds like there is water circulating in the mechanicals room.

Questions:

- Is this the way it should work? Does this mean that 24/7/365 the system will always be running?

- Is there a way to measure how efficiently the system is running, and if so, then are there ways that the technician can tweak the system to increase its efficiency?

I expect I have other questions, but this is a good starting point to better understand how the condensing gas boiler works.

Comments

  • jdbs3
    jdbs3 Member Posts: 27
    btw: We have radiant floor heating on the first floor (1 zone), radiant floor heating in the 2 upstairs bathrooms (2 more zones), and baseboard heating in the 4 bedrooms (2 more zones).
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,271
    edited March 2021
    There may be a circulator that is wired to run constantly. I'm not familiar with the Bosch model that you have however those pumps don't use very much electricity. The idea is whether the flame is firing. That is where the modulating feature is saving. And you are correct, if there is no call for heat or DHW, there should be no flame. Look on the display to see if the flame indicator is on. It should not be.

    As I am typing this, another thought occurred to me. Do you have a DHW recirculation pump? That pump is designed to keep hot water near your faucets. This is so you don't need to wait 30 seconds (or sometimes a lot more) for the hot water to reach the faucet. Less water wasted. If so, is that the pump you are hearing?
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,654
    @jdbs3

    @EdTheHeaterMan has some good suggestions. Also with the thermostats turned down and no zones calling possibly the outdoor air reset control (if used ) is keeping something running
  • bburd
    bburd Member Posts: 486
    Assuming this is piped primary/secondary, it may be the boiler loop circulating pump that is running. I have to wonder though if there was an error in setting up the wiring or controls.

    Bburd
  • hot_rod
    hot_rod Member Posts: 17,531
    Quite a bit of reading involved to learn all the control options. Really no need to run a circulator when there is no heat call. Some combi boilers have an option to keep the block hot for fastest DHW, but still the circ stops when the block is hot. Could be mis wired somewhere also.

    https://www.bosch-climate.us/files/Greenstar_Installation_&_Service_Instructions_en_US.pdf


    Be sure to maintain water quality to spec on those aluminum boilers.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jdbs3
    jdbs3 Member Posts: 27
    Thank you for all the feedback.

    RE: if no call for heat or DHW, should be no flame. see if flame indicator is on. It should not be.

    It is NOT on.

    RE: Do you have a DHW recirculation pump? That pump is designed to keep hot water near your faucets.

    Yes, there has to be a DHW recirculation pump. There is an ECO button that controls the DHW set to ON, i.e. ECO button lights up. As per the manual:

    "Pressing and holding ECO button until it lights up switches between Comfort mode and Economy mode.
    • Comfort mode (default setting) - The appliance is continually maintained at the set temperature. Consequently, DHW draws are immediate, however the appliance may run even if no DHW is being drawn.
    • Economy mode, ECO button lights up – DHW is only generated when DHW is drawn. For On demand: Quickly open and close a DHW tap to signal the appliance to heat to the selected temperature. After a short wait DHW will be available."

    RE: possibly the outdoor air reset control (if used ) is keeping something running

    I believe it is being used, since the Tekmar Mixing Control 360 is responding to the outside temperature.
    But at 70 degrees, the air reset control should not be running.

    Also, the manual notes "If an temperature sensor for an outdoor reset control is connected, pump control mode 04 is automatically set." And "Pump control mode 04: Intelligent control of central heating pump in heating systems with outdoor reset control. The heating zone pump is only switched on when needed."

    RE: may be the boiler loop circulating pump that is running. I have to wonder though if there was an error in setting up the wiring or controls.

    Hmm. Sounds like a call to the heating company that installed and is servicing the unit. I'll keep folks updated on what they say.

    RE: Be sure to maintain water quality to spec on those aluminum boilers.

    hot_rod, not sure what this means. The DHW boiler was installed in 2004 with no problems to date. I am on city water, so have no control over water quality.

    Can you expand on what you were thinking?

    Thanks to all. Stay tuned for an update.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,271
    edited March 2021
    FYI for @jdbs3 If you want a person to know that you answered their question or you want to ask a person like @hot_rod a question, place the @ symbol just before their user name. This way we get a notification in our inbox. I just happened upon this post to see if there were any updates. Thanks for the update.

