Filter pump causing heat?

[llamafilm]

I'd like to use my spa as a cold tub in the summer, but I can't get it to cool down! With the cover on, the water typically stays between 105-107 all summer, without running the heater. I started collecting some data on this and noticed something interesting: the water temperature increases only while the filter pump is running. I'm looking for an explanation of this. I know that every pump produces heat as a byproduct, but I'm quite surprised it would be enough to actually heat the water like this.

I attached 2 graphs showing the temperature over the last 6 days. On Tuesday night I reduced the set point from 104 to 80 and opened the cover, and by 7am the next day it was down to 80. The filter pump runs daily from 9am-5pm which corresponds exactly with the temperature increases.
The graph shows power consumption on Tuesday of 4500W when the heater was running, and 480W for the daily filter cycle. As an experiment, I'm going to try changing the filter cycle to run at night. But I really don't prefer that because of the noise.

 

[ajw22]

What model spa are we discussing?

Some standalone spas use the heat from the filter pump to heat the water.

 

[llamafilm]

It’s an Aquaterra Aryana

 

[Coconut Pete]

I'm surprised you didn't get more replies on this, but your suspicion is correct. Your pumps are absolutely transferring heat into the water when they are running, and the amount introduced is significant. When I am using my 500 gallon tub, I see 2-4 degrees Fahrenheit introduced during my session. I haven't analyzed the exact times, ambient temperature, etc., but I know that I will see at least a 2 degree rise, so I plan for this. To keep the temperature down in the summer, I cut a 3.5" diameter pool noodle in half and use it to prop the ends of the cover open a bit to let heat escape. It doesn't sound like much, but it has been EXTREMELY effective at keeping temperatures down! Without this, I would be 100 degrees or more, but by using this method I am able to keep temperatures under 90 degrees. I am generally able to maintain my desired temperatures using just one half of the pool noodle on the end of the cover. If I need a bit more cooling, I will use both halves of the noodle positioned farther in on each side of one cover end. This props that end of the cover up higher to let more heat out.

 

[llamafilm]

Wow, I'm surprised that small gap makes such a big difference, thanks for the tip!