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Gas Condensing Boiler - Coming from an oil boiler world

jdbs3
jdbs3 Member Posts: 6
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: 6
    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: 2,094
    edited March 27
    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 Shore.
    Cleaned & services first oil heating system at age 16
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,857
    @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: 178
    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: 14,755
    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: 6
    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: 2,094
    edited March 31
    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 Shore.
    Cleaned & services first oil heating system at age 16
  • Mike_Breault
    Mike_Breault Member Posts: 8
    edited April 9
    @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

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