Circulation Pump Based Heating Systems
The idea of the dedicated low volume circulation pump originated many years ago. This method of continuously circulating and heating the water is probably the most commonly manufacturered system today.
The following description is based on the popular Leisure Bay Spa, manufactured by Leisure Bay Industries. It uses a composite heater, made by RMF, and a digital spa control system by Balboa Instruments.
Your spa may be much like the one depicted here, with minor differences in the style of heater, plumbing, and component placement.
The basic components of the system consist of the heater "can" which contains the heater element, the high limit sensor, and the pressure switch.
On most systems of this type, the heater element is enclosed inside the stainless steel can, and is not replaceable. It must be replaced as an entire unit.
The high limit sensor is generally mounted in a pressure fit slot on the side of the heater to detect over temperature conditions.
The pressure switch, mounted somewhere on the side of the heater, or some place in-line with the water flow, detects the presence or absence of water pressure in the unit. This switch is what is responsible for the infamous "flow" or "flo" error on your side control panel.
The circulation pump, under control of the spa control system, is normally running most of the time. -Note that this is not always true, as in some systems, the software will shut down the pump according to filtration or heater parameter settings-.
The circulation pump will take water from the filter area, or from plumbing that leads to the filter, and discharge it directly into the heater case. Water flowing through the case creates pressure that is detected by the pressure switch.
The spa control system reads the pressure switch activation, and will activate the relays which send power to the heater element
Virtually all pressure switches have the capability to be adjusted. However, this is not recommended unless you know exactly what you're doing.
Properly calibrated for the correct pressure level at the factory, with proper care of your spa water quality, the pressure switch should operate for years without needing replacement, or adjustment.