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Spa pump/heater short-cycles when water gets near set point

Spa pump/heater short-cycles when water gets near set point

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  #1  
Old 03-09-13, 03:21 PM
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Spa pump/heater short-cycles when water gets near set point

I have a very old spa. The timer does not currently work so it is set to run the pump/heater whenever the temperature is below the set point, which is fine during cold weather. Recently, it started short-cycling, i.e., running for a minute then cutting off for a few minutes. I discovered it only does this when the water is hot, near the set-point. If the water is cool, it will keep running and heating, but then again when it gets near the set-point it starts short-cycling. It has good water flow. I suspected the heating element was encrusted with minerals considering how old it is, which would explain the short cycling if the heating element was not able to transfer heat to the water fast enough when the water got warmer. But the element is nice and clean, and it checks out at about 11 ohms.

The heater control has both a high limit switch with the standard push-button reset and a thermostat with a dial from low to high heat. Can adjusting the calibration screw on the thermostat address the symptom I have, or could there be some other control problem that I can check out while I have it torn apart? Thanks for
any suggestions!

Jim
 
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  #2  
Old 03-09-13, 03:40 PM
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Based on the type of thermostat it looks it may need to be replaced.
Thermostats usually have some sort of hysteresis (anticipation) built in to them so that they don't short cycle at the set point.
Readjusting the set point won't help.

Does that controller have a part number on it ?
 
  #3  
Old 03-09-13, 04:02 PM
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Older mechanical thermostats typically have a 1 degree setpoint...meaning if you set it for 100, it'll heat to 101, shut off, and turn back on at 99. yours sounds like it needs replacing.

HOWEVER, they measure the temp of the water inside the heater, NOT in the actual main body of the spa. IF, it's very cold out, and the equipment area is poorly insulated and getting wind blown into it, it's possible that the temp of the water in the heater is dropping very quickly, causing the thermostat to turn on.
 
  #4  
Old 03-09-13, 06:18 PM
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PJ, the label on it says:
275-3123-00
C1-25F
9929 MEXICO
MAX 111F
E16835

I googled "275-3123-00" and found numerous sites that pictured my thermostat by Eaton, although they give a 107 max. Maybe newer models have a lower limit than my old one.

The symptom does indicate that the hysteresis between turn on and turn off gets very small as the water heats up, but with the thermostat in my hand and turning the dial, there still seems to be a fair amount of hysteresis...turning counterclockwise, it clicks off about about the 6 o'clock position, and turning clockwise, clicks on at about the 7:30 position. I don't understand why the amount of hysteresis would decrease as the water gets hotter.
 
  #5  
Old 03-11-13, 02:49 PM
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I've ordered a new thermostat, but took advantage of the day off to put everything back together and cleaned/refilled the spa so it would be ready to go once the new thermostat arrives. The initial water temp was 50 after filling. It ran and heated non-stop for at least the 4 hours before I went to bed. When I checked it this morning, it had heated up to to 80 (it's only a 120v/1500w heater so it takes a while to heat), and it was then short-cycling. It would run for less than a minute, then cut off and back on again within a minute.

As a test, I turned the thermostat dial all the way counter-clockwise to the "No heat" setting, and could hear the switch click off. Yet, it still continued to cycle on and off. I would have thought turning it to "no heat" would stop it from running at all. (The Timer/Automatic switch is set to Auto, which means the pump only runs when the system is calling for heat.) Does this indicate that there is something else other than or in addition to the thermostat, or could a bad thermostat still explain this? Is there some other control that turns the pump? Also, for what it's worth, the pump won't run on the high speed setting, but that could be a bad switch at the topside control panel. Any thoughts?
 
  #6  
Old 03-11-13, 09:39 PM
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I'm just wondering if the problem could be due to where the bulb is located.
Is it against the plumbing pipe before the heater?
Is it well insulated ?

Temp range is from 47-107 degrees. +/- 4 degree calibration

There is actually not a "no heat" position. It may say that but it actually will activate at 47 degrees...... I presume to keep the spa from freezing.

So if the stat is cycling turned all the way down......it definitely sounds defective.
 
  #7  
Old 03-12-13, 03:24 PM
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The thermostat and high limit switch are located in a box mounted directly to the heater tube. Both bulbs insert into small tubes like thermowells that go into the heater tube in the same area where the heating element is, perpendicular to the flow path. There has been no change in the set up...it's worked fine for about 20 years. I'm amazed that the original heating element and pump still work, although I wonder now why the pump won't kick on high speed.

I say the "no heat" position because the label at that end of the dial calls it that, but you're right, it is supposed to be a freeze protect that will kick on heat to prevent water from freezing in the pipes.

Hopefully, the new thermostat will arrive by Friday, do the trick, and I'll be soaking in my spa again by Saturday night! I'll let you know what happens.

Thanks for the input, PJ!
 
