The Community Spa Mystery
Kevin Kennedy Associates was contacted by a law firm representing owners of a large condominium. In the recreation area of the condo was a community spa which had caused burns to a user due to overly high water temperature. The plaintiff user claimed that the owners knew or should have known that the spa created a hazard and was unsafe to people attempting to use it, and further, that the owners were negligent in failing to maintain, inspect, and correct the unsafe condition, and had failed to give warning to the potential users of the unsafe condition.
Our expert consultant’s task was to determine the root cause of the overheating, and to determine if there could have been any signs or indications that, with the exercise of ordinary care, the owners should have noticed and corrected before an injury could occur. To determine the root cause, our expert utilized Failure Mode and Effects Analysis (FMEA), which is a procedure to establish potential failure modes within a system and to determine the failure’s effect upon the system.
The established facts were that the spa’s heater that was in place at the time of the incident was no longer available (the 250,000 BTU model replaced by a 265,000 BTU model), that the gas valve and heat exchanger were replaced about three months before the incident, that the temperature of the water shortly after the incident was determined to be at least 120°F, and lastly, that replacement parts were purchased the day after the incident. Our expert’s investigation revealed additional information that the type of thermostat that was in the heater at the time of the incident was a bulb and capillary type, and that the spa manufacturer estimated that the time to heat the spa to over 120°F would have been approximately 64 minutes.
Despite the fact that there are many different style spas, most operate in the same manner. There is a body of water, a pump, heater, filter, chemicals, sometimes a blower, plumbing, and a control system that regulates the interactions of the various parts, all of which are designed to heat water to a temperature that is pleasingly warm.
Many of these components within a spa do not contribute to heating the spa, and since plaintiffs’ allegations center on the spa overheating, all these items were ruled out. However, the heater and the heater control system both contribute to the heating of the spa and therefore, both items were investigated in depth.
The spa in question operated as do most commercial spas. The spa heater does not run continuously. If the water coming from the spa is at or above the set point, (the temperature that the operator would like to maintain in the spa), the heater is off. When the water coming back from the spa drops below the set point, the heater control system turns the heater on. As warmer water being pumped back to the heater again reaches the set point, the heater control system will again turn the heater off.
If a spa fails to maintain the correct set point then there is a problem with the heater or the control system. It is important to note that some of the ways that spas can fail to keep the correct temperature will result in a gradual inability to keep the temperature at a constant set point, while other failure modes will result in a sudden inability to maintain that set point.
Bulb-and-capillary type thermostats have a fluid-filled bulb that is placed into the stream of water. The fluid expands when the bulb heats up and contracts when the bulb cools. Once it contracts, the plunger on the switch end of the tube draws back and signals the heater to come on to begin warming the water. As the water temperature rises, the fluid in the bulb expands, and the fluid pushes the plunger, causing the electrical connection to open, signaling the heater to turn off.
These thermostats have a certain range of temperatures that they can operate within. The distance between the plunger and the electrical contact is adjustable by the end user, so that the set point can be anywhere within its range. By rotating the knob, the distance that the plunger must travel to open the electrical connection and turn off the heater increases. This means that the fluid in the bulb must expand even further to push the plunger that additional distance. Conversely, the end user can decrease the set point of the spa by rotating a knob on the thermostat the other direction.
This expanding and contracting of the bulb fluid, with its accompanying extending and retracting of the plunger, happens thousands and thousands of times over the life of the heater. This happens automatically and without intervention from the spa operator; in fact, the spa operator only comes into contact with the thermostat via the knob that is rotated to establish the set point. Once the correct knob position for the thermostat has been set, the knob does not have to be adjusted further.
It was quickly realized that only failure modes that would cause the thermostat to continue sending command current to the heater needed to be evaluated. The first step in analyzing possible failure modes was to identify the potential failure modes of each component. In a bulb and capillary tube thermostat, each component of the thermostat had to be evaluated with regards to function, to see what failure mechanism(s) might affect that component. The thermostat has a bulb, capillary tube, bulb fluid, plunger, welded joints, rocker arm, rocker arm hinge, electrical switch, internal contacts, external contacts, factory adjusted set screw, and a housing.
