The hot surface ignitor
The hot surface ignitor lights the gas mixture in the combustion chamber or box. There are different shapes and styles of ignitors, but overall, they can be tested by performing a simple resistance test. First, set the voltmeter to read ohms and disconnect the ignitor from the wire harness. Make sure the ignitor is not hot as this will affect the reading. (If the heater had just attempted to fire, give it some time to cool down). After it has cooled, place one of the voltmeter’s leads on one pin of the hot surface ignitor and the other lead on the remaining pin. It is important to know what the proper resistance is for the ignitor; most manufacturers’ specifications will include this information. If the resistance reading is anywhere outside the specified range, the ignitor is defective and should be replaced.
The thermistor
The thermistor senses the water temperature and is also measured by a resistance reading. This reading will change depending on the actual water temperature. For example, a 10-kelvin (k) thermistor should read 10,000 ohms at 25.5 C (78 F). (Most of the leading manufacturers provide charts showing the proper resistance reading at various water temperatures.)
Using an accurate thermometer, test the water temperature. Set the meter to read ohms then, with the thermistor in place on the heater, remove the wires from the thermistor. Place one lead on each of the thermistor terminals and take the resistance reading. Refer to the manufacturer’s chart to determine if the thermistor is defective. An ohm reading outside the range specified on the chart would mean the thermistor is defective.
Sensor resistance at various temperatures |
|||||||||
|---|---|---|---|---|---|---|---|---|---|
| Temperature C | 16 | 20 | 21 | 25 | 30 | 31 | 35 | 40 | 41 |
| Temperature F | 61 | 68 | 70 | 77 | 86 | 88 | 95 | 104 | 106 |
| Resistance (k ohms) | 14.92 | 12.49 | 11.88 | 10 | 8.06 | 7.69 | 6.53 | 5.33 | 5.1 |
Wrapping it up
Once it is known which part(s) require replacement, it is a good idea to report back to the pool owner, unless it was determined in advance how much they were willing to spend to repair the heater.
For instance, the conversation might go something like this: “The good news is I know why your heater wasn’t turning on. The better news is, I can fix it for (insert total cost, including labour, here) and I can take care of it (right away, or name a time frame).” Do not apologize for the cost (no matter the price) or wait time (if applicable) for the repair. Always focus on the good news, which is their heater can be fixed.
Sue Robach is the national training manager for Pentair Aquatic Systems. She has been leading technical seminars and preparing training materials for more than 20 years. Prior to joining Sta-Rite, which was later purchased by Pentair, she ran her own pool service firm in Sacramento, Calif. She can be reached via e-mail at sue.robach@pentair.com.
