Hot Tub Heaters

Heaters for Hot Tubs

ON/OFF SWITCH

As the name implies, the on/off switch is usually a simple, small rocker-type or toggle-type switch on the face of the heater next to the thermostat control. On older models, the switch may be located on the side of the heater or remotely mounted, so the user can switch the unit on or off from a more convenient location. Manufacturers recommend that a remote on or off switch for a millivolt heater be located no more than 20 to 25 feet (6 to 7 meters) away from the heater. With less than 1/4 of 1 volt passing through the control circuit, any loss of power due to heat loss from running along extended wiring means that there may not be enough electricity left to power the gas valve when the circuit is completed.

Also, as the thermocouple wears out and the initial electricity generated decreases, the chance that there won't be enough power becomes very real. Therefore, I suggest from my own experience that remote switches not be located more than 10 feet (3 meters) from the heater and that they be run through heavily insulated wiring to avoid heat loss.

THERMOSTATS

Thermostats also called temperature controls, fall into two categories, mechanical and electronic.

The mechanical thermostat is a rheostat dial connected to a metal tube which ends in a slender metal bulb. The tube is filled with oil, and the bulb is inserted in either a wet or dry location, where it can sense the temperature of the water coming out of the heater. These thermostats are not precisely calibrated because so many installation factors will affect the temperature results. In other words, setting the dial at a certain point may result in 95°F (35°C) water for one spa while the exact same setting may result in 105°F (41°C) water for another.

Therefore, spa heater thermostats are generally colour-coded around the face of the dial, showing blue at one end for cool and red at the other end for hot. Settings in between are used by trial and error to achieve desired results.

As shipped from the factory, thermostats will not allow water in the spa to exceed 105°F (41°C). Also, they do not generally register water cooler than 60°F (15°C), so if the water is cooler than that, you may turn the thermostat all the way down and the heater will continue to burn. Therefore, the only way to be sure a heater is off is to use the on/off switch.

The electronic thermostat uses an electronic temperature sensor that feeds information to a solid-state control board. These are more precise than mechanical types; however, due to the same factors noted above, they are also not given specific temperatures, but rather the cool to hot, blue to red, graduated dials for settings. Some manufacturers of spa controls make specifically calibrated digital thermostats, but my experience is that no matter what the readout says, the actual temperature may vary greatly.

Some heaters are equipped with dual thermostats, so you can set one for a comfortable pool temperature and the other for the spa.

High Limit Switches

While the fusible link detects excessive air temperatures, the high-limit switch detects excessive water temperatures. High-limit switches are small, bimetal switches designed to maintain a connection in the circuit as long as their temperature does not exceed a pre-designed limit, usually 120 to 150°F (49 to 65°C).

Often the first high-limit switch is a 120°F and the other a 150°F switch. They are mounted in dry wells in the heat exchanger header for this reason. Sometimes a third switch, called the redundant high limit switch is mounted on the opposite side of the heat exchanger for added safety.

PRESSURE SWITCH

The pressure switch is a simple switching device at end of a hollow metal tube (siphon loop). The tube is connected to header so that water will flow to the switch. If there is not adequate water flow in the header, there will not be enough resulting pressure to close the switch. Thus, the circuit will be broken, and the heater will die down.

Although preset by the factory (usually for 2 pounds per square inch, or 1 millibar), most pressure switches can be adjusted to compensate for abnormal pressures caused by the heater being located unusually high above or below the water level of the spa.

AUTOMATIC GAS VALVE

Often this is called the combination gas valve because combines a separately activated pilot gas valve with a main burner ay gas valve (and sometimes a separate pilot-lighting gas line combined with the pilot gas valve).

After the circuit is complete, the electricity activates the main gas valve which opens, flooding the burner tray. The gas is ignited by the pilot, and the heater burns until the control circuit is broken at any Joint, for example, when the desired temperature is reached and the thermostat switch opens; when the fuse link detects abnormal neat and breaks the circuit; when the on/off switch is turned to off; or when the pressure drops (when the time clock turns off the pump/motor) and the pressure switch opens and breaks the circuit.

The control circuit for a typical combination gas valve is connected to the terminals on the valve. The gas plumbing of the automatic gas valve is self-explanatory. The large opening (1/2- or 3h-inch; 13- or 19-millimeter) on one end, with an embossed arrow pointing inward, is the gas supply from the meter. Note that it has a small screen to filter out impurities in the gas, such as rust flakes from the pipe. The hole on the opposite end feeds gas to the main burner. The small threaded opening is for the pilot tube, and a similar hole is for testing the gas pressure. These are clearly marked.

Automatic gas valves are also clearly marked with their electrical specifications, model numbers, and, most important, "Natural gas" or "Propane." Black components or markings usually indicate propane.

All combination gas valves have on/off knobs. On 25-volt units, the knob is only on or off. With standing pilot units, there is an added position "pilot" for when you are lighting the pilot.

Natural versus Propane Gas

The differences between heaters using natural gas and those using propane gas are nominal. Most manufacturers make propane heaters in standing pilot/millivolt models only. Because of different operating pressures, the gas valve is slightly different (although it looks the same as a natural gas model), as are the pilot light and the burner tray orifices. The gas valve is clearly labelled "Propane." The heater case, control circuit, and heat exchanger are all the same as those for a natural gas model.

Although there is a section later in this chapter regarding heater safety, it is essential to mention the danger of working with propane right here. Natural gas is lighter than air and will thus dissipate somewhat if the burner tray is flooded with gas but not ignited for some reason. Similarly, the odour perfume added to natural gas will be detected if you are working nearby as the gas floats out and upward. Make no mistake, this is still a serious situation, and explosions can occur.

With propane, however, the gas is heavier than air, and as it floods the burner tray without being ignited, it tends to sit on the bottom of the heater. Because it remains undisputed and because you are less likely to smell it since it is not floating out and upward, if it does suddenly ignite, it will do so with violent, explosive force. Rarely is the heater itself damaged—the explosion takes the line of least resistance, which is out through the open front door panel and into your face, which is probably poised in front of the opening trying to learn why the heater hasn't yet fired. There is more on safety procedures later, but for now just remembering treat propane with great respect.

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