Disclaimer: I am not a physician and hold no credentials qualifying me to proclaim any information as presented here as gospel. The comments presented here are gleaned from reading articles and personal experience.
In a previous comment I stated that 105 degrees F (40.5 degrees C) is the absolute upper limit I will use. The decision to set 105 degrees F as the upper limit is driven by four considerations:
a: by the time the water reservoir is filled, prepared and water flowing into the rectum enough time will elapse to allow the water temperature to drop two to three degrees. If you happen to use a product that directly introduces water from the tap it seems it would be wise to lower the temperature by at least five degrees.
b: the water temperature is measured with a lab grade thermometer with calibration traceable to a national standard. The lab grade thermometer is cross checked with a second lab grade thermometer at least once per encounter with enemas. As a backup check, the water filling the enema container is felt with the skin on the top of the forearm for comfort. If it seems too warm the temperature is rechecked. I also own a thermocouple type thermometer for triple checking if I have any doubts the first two thermometers are in error and the skin test fails.
c: professional water system installers have established the safe upper limit on water temperature is 110 degree F where water may contact human or animal tissues.
d: pain is reported to occur in most people when exposed to temperatures of between 105 degrees F (40.5 degrees C) and 110 degrees F (43.3 degrees F).
Studies conducted to evaluate the use of elevated temperatures to cause cellular death in human tumors has determined that temperatures of 46 degrees C (114.8 degrees F) to 60 degrees C (140 degrees F) are associated with non-reversible cellular death. https://ar.iiarjournals.org/content/32/3/915.full . No tolerance for temperature variance was specified in reviewed articles. The quoted study above is for tumor cells, not normal cells in the body. No mention was found in the article that indicated tumor cells were any more or any less susceptible to elevated temperatures than normal cells.
Temperatures above 105 degrees F (40.5 degrees C) just seem to risky.