The bulb, bulb fluid, capillary tube, and welded joints have failure modes that would bring about a sudden and instantaneous change in the thermostat’s ability to maintain a set temperature. Failure mechanisms that would result in an instantaneous failure mode would include structural overload or electrical overload.
An electrical overload could have caused the electrical contacts to stick together so that the plunger was not able to open the circuit which would keep the heater running. This failure mode is not impossible, but it is rarely the mode by which these thermostats fail. Usually the opposite happens. The contacts get corroded or covered with carbon deposits and are no longer able to conduct electricity, which in turn does not signal the heater to come on.
The most common method of instantaneous failure in a bulb and capillary thermostat is that of a leaking of bulb fluid. The fluid in the bulb is always under pressure, contained within a relatively rigid conduit, and there is a very small, fixed amount of fluid that opens the electrical circuit and turns off the heater. Since there is such a small amount of fluid (just over 1/10th of an ounce), any leak at all will cause the thermostat to be unable to turn the heater off.
By taking the presented facts and newly acquired facts, plus sound engineering principles and experience, the root cause of the failure was determined. There are only two parts of a spa that can cause the water to overheat; the gas burner and the control system. In this case it was the control system (the thermostat). This is based upon two main points:
- First, the gas valve and the heat exchanger, the main parts of the burner, were less than four months old. The heater itself was older than that, but maintenance replaced the gas valve and the heat exchanger in the middle of November, 2003.
- Second, the day after the incident, a new thermostat and some other items were purchased. Of these items, the thermostat is the only one that regulates temperature, and after the new thermostat was installed the spa was once again able to accurately hold a set temperature.
If there was something other than the thermostat involved in the failure, the owners would have had to replace other temperature-regulating related items. Since they did not, that left the thermostat as the root cause of the overheating.
Since it has been established that the root cause of the overheating was the thermostat, the final issue is one of signs of imminent failure. Was there anything that the owners could have done or should have observed that would have led them to believe there might be a problem? The answer to that was no. The bulb, bulb fluid, capillary tube, and welded joints are all subject to instantaneous failure from either an electrical or structural failure, neither of which could have been anticipated. That is why the manufacturer of the heater plainly states in the maintenance section of their manual that the thermostat is not serviceable. There is no maintenance schedule for this part, no replacement schedule for this part, and no ways to field calibrate this part.
The Kevin Kennedy Associates expert concluded that, in the course of exercising ordinary care, or even extra ordinary care, neither the maintenance men nor a trained pool professional could have anticipated either of these failure modes, due to the fact that neither mode would have provided any prior indication of failure. There is no way to determine, nor are there any signs, that there is an impending leak when using this type of thermostat. Unlike other components that can be removed, checked, and then reassembled, these types of thermostats are not designed to be removed, but are designed to be installed as subcomponents of the heater and to work for the full life of the heater.
Finally, there is the issue of time from failure to time to reach a high temperature. As previously noted, the owners replaced both the gas valve and the heat exchanger less than four months before the incident. This means that this spa heater was able to heat water as fast as a brand new heater. In addition to installing the new components, the maintenance person started up the system and checked the operation. So, as of three months prior to the incident, the owners had a properly functioning spa heater that had been set up, checked and given a clean bill of health from a professional pool and spa installer.
Allowing that the gas valve and the heat exchanger were virtually brand new, we needed to determine how long from the thermostat’s failure would it have taken to reach 120°F. Given that the water was at the set point when the thermostat failed, (103°F), the manufacturer of the heater estimated that it would take approximately sitxy-four minutes to heat a 1280 gallon spa the additional 20°F. We concluded therefore that an hour (from the time of failure to the point where the water reached 120°F) was logically too short a time for the owners to have determined that failure occurred, and closed the spa.
This article was written by Lead Consultant,
Glenn Akhavein, and edited by Lead Consultant,
Tom Weisgerber. Glenn can be reached at 317-536-7029 or via
email. Tom can be reached at 317-536-7009 or via
email.