While I do not own a pool, it occurs to me that when the filter system is running, you should be able to stick your hand in where the water is returned to the pool. If it is warm or hot, I would re-check the heater controls and valves. If you have an electric heater it is possible for the heater element to be partially on due to a short, when the thermostat is off.
If it is same temperature as the pool water, then I would guess you are getting a lot of solar heating from just sunlight on the pool. Do you have a dark colored lining on the pool? Can you shade it?,
We have a hot tub, with the heaters off 7 months of the year or more, and it sitting in the shade of the lanai, it will still get up to 92 degrees just from the pumps circulating the water. I use the bubble aeration to cool it off if it gets to warm.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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