The above diagram is of a 220-240 volts heater circuit. You will notice that it is made up of five components;
(1) A contactor which is the switch that actually turns on the current to the (2) heater element. The heater element is similar to the heater elements on an electric stove. It converts electricity into heat by electrical resistance in the heater core.
The (3) thermostat, is an on/off switch that is adjusted by
spa user to the desired temperature. When the temperature drops, it
to "on" until the desired temperature is reached then it returns to
off". The (4) high limit switch is similar to the thermostat,
is set to one temperature, the highest allowable temperature (usually
degrees F.) before things start to melt. It usually has a reset button,
but it can also be automatic and reset itself after things cool down. I
much prefer the manual reset type, because the high limit only trips
something is wrong. The system should be checked before restarting. If
your high limit trips give us a call
Last, but in no way least, is the(5) pressure switch or flow sensor. There are two types available, one senses an increase in the water pressure in the heater manifold, the other type is a flow sensor, that actually uses a "paddle" placed in the plumbing pipe. As the water starts to move in the pipe, the paddle is pressed against and turns on the flow switch, indicating that water is moving through the heater.
The pressure type senses that there is water moving because the water pressure in the heater manifold is higher. When the pressure reaches a preset value, usually 1 to 2 pounds above the still water condition, it switches to the "on"position and stays there until the pressure drops.
One thing we absolutely do not want is having the heater on with no water flowing. Even though the "high limit" switch is there to shut things off, it is much better to never reach the high limit temperature.
If the heater is in a dry manifold, as during a spa water refill before the water flows, the high limit often will not sense the rapid change of the air temperature, and the heater turns bright red (just like the electric stove burner on high). This melts the PVC parts around the heater, or in the case of some with plastic heater manifolds, also melts the heater manifold housing.
In the above drawing, all three of the switching devices must be on in order for the heater contactor to switch on. This is the best scenario. If you eliminate either the high limit or the pressure switch, the heater can go on and melt down the spa equipment. This is one reason why a low flow circ pump is a really bad idea.
Those low flow "circ" pumps, the kinds that some spas are running 24 hours per day, (presented as some sort of "clean" system,) have such low pressure and flow that they can't operate a pressure switch. Often the heaters used with these spas are not very powerful (less than 4500 watts). The standard is 5500 watts and every watt is needed when the weather is cold!
The tiny "circ" pumps are responsible for heater element burn-out, heater meltdown, and a lot of wasted repair costs. I have known customers who have reset the high limit switch several times in an attempt to keep their spas from freezing. They did not know the tiny pump was not working. If the heater is turned back on several times with no water flowing, it begins to weaken and in some cases burns out. This really shortens the life of heating elements. Since customers are not technicians, when they see a "reset" button, they are going to press it. In this way they are unknowingly destroying their own heater.
For proper heater operation, the heater only comes on when there is water flowing. There must be three conditions met in order for the heater to be turned on. 1. The thermostat is switched on telling the system that the spa water is too cool. 2. The spa has not overheated and "tripped" the high limit switch. And 3. There is water flowing, thus the pressure or flow switch is set to "on". When all conditions are met then the contactor is allowed to turn on the heater.
Having a "current collector" around the heater is also very important. Spa heaters that don't have a metal shield directly around them, are not as safe. When spas are designed so that the nearest material to the heater is plastic or even a polymer, can ground out into the spa water, instead of grounding out to a metal shield very near the heater element.
It doesn't matter what the heater warranty* is, if your spa water isn't in balance, it is not good for you!
I have also seen heater elements that were 12 to fifteen years old, still functioning, without a spec of calcium (boiler scale) or rust. If you just take care of the water, your heater will last for a very long time. By the way, these spas all had pressure switches, and a standard modern two speed pump for filtering and heating.
I always recommend some sort of sequestering agent be added to the water. This will help keep the spa shell as well as the heater free from deposits.
If you own a spa with a tiny circ pump, we highly recommend that you have it removed and replaced with a more modern pump system! We also recommend that you install a pressure switch to insure your heater life.
*Often times spa companies will try to throw a spa buyer off track by having a special heater warranty, or component warranty. They do this because their shell/structure warranty is so bad. In their brochures they keep repeating over and over the heater warranty, so you might not notice how bad the main structure and surface warranty is; or how quickly they freeze.
For a really detailed explanation on spas today, order the booklet "How
Spas are Made", published by the Spa Specialist Inc.
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