Hot Tub Heaters
Heaters for Hot Tubs
The heater is arguably the most important component of the hot tubs equipment system, since hot water is the main reason people invest in a spa or hot tub in the first place. But let's begin with a point about heaters that can create confusion. The hot tub heater doesn't work as the water heater in the home does. Customers have asked me countless times why the water coming out of the return line isn't hot, like tap water in the home. The assumption is that the pool or hot tub heater holds a large reservoir of preheated water, as a household water heater does.
Instead, the hot tub heater warms the water as it passes over copper coils inside the heater unit. The coils are heated by electric current and the resulting water flow out of the heater is typically 10 to 25°F (5 to 10°C) warmer than the water that originally went in. Electric fuelled heaters can take several hours to heat even a small hot tub. A cover will help retain heat in hot tub water regardless of how the water is heated, speeding up the process somewhat.
Larger hot tubs, or those connected to a pool, are typically heated by gas-fuelled heaters, but the operational concept is the same as that of an electric-fuelled heater. Heat rising from a gas burner warms a copper heat exchanger. Water passing through the exchanger is heated with each successive pass.
Gas-fuelled heaters typically include a sophisticated control circuit, which ensures the proper operation of the unit. Electric fuelled heaters are also designed with the same features, although small electric fuelled heaters (for compact hot tubs) are normally controlled only by a thermostat and the other safety and operational controls are built into the hot tub control panel.
Finally, a very efficient and cost-effective way to heat a hot tub is with solar fuel. Much like the gas and electric counterparts, the solar heater works by passing water through pipes that are heated, thus warming the water. With a solar heater, the pipes are laid out inside of panels, which attach to a roof or can be set up on the ground. The sun warms the panels, and as the water passes through the panels, the sun's warmth is transferred to the water. Solar heaters cost no more than their gas or electric counterparts, and once installed, solar heaters cost nothing for the fuel itself.
Let's take a closer look at the components and operation of each type of heater and then examine the ways to keep them operating at peak performance.
Gas-Fuelled Heaters
In a typical gas-fuelled heater the water passes in one port of the water header (item 13) and through the heating coil (heat exchanger), then out the other port.
The heat exchanger tubes are made of copper, which conducts heat very efficiently, so the water picks up 6 to 9°F (3 to 5°C) on each pass. Heat rising from the burner tray (item 1) is effectively transferred to the water in the exchanger because of the excellent conductivity of copper. The tubes have fins to absorb heat even more efficiently and are topped with sheet metal baffles to retain the heat.
The front header has a flow control assembly. This spring-loaded valve is pressure-sensitive, designed to mix cool, incoming water with hot, outgoing water to keep the temperature in the exchanger from becoming excessive. This design keeps the outgoing water no more than 10 to 25°F (5 to 10°C) above the temperature of the incoming water to prevent condensation and other problems that greater differentials would create.
The other major component of the gas-fuelled heater is the burner ray. This entire assembly can be disconnected from the cabinet and drawn out for maintenance or inspection. Depending on the size of the heater, there will be 6 to 16 burners, the last one with a pilot mounted on it (item 3). Individual burners can be removed for replacement. The combination gas valve (item 2) regulates the flow of gas to the burner tray and pilot and is itself regulated by the control circuit.
Gas-fuelled heaters are divided into two categories based on the method of ignition.
The Millivolt/Standing Pilot Heater
As the name implies, the standing pilot system of ignition uses a pilot light that is always burning (Fig. 5-4, item 3). The heat of the pilot is converted to a small amount of electricity (1/4 of 1 volt, expressed as 0.750 volt or 750 millivolts) by a thermocouple (item 4) which in turn powers the control circuit. The positive and negative wires of the thermocouple (also called the pilot generator) are connected to a circuit board on the main gas valve.
When you light the pilot, it is necessary to hold down the gas control knob to maintain a flow of gas to the pilot itself. When the heat has generated enough electricity (usually a minimum of 200 millivolts), the pilot will remain lighted without you holding the gas control knob down. The positive side of the thermocouple also begins the electrical flow for the control circuit. When electricity has passed through the entire control circuit, the main gas valve opens and floods the burner tray with gas, which is ignited by the pilot.
The Electronic Ignition Heater
When the heater with electronic ignition is turned on, an electronic spark ignites the pilot, which in turn ignites the gas burner tray in the same manner as described above. In all other respects, these types of heaters operate the same as millivolt units.
Regular line current (at 120 or 240 volts) is brought into the heater and connected to a transformer, which reduces the current to 25 volts. This voltage is first routed into an electronic switching device called the intermittent ignition device (IID), which acts as a pathway to and from the control circuit. From here the current follows the path through the control circuit switches as described below.
When the circuit is completed, the current returns to the IID, which sends a charge along a special wire to the pilot ignition electrode, creating a spark that ignites the pilot flame. The IID has simultaneously sent current to the gas valve to open the pilot gas line.
When the pilot has lighted, the heat itself generates a current that is sensed by the IID through the pilot ignition wire. This information allows the IID to open the gas line to the burner tray, which is flooded with gas ignited by the pilot.
The Control Circuit
The control circuit is similar for both millivolt and electronic ignition heaters. The only difference is that one is powered by 25 volts of electricity and the other by less than 1 volt. The control circuit is simply a series of safety switches—devices that test for various conditions in the heater to be correct before allowing the electric current to pass on to the main gas valve and fire up the unit. Following the flow of electricity, a control circuit includes the following parts:
FUSIBLE LINK
The fusible or fuse link (also called a thermal fuse); a simple heat-sensitive device located on a ceramic holder near the front of the gas burner tray. If the heat becomes too intense, the link melts and the circuit is broken. This most commonly occurs when debris (such as a rat's nest or leaves) is burning on the tray or when part of a burner has rusted out, causing high flames. Other causes are improper venting (excessive heat builds up in the tray area), extremely windy conditions, or low gas pressure, causing the burner tray flame to "roll out" toward the link.
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