I am the proud owner of a Weil Maclean Ultra 105 boiler; it was
insalled this past May. Generally, I like the unit, but I am confused
about modulation: The literature says it has "20-100% modulation."
Does that mean it fires at any rate between those two? Or at JUST 20%
Yes. Most of this type of boiler senses the temperature and will trim the
fire to maintain a steady water temperature as opposed to the older methods
of turning the burner off, starting it back up again when the temperature
comes down, shutting off again, and repeating.
I don't know the exact setup WM uses, but let's say the maximum temperature
is going to be 185 degrees. At about 180, it may go down to 20% fire and
let it creep up slowly. On a very cold day, the water coming back to the
boiler may be 120 degrees and that will cause the burner to go back up to
100% until it reaches a warm enough water temp to start backing down. The
result is more even heating, less temperature swings.
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