Swimming Pool Blog

Digital Pool Heater Troubleshooting Guide

Millivolt Pool Heater Troubleshooting Guide
by Mark Garcia October 20, 2015

Third in our series of Gas pool heater troubleshooting guides, today we cover digital pool heaters introduced in the early 2000's.

Digital pool heaters aren't much different from their predecessor, the Spark Ignition generation. These heaters are still powered by incoming 115 or 230 volts, into a transformer that steps down the power to 24 volts.

Like previous electronic heaters, the ignition and firing of the heater is controlled by an IID (Intermittent Ignition Device), also called a module. The grey box houses a PCB (Printed Circuit Board) that monitors the heater's safety circuitry and combustion.

Unlike previous electronic heaters, the new Digital generation sports a fancy UI (User Interface) Control Panel, an LCD screen that displays temperature, error codes, time of day. It also has built-in freeze protection and programmable heating modes.

But, aside from these bells and whistles, digital pool heaters still function in the same way as millivolt heaters. Once the safety circuitry checks out, the gas valve opens and releases gas to the burners. The pilot light ignites the first burner (on the far right), and the others follow - whoosh!

The diagram on the right shows how air is drawn into a gas heater, fanning the flames of the burner tray, and heating the 9 copper finned heat exchanger tubes at the top of the heater. When all is working well, that is - and you probably are here because you are having pool heater problems.

So, let's get on with it then!


For our example heater, we will be using the Laars Lite 2 Digital heaters, made from 2003-2009; but the concepts apply to digital heaters made by Raypak Digital, Hayward Universal or Pentair Minimax NT and MasterTemp pool heaters.


What we have here is a schematic diagram of the safety circuit in an electronic gas pool heater. The power does a circuit, or a loop, from the transformer blue wire (1) to the red, then white wires of the hot surface igniter (7).

When the power successfully passes through all of the components - fusible link (4), high limits (5), pressure switch (6), thermistor and again returns to the Power Control Board, the gas valve receives an all-clear signal (8) and opens to allow gas to flow into the burners which is lit by a fuse protected (7 c/d) hot surface ignitor (glow plug). Whooosh!

Let's take it step by step...



Set your multi-meter to 250 VAC, and check for voltage between the wires coming from the transformer. If no power is found, check the incoming power (circuit breaker), and also check that the conversion block is set to match the incoming voltage (either 115V or 230V). If power is proper coming into the transformer (115V or 230V), but not coming out of the transformer (24V and 12V), replace the transformer, pn R0366700. NOTE: All troubleshooting steps are done with the heater on and the thermostat turned up past the current water temperature. 


With your multi-meter still set on 200-250VAC, keep the common lead (black probe) touching the blue wire (1). Open the black igniter wire fuse case, and hold the red probe on the far end of the fuse and check for 24 V. You can also visually inspect the fuse and red wire for damage. After this test, wedge the probe into the terminal if possible, it will stay on the blue wire (2) buss bar terminal for the next several tests.


Next, find the Fireman's Switch terminal on the power control board and move the black probe to the blue wire terminal (2), and use the other meter probe to test for 24V at both terminals of the fireman's switch. If your pool pump uses an external timeclock or controller, these wires may run out of the heater to the timeclock, with the purpose of shutting off the heater 20 minutes before the pool pump shuts off - for a cool down period. If not, two short wires will be joined with a wire nut, as shown in the diagram above. If there is no power at 3a, check again to confirm a faulty Control Board, pn R0366800. If no power at 3b, tighten the wire nut, or follow the wires out of the heater and find the short, or where the power dies. 



With the common probe still touching the blue wire (2), touch the other meter lead to both sides of the Fusible link. If there is no power coming in, check wire connections to be sure they are tight, clean and not broken. Also inspect wires for heat or rodent damage. Barring those possibilities, replace the Control Board if no power is coming from the board to the Fusible Link. If there is power coming in (red wire), but no power coming out (white wire), replace the Fusible Link, which fails (on purpose) when it overheats.

The Fusible Link is really a mechanical flame roll-out sensor. If it gets too hot in the front of the heater, a small wax pellet inside of the link will melt and open the circuit. If your Fusible Link has failed, it's important to investigate the cause. Flames (or excessive heat) coming out the front of the heater means that something is blocking the heat from rising up through the heat exchanger, which could be sooted or covered in leaves. The burner tray could also be blocked with nesting material or insulation material.



Pool heaters use two High Limit switches to shut off the heater if excessive temperatures are sensed. You can test both High Limits at once by bypassing them and checking for 24V at the white wire (incoming wire) on the pressure switch. If 24V is found on the white pressure switch wire, then both High Limits are good. If there is not 24V coming into the pressure switch, use your meter to test each High Limit individually, to find out where the power dies. If the high limits are open, you should investigate the cause. Open up the front header side port to inspect the internal bypass assembly, which may be bypassing too much water. Also inspect the heat exchanger for bowed tubes or lime deposits on the inside. A temperature rise test can be performed, by inserting a Laars thermometer, pn R0366000. If you find that one High Limit has gone bad, replace both with the High Limit Assy, pn R0023200. 



Now you can test the black wire of the pressure switch, which lets you know if the pressure switch is the problem. As with High Limit and Fusible Link failures, if you find no power (24V) coming out of the pressure switch, you should investigate the reason why. Usually, it's a dirty filter, clogged pump basket or impeller, open internal or external bypass assembly, or other water flow problems. The pressure switch will open if there is not a safe volume of water flowing through the heater. A Back Pressure test can be performed by screwing a pressure gauge into the drain plug on the front header. If you find over 2.5 psi, the flow is good and the pressure switch is bad. If however, there is 24V on both sides of the pressure switch, you can move on to the next step.


For Step 7, remove the meter probe from blue wire (2), and make a visual inspection of the Hot Surface Igniter. It should glow red hot within 15 seconds (you may need to shield the sunshine to see the glow). If the igniter does not heat up, disconnect the white and red wires that lead to the igniter and set your multi-meter to Ohms and check the resistance. A good igniter usually will test at 60-80 Ohms. If zero or very low resistance is found, replace the Igniter, pn R0449800. If there is resistance, check each fuse for continuity (or visually check for a blown fuse filament). If both fuses are OK, replace the igniter (carefully, they are very fragile items). Small cracks in the carbon igniter surface can prevent heat up, while still producing resistance reading on your meter.



Reinsert the common probe of your meter into the or onto the blue wire (2), and place your other meter on the brown wire, where it connects to the gas valve. As with all tests, the heater should be turned on, with the thermostat turned up. After 15 seconds or less, you should see the Igniter heat up  (red hot) and you should test 24V at the brown wire, on the gas valve. Be sure to touch your meter lead to the wire metal or terminal metals, to obtain a good reading. If there is not 24V - check through everything again before you plunk down $250 on a new Control Board, pn R0366800. If there is 24V at the brown wire, but the heater still won't fire you may have a bad gas valve. Check the gas supply and be sure that gas volume and pressure are proper, before you plunk down $200 on a new gas valve, pn R0317100.



After running through all of the steps at least twice - if you want some confirmation of your Laars heater troubleshooting, you can call us at 877-766-5287 (1-877-POOLCTR), to speak with a pool tech, who can answer any questions on the process. Jandy Laars (Owned by Zodiac) also has homeowner technical support, and heater manuals online, or call them at 800-227-1442. 

And.... here's the link for over 150 gas pool heater parts schematics! Support your local swimming pool blog! If you've found our digital pool heater troubleshooting process helpful, please pay a visit to our store for any parts or supplies you may need.


Thanks for Reading!
Mark Garcia