    As far as water quality is concerned, you can have the water from the boiler tested and see if the pH and other parameters are in a safe range for your boiler. If there is too much or too little of something, there are additives for that. Hot Rod Bob is an expert on things boiler-related and is a great resource. Perhaps he will respond to your Query since I tagged him herein
    Mr. Ed
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org

  • Mike_Breault
    Mike_Breault Member Posts: 36
    edited April 2021
    @jdbs3 the 360 is a mix control, not a boiler control.. it may stop calling for heat in your mix zone, I assume based on the OP, you have a mix of hi/lo temps.. Something is initating a call. possibly a control set up for the oil boiler you removed that maintains a 140°F Primary loop temp? the 360 looks at the boiler return piping temperature, and as si common on oil boilers likely had a setting of 140 Min. below that the mix shuts down to protect the boiler

  • jdbs3
    jdbs3 Member Posts: 27
    Update:

    Again thanks for all the feedback. On and off I continued to learn about my system.
    - The hum that I thought was the gas boiler continuing to run even when the thermostats were all off was actually the 2 Taco controllers seen to the right of the boiler in Picture 1. They each have a metal cover which is also hot to the touch, not burning hot though.

    I talked with Taco tech support; he noted that what I am hearing is the 24v transformer hum; it just means the controllers are showing their age. He noted it is probably the varnish breaking down between the steel laminate of the transformer. The hardwood backing it is mounted on helps resonate the vibration.

    So sound source resolved.

    - BUT, in Pictures 2 and 3 there is a Buderus Quick-Fit Modular Piping System, a fully assembled radiant heat mixing station that conveniently combines a motorized mixing valve, circulator pump, shutoff valves, a check valve and temperature gauges in a compact, attractive foam-insulated unit. It includes everything needed to automatically adjust water temperature for radiant floor heating.

    In late march and a good part of April and the start of May, we would turn off all thermostats. The house is very well insulated and retains heat well. The supply and return temperature gauges on the Buedrus continued to show a water temperature of ~118° F. Yet all thermostats were turned off.

    Sometime in May, I turned the Bosch ZBR 21-3 Greenstar condensing boiler to Summer Mode. Soon after, the temperature gauges on the Buderus dropped to ~90°, and even as low as ~74°.

    So the boiler was not making the humming noise, BUT it was continuing to run with the thermostats all off. This is an unnecessary use of gas, especially since the further we got in April, the more the system was never on or only on for a very brief time.

    - I also tried an experiment today. With the system in Summer Mode, I turned on a thermostat and called for heat (yes, it was 79°, so I pushed the thermostat up to 84°). Interestingly, the ZVC 403 controller for the radiant floor heat came on, but in 9 minutes, I never heard the boiler go on, nor did the piping through the Buderus get hot. So it looks like in Summer Mode, I can NOT get heat.

    Questions:

    1. Is my assumption about Summer Mode correct: No heat when in Summer Mode?
    2. When not in Summer Mode, why does the boiler continue to maintain heat that circulates all the way through the Buderus mixing station?
    3. Is there any way for me to stop the action in question 2 when I get to that in between season where most of the time I do not need heat, but want it when it is needed?
    4. And what is the purpose of the Erie Pop top Zone Valve in Picture 3 that sits above the Buderus mixing station? The 3 radiant floor zones connect to the Taco ZVC 403 box (top box), the 2 baseboard zones connect to the Taco SR 503 (bottom box). The Tekmar 360 controller sits between them.

    Again thank for all the input. My goal is to understand all of it so that I can use it in the most efficient way while reducing my gas use.





  • jdbs3
    jdbs3 Member Posts: 27
    Any feedback?

    thanks!
  • gennady
    gennady Member Posts: 839
    edited June 2021
    Mod cons work on outdoor reset principle. Constant flow with water temperature changing according outdoor temperature. System must be balanced, trvs and balansing valves must be installed on each radiator. If radiators or baseboards are piped in series, by passes must be installed. System pump must be vfd type if no bypasses present. That’s a very basic path to a good heating system. There are more things to do as well. No thermostats needed, and if left in place, it makes system work improperly
  • jdbs3
    jdbs3 Member Posts: 27
    Is there anyone who can read my comments on May 26 and answer the 4 questions I have?

