5:1 turndown is soon a thing of the past. It was only limited to that for testing reasons but they have been resolved so look to see some 20:1 turn down coming soon.
That said, if a house has a design heat loss of less than 50Mbtu, it is better to use a vertex or polaris HE tank, if you want to use gas. Still put them in but getting away from boilers, more and more.
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