I had an incubator die on day 10 once and so we finished the hatch in a food warmer (that was back in the days when we only had on incubator). The controls were pretty touch on the warming tray and we had temps up to 110 deg F. but still had chicks pip right on day 21 and hatch without any problems.
We also treated eggs with heat to kill Mycoplasm one year. The research said that if the eggs internal temperature reached 114 deg F that the Mycoplsm wouldn't survive. We put the eggs in 117 deg for half a day to get the internal tempurature up to 114. The research we read indicated that hatchrates would drop by 10% but we saw more like a 30% drop in hatchrate.
Yes, I fully agree that too hot is worse than cool temps.
Thanks for the info, the food warmer is a good option to have just in case. I use a workshop-type heat gun to check temps in odd situations, plus a small conventional thermometer.
Do you ever use other methods to eliminate problems that may be passed through the hen to the hatching eggs? I'm thinking options have been posted on here in the past.