ok - help me out here as i've not yet ordered either. i was going to order the t-stat circuit board. i have already replaced one of the thermistors 38416 on the outlet side of the heater. i tested the inlet thermistor 38415 (the one you mentioned) and at any given temperature its ohm readings are consistently just above the ohm readings of the outlet thermistor 38416 (which was recently replaced).

for a total of around $75 i could replace them both - the inlet thermistor and the control circuit board...

what do you think?

alright - i broke down and ordered both parts. i'll report back once they're installed...

for the record, i was able to disconnect the heater from the spa and connect it directly to my GFCI outlet using an adapter - while ensuring that the 24/7 circ-pump continued running. i got the tub up to 103 degrees in less than an hour (from 99 degrees). so i know now that heat loss is definitely not an issue, and i also know that at 103 a person can really lose themselves in a nice dip in the tub ...

thanks for all of your help swine and charger_1. i'll report back when i know more.

Don't worry about buying both-in the long run- it's small potatoes. Though it does sound like you have perfected the art of troubleshooting your spa (ever in SW florida- I need good technicians! ) -the electronic thermostat is your problem- it won't hurt to replace the thermistor- if it's the original, chances are you'd need a new one anyway sooner than later.

One more thing of note when installing the new thermistor (though it sounds like you have a great handle on things already replacing the hi-limit thermistor)- be fairly gentle in removing and installing... The plastic heater housing can crack (if you still have the original no-fault-1500 heater) if the thermistor is overtightened.

Good luck, and enjoy- wonderful spa weather!

the unknown item is the flow switch. Used with the original cartridge heater. It's no longer needed with your new PDR heater. You need the jumper that plugs in where the flow switch plugs in. You may need a new control thermistor, or electronic thermostat. An Ohm reading of both thermistors with an accurate water tempature would tell which one you need. You can email us the results if you'd like. scarritt@snet.net

hottub.pool_boy wrote:

the unknown item is the flow switch. Used with the original cartridge heater. It's no longer needed with your new PDR heater. You need the jumper that plugs in where the flow switch plugs in.

i actually received two jumpers with the heater, one with a red wire, and one with a blue wire - the blue wire is slightly larger than the red wire. both are designed to jumper the flow switch - does it matter which one i use? they both have the same end - can't be used for any other purpose or in any other plug-in receptacle.

hottub.pool_boy wrote:

You may need a new control thermistor, or electronic thermostat. An Ohm reading of both thermistors with an accurate water temperature would tell which one you need.

two weeks ago i replaced the outlet thermistor because i broke it while installing it into the new heater . since replacing the outlet thermistor and the heater, i've done three separate ohm readings at various temperatures on the intake & outlet thermistors. while they tracked each other fairly and relatively close - i went ahead and replaced the intake thermistor anyway - along with the thermostat.

hottub.pool_boy wrote:

You can email us the results if you'd like. scarritt@snet.net

oddly enough - i purchased my new heater, intake thermistor and thermostat from scarritt - the latter two parts just arrived here a few hours ago. you guys did a good job of getting the parts here quickly - and they all worked great.

MY FINAL REPORT
IT WORKS!!! with the thermostat dial set at 8 (out of 10) the tub is holding at a baking 104 degrees!!! ... and when the thermostat is turned up to 9 - you can hear the magical "click" indicating a call for more heat. i have grown to love that sound (now that it works).

THANKS TO ALL WHO PARTICIPATED IN THIS THREAD - COULDN'T HAVE DONE THIS WITHOUT YOU

final note - although, as it turns out, i did not need a new heater, i am glad that i replaced it in the process. the old heater was 11+ years old and did not heat as efficiently as the new heater.

peace

NICE!