My Eternal also "foghorns" sometimes, but at it's lowest modulation while on, around 26kbtu. Spooky and comical low hoot sound-almost as if the flamefront has resonant pulsation. Ignition is sometimes a problem so I get an occasional failure to ignite code if it is shortcycling frequently while the inlet temp is warm, it's fine if igniting with cooler inlet temps. Seems like warm inlet temps show less of a temp change for it to recognize the burner has lit. If it ever shut off due to a code while I was away for a while in subfreezing temps....oh my.
I imagine the condensing storage heaters I describe would be more reliable, as they don't modulate or don't modulate very far (say a 3:1 modulation ratio, instead of 5.5:1 like my Eternal) and are meant to ignite while already seeing warm temps because the tank is already warm. I likely wouldn't choose an Eternal again and BBP's comments would make me yearn for field knowledge if I were choosing a new heater. I know many utterly reliable tankless water heaters with high modulation exist. My Grandparents had at 20+ year old Bosch that never had a problem.
For those considering gas for their heat source, anyone want to chime in with practical experience on condensing water heaters similar to BBP's reports on boilers?
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