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.
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