Heater says that water temperature is at 126 degrees and won't turn on

I have a couple of thoughts. You may have air in the boiler water circuit at the loop that feeds the tank. You'll have to find the vent in the loop to get it out. Second, the automatic valve that opens for hot water may not be opening fully. Have you felt both the boiler water lines that go in and out of the tank? If the water going in is hot and the line out is very cold you have very little boiler water going through the loop. Make sure the isolation valves on the loop are open. Is your boiler full of water? If the boiler loop lines are hot going in and coming out, the tank is full of rust or debris and no water can get to the fins on the internal tubing to pick up heat. If that's the case there is no repair and its time to get a new tank. Hope this helps good luck.

How big a pool? You normally will not notice a temperature rise immediately. On average, swimming pools will see a one degree rise in temperature every 45 - 90 minutes.

I recommend safety changes to prevent ruptured water heater and assorted leaks.
135 degree hot water.
Maybe replace TP valve if it is dripping.
Drip pan under water heater with 3/4" drain line to outside.
Expansion tank on incoming cold water line to absorb pressure build-up, and prolong life of tank and plumbing.

Here's why:

1) Yes you can have scalding hot water.

2) I recommend 135 degrees since that will burn the evil right out of ya. And generally kill clothes washing bacteria.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) 150 is dangerous and can seriously burn skin in 1.5 seconds. Not safe around children and elderly.

4) Temperature settings on electric water heater are approximate, since thermostats are calibrated approximately, and read temperature through tank wall.

5) Also if you want to increase amount of hot water for high-consumption household, then setting tank temperature upward is one solution since more cold water is mixed with hot, so amount of hot inside tank goes further.

6) High temperature can cause TP valve to release water.
TP valve is pressure-temperature safety valve. Do NOT cap off.
Rating should be 125 psi and 180 degrees ... but all things water heater are approximate, and TP valves need replacement now and then.
It is not recommended by water heater manufactures to use 150 psi TP valve when water heater came with 125 psi. Rating shows in small print on end of TP. New TP valves sold at local hardware. Let tank cool before installing new one.

7) Higher temperature and high pressure can rupture tank.
Pressure over 80 psi shortens life of water heaters and plumbing fixtures. Put BTG100 pressure gauge on water heater drain valve to check pressure.

8) You need a drip pan with 3/4" drain line to outdoors, or to drain vent line, to prevent flood caused by ruptured water heater.
Turn water heater off when going on vacation.
Inspect water heater each 6 months.
Drip pan available at local home center or hardware:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#drip

9) Heated water expands and puts pressure on tank and pipes.
Water does not compress... so when water gets hotter it expands against tank wall and against pipes.
Especially in closed system (check valve present) when system is without expansion tank.
Expansion tank is installed on incoming cold water line.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#expansion

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Definitely bad thermistor Pentair reps have seen this happen a lot. Lick finger and touch the two contacts where the thermistor plugs in it should fire up if it's a bad thermistor. If you do an ohm reading you'll find that the ohms do not correlate with the temperature that's displayed. I just trouble shot one and I'm getting 51.2 ohms which equates to 10 - 20 degrees Fahrenheit which is obviously not what is being displayed because it's 126 which is a common number that's displayed when a thermistor is not reading properly

Use a thermometer to check the actual temp of the water to see if the units temp probe is accurate. Also what is the max temp allowed by the spas safety program...owners manual.
Thanks,
RSMITH972278

This is a problem with your thermister (heat sensor) You will find this on the left side of the water manifold about half way down. I'm not sure but I think it is a small tube that slides into the manifold and if you follow the wires back to the board it just plugs in. The part is reasonable priced for a heater part and very easy to install. Good luck.

I am not sure if you have resolved this issue, or not, but just in case. Yes, the problem is the Thermistor. This is a sensor that "reads" the incoming water temp. It is really easy to replace as well.

The 126 is not an error code, but it maybe the temp of the water that the heater is reading. If the heater thinks the water is 126 degrees, it will not start at all.

If the heater is reading the wrong water temp, the problem is the Thermister, which is the sensor that is suppose to read the water temp. You will need to replace this sensor in order to correct the problem with the heater. It is really easy to do!

If you have a manual for the heater, if you go to the Replacement Parts Listing for the Electrical System, page 53 in the online version, the Thermistor is Item #9. It goes into the left side of the manifold in the to place, between the first two manifold bolts.

would not suggest trying to manually light,its an electronic ign model so you may damage some wires with an open flame.try powering off then turning back on and see if display retuns to normal

Anytime the heater is reading the water temp to be above what it actually is, especially 126, the problem is the Thermistor, or Temp Sensor, for the heater. This is a very easy part to replace. The Thermistor is located on the manifold, on the left side, between the top two bolts.

Sorry this is a late response! Hope that it helps!