    As noted, I appreciate the feedback on this forum, but am looking for feedback to those questions. I've learned much; a reply to these questions will further my knowledge.

    thanks
  • Zman
    Zman Member Posts: 7,406
    jdbs3 said:

    Update:



    Questions:

    1. Is my assumption about Summer Mode correct: No heat when in Summer Mode?
    2. When not in Summer Mode, why does the boiler continue to maintain heat that circulates all the way through the Buderus mixing station?
    3. Is there any way for me to stop the action in question 2 when I get to that in between season where most of the time I do not need heat, but want it when it is needed?
    4. And what is the purpose of the Erie Pop top Zone Valve in Picture 3 that sits above the Buderus mixing station? The 3 radiant floor zones connect to the Taco ZVC 403 box (top box), the 2 baseboard zones connect to the Taco SR 503 (bottom box). The Tekmar 360 controller sits between them.

    Again thank for all the input. My goal is to understand all of it so that I can use it in the most efficient way while reducing my gas use.

    />

    1. I don't work on Buderus, I think you have answered this yourself. This makes sense, no heat in the summer...
    2. Either something is telling the boiler to stay on (a wire attached to T-T has closed the circuit), or the boiler is programmed to always heat. I would start by pulling the wire and if continues to heat, I would dig into the manual.
    3. I am sure there is.
    4. What is the valve wired to? Where does the attached pipe go?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • jdbs3
    jdbs3 Member Posts: 27
    Zman,

    Thanks for your replies; very much appreciated.

    RE: What is the valve wired to? Where does the attached pipe go?

    See the last picture on my May 26 entry and the additional 3 attached pictures.

    The last picture on May 26 shows the Erie on the supply side line above the Buderus mixing station.

    The Erie picture attached shows a copper pipe behind the Erie going into the wall. This is the start of the supply side run from the attic (where the boiler system is) and the crawl space where there is a manifold with 8 loops for the 1st floor radiant floor heat.

    Just below the Erie, the copper pipe feeds into the supply side of the 2 loop manifold for the the 2 bathrooms on the second floor.

    So the Erie pop top zone valve is only in-line on the supply side for the 1st floor radiant floor heat.

    The Erie also shows a brown wire feeding into the wall. This brown wire is connected to the Taco ZVC 403 in the Zone Valve 1 slot (see 2 TACO pictures attached). And this Zone 1 connection is then connected with the red wire to the end switch slot, and (I guess ??) on to the circulator.

    Note that the 3 thermostats are wired to the thermostat slots at the top of the ZVC box.

    And there are connections to the bottom Zone Valve slots 2 and 3 from somewhere. For Zone Valve slots 2 and 3, there is a jumper wire between connections 3 and 4 meaning a circulator is being used, not a zone valve.

    Hope these pictures help.

    All my questions are to be able to understand the system, as much as possible, before calling the HVAC company that installed the system. I am also calling the manufacturers of each component to ask a few questions of them.

    It is not that I think the system was installed incorrectly; actually, I am quite impressed with it.

    But if I really understand it, then I can assure it is being used optimally. For example, it already looks like I may have an issue with my domestic hot water hardness that will lead to a premature life span of the boiler if i do not install a water softener. I'm waiting for a call back from my local water department.

    Follow-up questions:

    1. What is the purpose of the Erie Pop top Zone Valve
    2. Why is the Erie only on the 8 loops for the 1st floor radiant heat, and not the 2nd floor bathroom loops?
  • Jamie Hall
    Jamie Hall Member Posts: 19,954
    Unless your domestic water is very hard, softened water is actually more corrosive for the boiler than tap. For the heating side, you can (and possibly should) use deionized water with appropriate chemical treatment -- always assuming that you don't have leakage. On the domestic hot water side, moderately softened water may be desirable, but even with hard water simply descaling the heat exchanger from time to time (the manual will have the information on how to do that) is fine.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England