Spa heater keeps tripping the high limit, chasing my tail here

Started by btu_frank · · 2 replies
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#1
Gas heater on the tub trips the high limit switch about ten minutes into a heat cycle. Flow looks fine, cleaned the filter, checked for airlock. Did the math on BTU rise for the tub volume and it's not oversized for what it's heating, so it's not that.

In the trade we'd look at a restricted heat exchanger or a bad flow switch before condemning the board. Anybody run into this on a tub specifically. Wondering if it's just scale built up in the exchanger from years of not watching calcium hardness close enough, wouldn't be the first time I've seen that on a residential unit. Not ready to call a tech yet, want to rule out the obvious first. FN

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#2
Frank, this is way over my head on the technical side, but the calcium hardness comment jumped out at me, ours crept up over 300 ppm one summer before I started actually watching it and I do wonder now if that was doing some quiet damage we never traced back.
#3
Scale in a heat exchanger builds narrow first, restricts flow through the tight passages before your overall flow switch even notices anything wrong at the pump. That's exactly a high limit trip pattern, not a bad board. If your fill water runs hard get it descaled before you replace anything electrical.