Rinnai PN-721 N Series Anti Frost Heater (External) - (PN-721) Rinnai N Series external continuous flow water heaters use an anti-frost protection system to keep internal components from freezing in cold conditions — electrically powered heating elements that activate automatically as temperature drops, protecting the appliance down to -20°C. It's what allows an external unit to keep working reliably through harsh winter conditions rather than seizing up the first cold snap. If this anti-frost heater fails, that protection is gone, and a hard freeze can put the appliance out of action with a repair bill well beyond the cost of the part itself. The PN-721 is the genuine Rinnai replacement for the N Series, made to the original specification, restoring frost protection and letting the appliance run as intended through the coldest part of the year.