  #8  
Old 03-12-13, 07:32 PM
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Your welcome

I'm pretty sure the pump won't switch to high speed when the heater is active.
 
  #9  
Old 03-12-13, 08:21 PM
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Actually it's, the heater may not activate when the pump is running on high.
 
  #10  
Old 03-14-13, 02:41 PM
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Yes, Dr. Spa...my control panel can be wired for either 120v or 240v. Mine is 120v, which limits it to either heating OR high speed pump...if it is in heating mode, the heater is overridden and shuts off when I turn the pump to HI. But currently, pushing the pump button to HI does nothing...not sure if that's because the switch signal isn't reaching the control panel due to a bad switch or the pump won't go to high speed for some reason. That's the next thing to figure out after I get it to heat up to the desired temperature.
 
  #11  
Old 03-25-13, 06:25 PM
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Well, I got the new thermostat installed, but it is doing the same thing. Like before, when the water was cold, it would keep running and heating. After running over night, it had heated up to about 80, and had already started short cycling--roughly a minute on, then a minute off. I let it run for 2 full days and it got up to 94, and the cycles got shorter. I timed them...it would run for 18 seconds, then turn off for 35 seconds.

So it isn't the thermostat. I'm baffled as to what could cause this short-cycling, and that the cycles get shorter as the water gets warmer. I'm really bummed without a working hot tub, and can't swing a new one with 3 more years of college and two more weddings to pay for! Any other ideas to try?
 
  #12  
Old 03-25-13, 10:13 PM
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You can actually hear the new thermostat short cycling ?

If you can, there is an issue where the bulb is located. If the thermostat is shutting the heat off before it should then the sensor bulb thinks it's satisfied. That sensor is getting too hot. It's not seeing the correct water temperature......it sounds like it is sensing the heater.

Can you feel the area where the sensor is mounted. It should be warm but not hot. There may be a waterflow issue thru the heater.
 
  #13  
Old 03-26-13, 06:29 PM
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I didn't think to try listening for the thermostat click to associate that with the pump/heater starting up. But if the sensor bulb is getting too hot as the water warms up, I don't know what could have changed between it working normally and short-cycling--the bulb would have been in the exact same position before as it was before I replaced the thermostat.

My heater has a terminal box mounted directly to it. The heater wires exit the heater tube via sealed openings into the box. The thermostat and high limit switches are inside the box. Both the thermostat and high limit sensor bulbs slide into a thermowell that is open to the box, but sealed from the water. The water heats the thermowell which in turn heats the sensor bulb.

You're right that is seems like it is sensing the heater too much, as if the water flow is insufficient. If the water is initially cold, the heater can't heat it enough to trip the sensor, but as the water warms up, it is able to. I initially thought the heating element was fouled with mineral deposits, but when I took it apart, both the heating element and heater tube was clean as a whistle. Water is circulating into the spa so there is definitely SOME flow, but maybe there is a partial blockage somewhere else that is limiting the flowrate. Not sure how to test that theory or to find the blockage. Any thoughts?
 
  #14  
Old 03-26-13, 06:49 PM
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The reason I asked if you could hear if the new thermostat was cycling was because if it's not.....then maybe the hi limit sensor is actually shutting the heater down.
 
  #15  
Old 03-26-13, 10:10 PM
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Low flow, at this point, is a good suspicion (your original post though did say there was good flow). If you have a small circ pump that moves the water through the heater, it's impeller may be partially clogged (hair a likely culprit). If you have a 2 speed pump moving the water, it may be as simple as a clogged filter, reducing the flow enough for the heater to over heat. Try removing the filter cartridge and running the system without it.
 
  #16  
Old 03-27-13, 07:49 PM
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PJ, I started wondering about the high limit switch too. I had previously ruled that out since pushing the reset button hasn't resulted in a "click" like it's supposed to if it had been tripped, but that doesn't mean it hasn't failed in some way to stop the heating cycle periodically without fully tripping the switch. I will check that out.

Dr. Spa, that's a good thought about the impeller being partially clogged. I assumed the flow was normal since I can see the return jets flowing some water, but guess I can't say for sure it is really the normal flow. Since the heater tube was completely clean, I couldn't figure out what else could possibly be limiting flow. So that's some thing else to check out. Boy, that's going to be a pain getting to the pump I'm afraid. I'll test the high limit switch first. About the filter, I think my spa takes suction from one line directly from the bottom of the spa and a separate suction from the filter via a surface skimmer, so a totally clogged filter will not prevent enough flow via the direct spa bottom connection. I'll trace the piping to verify this.

I was wondering too if the motor itself has a separate thermal cut-out that could be turning it on and off. But don't see a connection to higher water temperature causing it to kick in.

Thanks for the additional thoughts, guys!
 
  #17  
Old 03-27-13, 10:09 PM
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Those hi limit switches, once they trip they stay tripped. You have to push the red button to reset them (turn them back on).

Motors do have a thermal overload, but I can't imagine the temp of the water would effect it.
 
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