Yoma, Chapter Three, Mishnah Five
The mishnah now makes a note about the precise timing of the morning and dusk incense.
1) The morning incense was offered up between the blood and the limbs,
2) The dusk [incense was offered] between the limbs and the drink-offerings.
3) If the high priest was either old or of delicate health warm water they would heat some water for him and pour into the cold [water], to temper its coldness.
Section one: The morning incense is, as we learned yesterday, offered up after the blood of the tamid has been sprinkled on the altar and before the limbs have been offered.
Section two: The incense which is offered at dusk (Exodus 30:8) is offered after the limbs of the dusk tamid have been put onto the altar but before the wine of this tamid is offered.
Section three: If the high priest cannot handle the frequent immersions because the water is just too cold, they heat up some water on the day before Yom Kippur and put it into the cold water to temper its coldness. The Talmud explains that they would put some iron rods into the fire on the day before Yom Kippur and then on Yom Kippur put them into the water to heat it up. We should note that doing this outside of the Temple would be prohibited because of shvutrabbinic Shabbat prohibitions. However, in the Temple such prohibitions are waived.