Water temp stays higher than the setting

[psf91745]

I got a ten year old used Aquaterra hot tub recently. I set the thermostat to 90F, thinking I’d raise to about 100F before I use it every couple of days, but the temperature hovers around 95-96F. Why is that? The temperature reading is from both the display on the touchpad and a floating thermometer I have in there. When I press the buttons on the touchpad, it does show that the thermostat is still set at 90F, so it’s not the setting that has changed. The tub just wants to heat the water higher than the setting! It’s about 40F outside at night and 60F during the day here these days, so it’s not the ambient air temperature. What part could be malfunctioning in my hot tub? Thanks.

Edited November 3, 2021 by psf91745
Typo

[RDspaguy]

Check filter cycles. The pump by itself can heat a spa if running too much.

[psf91745]

6 hours ago, RDspaguy said:

Check filter cycles. The pump by itself can heat a spa if running too much.

Good tip! Thanks.

I checked and it's set to "F1". I believe that means it runs for 1 hour every 24 hours (or 30 minutes every 12 hours).

Any other possible reason why the water temp is 3-5 degrees hotter than its setting?

Thank you.

Edited November 3, 2021 by psf91745
Figured out what F1 means.

[CanadianSpaTech]

If your temp sensor is located through the sidewall of the spa and touching the water in the bather area make sure the back of the temp probe (inside the spa) is covered with a grommet/plug or foam. If the back of the probe is open to the air inside the cabinet it might pick up that ambient temp in the cabinet. Have even seen the pump running and blowing cooler air in the probe's direction and causing an off temp reading. If the sensor gets tucked inside a sleeve that goes through the sidewall of the spa and the sensor then gets pushed into the sleeve make sure it is all the way in and toughing the front of the sleeve where the water is on the other side. If it's not pushed all the way in you won't get a good reading and it might read off a few degrees... or you may just have a temp sensor going bad.

[psf91745]

2 hours ago, CanadianSpaTech said:

If your temp sensor is located through the sidewall of the spa and touching the water in the bather area make sure the back of the temp probe (inside the spa) is covered with a grommet/plug or foam. If the back of the probe is open to the air inside the cabinet it might pick up that ambient temp in the cabinet. Have even seen the pump running and blowing cooler air in the probe's direction and causing an off temp reading. If the sensor gets tucked inside a sleeve that goes through the sidewall of the spa and the sensor then gets pushed into the sleeve make sure it is all the way in and toughing the front of the sleeve where the water is on the other side. If it's not pushed all the way in you won't get a good reading and it might read off a few degrees... or you may just have a temp sensor going bad.

Thanks for that idea.

If the temp sensor is exposed to air and reading the wrong temperature, or is just plain old failing, wouldn't the hot tub display show the wrong temperature? It's showing the correct temperature -- at least the same temperature as the digital thermometer that's floating in the water.

[CanadianSpaTech]

Perhaps you have a well insulated spa and the temp just hasn't got down to 90. Suggest add cold water and get spa to 90 degrees and just leave it there and see if it increases above 90 and back to 95. Not sure what else to tell you.