Rinnai PN-721 N Series Anti Frost Heater (External) - The Genuine Article The Rinnai PN-721 Anti Frost Heater is a replacement part you can trust, specifically designed to fit Rinnai N Series external continuous flow water heaters. This heater is a critical component of the unit's built-in anti-frost system, and its job is to keep crucial internal bits from turning into ice in really cold weather. These Rinnai N Series water heaters are pretty clever - they've got a built-in system that kicks in the heating elements when the temperature plummets so the internal pipework and components don't freeze up. That means even in the really harsh winter conditions the external units will still work like a dream. But what if the PN-721 Anti Frost Heater decides to pack up on you? The water heater's a gonner then, and you're left with a