Hello, pentair mastertemp 200. Turned it on for first time. Was getting crazy high water temp reading and it wouldn’t fire. Replaced thermistor and this allowed it to finally fire again. However now it’s still doing same as last season. On for about 2 mins… then off… off lasts 30-45 secs and then on again. Cycles forever. Is there more troubleshooting or parts to swap out?
Thermistor reading
- Thread starter aaron_w_o
- Start date
So my heater showed a super high water temp and wouldn’t come on. Easy google find to determine the thermistor was bad. Swapped in a new one. Heater worked! For fun I wanted to measure the resistance on the old one (which looked very good I might add). The resistance with water at 71 was 11.5(which must be k?) my meter was set to 200kohm.
I then pulled the new one out to see what it showed for water at 71. It was almost same reading! Maybe slightly less. 10.9
Now I know the new thermistor works and the old one doesn’t, but why would both give same reading on my meter?
Maybe old one is fine and just needed terminals cleaned? There was mild corrosion.
I then pulled the new one out to see what it showed for water at 71. It was almost same reading! Maybe slightly less. 10.9
Now I know the new thermistor works and the old one doesn’t, but why would both give same reading on my meter?
Maybe old one is fine and just needed terminals cleaned? There was mild corrosion.
Anyone ever try just leaving a bad thermistor in their heater and shorting the wires across a potentiometer? You could then dial in whatever resistance was needed to sustain the temp you want.
If the temperature starts out correct and quickly rises on startup, then the hot water is backing up into the inlet, which is usually an external bypass issue or maybe the heater is plumbed backwards.
Can you post pictures of the system?
Can you post pictures of the system?
Bad thermal regulator. Bad internal bypass. Both can lead to that condition.
Is there a way to troubleshoot which is causing issue? Should I just replace the parts? Believe there is also a high limit sensor in the manifold I can replace too.
Yes I can tomorrow when it’s daylight. But my system goes: pump->filter->heater inlet. I think I owned the heater for 5 years and it was last season that shirt cycling became a thing.
Yes, by changing any of the operating system you can bypass all the safeties, which is what appears to be working fine, and scald someone.
Sounds like the high limit is doing what it is supposed to do. Its just a "dumb" open/close switch that operates on heat. Look on the back of the board for a red trouble LED. The AGS is also a high-limit "dumb" switch. One may be lit saying the switch has opened. You may have to run the heater with the back of the board visible and see if one of the LEDs lights.
You can replace them both (high-limit and AGS) if you want, but since the heater does operate for a while, they seem to be working properly. Depending on what I find in the manifold I sometimes recommend it, they are just brass and subject to chemical damage sooner than the other parts.
What you are describing is usually a waterflow issue, either not enough to the heater or too much is bypassing the heat exchanger, which can be caused by the two items mentioned. You can test the thermal regulator, but have to physically examine the internal bypass. It is just a piston on a spring. If not enough water gets to the heat exchanger it can "flash boil" but usually just gets way too hot. the internal temperature of the firebox is upwards of 2000F (1093C).
The quick way to test the thermal regulator is to remove it (use a screwdriver shaft in the castle) run it under very hot water and see it if opens. If it comes out open, it is bad and no test need be done. There are more exact numbers (temperatures) for the test, but hot water from a home faucet is usually enough to see it operate. Since it is in the same water, I always replace the bypass valve if I work on one of these heaters. Learned that the hard way. That takes removing the manifold and dismantling it.
Can anyone explain why the old and the new thermistor gave me same readings for resistance on 70F? I thought on account of the heater showing like 126F that the resistance would also be wrong…
Temperature vs. Resistance Data The IntelliCenter® Control System use 10k Ohm thermistor sensors. When the solar sensor is disconnected from the control system, the sensor will read 10k Ohm at 77º F (25ºC). Refer to the following table for the resistance at other temperatures. An accurate reading should give a temperature setting that is accurate to ±0.2º C from 0 to 70 º C.
When out of the water you may get the same reading, but in operation the old one may have a slight crack in the probe allowing pool water to short it internally (pool water is very conductive), which will show the board as less resistance and, therefore higher temperature. Or, the sensor may just be bad.
You mentioned that this was the first time you operated the heater. Pentair/StaRite heaters have had a bad batch of sensors. There are two actual sensors in the heater, the thermistor for temp control and the stack-flue sensor for exhaust temp readings.
After going over my troubleshooting on a heater I installed six months ago with Pentair tech support, the man recommended that I buy two because they knew they had bad sensors still in the supply line and if one of the new ones was bad the other probably wouldn't be.
I buy the aftermarket ones available from Optimum Pool Technologies or Wholesale Sensors on Amazon. Have yet to have one fail.
surprisingly, the sensor I pulled out looked very good condition wise... almost new really. I was expecting it to be corroded like what is often seen on water tank anodes. Nevertheless, despite me getting the correct reading when *not* in the heater and getting ambient air 70F, the resistance was more or less correct according to these charts. So we have to assume the short you described, or, a bad sensor in some way... or that the wire spades were just scuzzed up and needed to be sanded. I suppose all I care about is the aftermarket one I put in works just fine and no longer reads the max water temp on startup. See photo of the "bad" thermistor attached.
correction: first time I operated the heater this season... not first time ever. for a few years the heater worked perfect. last year it started short cycling. this year it had a bad thermistor (now repaired) but still short cycling.
here's the pluming to the heater... nothing
special. top port is from the filter, bottom port is to the pool. believe that to be correct flow directions.