By Brad Duncan, National Pool Product Trainer, Raypak, Inc.
“Oh NO! The party is tomorrow, and I forgot to turn the pool heater on! That’s OK; I’ll just turn the temperature all the way up so that it heats faster.” I’ve seen this cringeworthy misconception many times, and it never fails to make me, well, cringe.
I think this idea comes from the way that automobiles function. The more you push down on that long pedal on the right, the more the engine revs and the more power you deliver to the wheels, which generally results in the car moving faster. So, one might conclude that your pool heater burns hotter when you set the temp higher, and your pool heats faster.
Thermostat is not a gas pedal
The problem is that your heater does not have a gas pedal. Residential gas pool heaters have two speeds: “wide open” and “dead off”. When the heater is burning, the fuel delivery is fully open, and all available gas is burned immediately, creating as much heat as possible for the size of the heater. This on-and-off procedure does not change by raising the temperature setting. The set temp is just there to tell the heater when to stop!
When the desired water temperature is reached, the electronics close the gas valve and shut down the fire. However, there are other functions that will shut down the heater or keep it from firing. These are mostly for safety reasons, and to ensure proper function. The most common of these is water flow, or open pressure switch.
Water flow is important
A pool heater is not a pot on a stove. It’s not a vessel of water being subjected to heat in close proximity. It is an appliance that heats water flowing through a series of tubes called a heat exchanger. Fire heats the tubes, and the water carries that heat back to the pool. The rate of the water flow through the heat exchanger is one of the most important variables in the safe and efficient function of a pool heater.
Campfire analogy
Let’s say you’re camping. Your hands have gotten a little cold, and you’d like to warm them over your fire. You approach the fire with your hands held out until you find a distance that provides the right temperature, by your estimation. You will then hold your hands in that position until they have gathered enough heat, then you will remove them. The temperature of the fire never changed, only the amount of time your hands were exposed to the heat. Exposure to heat over time is how we regulate water temperature inside the pool heater.
Exposure to heat over time
Because the temperature of the fire remains the same, the amount of heat gleaned by the water will vary based on how much time is spent exposed to the heat. And because the water is flowing, that time is regulated by the rate of flow; the faster the flow, the less heat is collected. Conversely, a slower rate of flow forces the water to gather more heat as it spends more time in the heat exchanger and is exposed longer.
These heaters are designed to accept a certain rate of flow, which varies based on the model and the BTU input, generally around 20 gallons per minute at the low end, and 125 gpm maximum. Between 60 and 80 gpm will generally accommodate a residential-sized pool heater. Within the proper incoming flow range, the flow inside the heater is further regulated by internal devices that operate on pressure and temperature.
Internal Bypass and Temperature Regulating Valve
Inside the front, or “in-out” header, there is an internal automatic bypass and a temperature-actuated valve. The bypass is spring loaded, so that pressure caused by excess flow will push the valve open and relieve the pressure as it allows some of the water to bypass the exchanger tubes and simply return to the pool. The temperature-actuated valve holds water inside the heat exchanger until it gathers enough heat, and then it opens to let the warmed water out. These devices work together to achieve the optimal flow. Not all heaters have both, but most do.
Error Messages
Remember, the pressure switch keeps the heater from firing unless there’s enough flow coming into the header, but improper flow within can throw you an error message. This is why heaters have High Limit sensors to shut the heater down when the internal temperature is too high, often due to failure of the internal bypass or temperature regulating valve (unitherm governor, thermal regulator, etc.) These High Limits will disable the heater, and an error message or code will appear on the display to tell you why the heater is not firing. Devices like these are integral to maintaining the safe operation of the heater.
Having good general knowledge of the operational limits of the heater will help clarify the expectations of how and when the heater will perform. While we in the industry may easily recognize these limits, we should bear in mind that homeowners and other pool equipment operators may not, and perhaps take a few minutes to explain how they work.
