All of the K-mahines with R.O.'s attached at my facility drop temp. in the last 12 or so Minutes of heat disinfect when the R.O. is turned off.
The K-machines we hook up to portable R.O.'s complete the heat cycle with the R.O. off.
Any ideas?

Hello SCtech, I don't have any like that but have an idea what may be causing it or at least contributing to it. My thought is that what you are seeing is the loss of heat that is/was being given off by the RO system (pump and valve heat). Beyond that, I would need a little more information before I could offer any more suggestions. What is your incoming water temperature? How high does your temperature reach before turning off the RO's? How far does the temperature drop and to what does it drop? Anyway, that's my two cents worth!

The Temp is 80-85. It is half way done with heat.At this point there is no incoming water.It drops all the way to 33 if the nurse leaves it all night. Most of the time when I catch the problem the temp is around 65.
What could be triggering the heat rod to cut off? Is there communication(sensor)with the machine and attached R.O.?(I don't see any common circuitry)

Have you tried leaving the RO's on to see for sure if that is creating the problem? How long has this been going on? How many machines?

All of my combo machines do it. As of now I do have the nurses leave on the RO so they will complete the cycle but that 20 Min. helps them on busy days.

Are they in the shade? .....Sorry.

As was mentioned, there is no water circulating at that point, so the RO source should not have any bearing on the problem. Are these 2008Ks configured exactly the same as the others as far as times, options, etc.?

This is a curious one. Be sure to let us know what it is.

You did not mention what flavor RO,s these are.
If they are the Zyzatec/Osmonics/GE/Marcor 801 integrated RO I had a similar issue with H machines

It seems that during TX/Disinfect while the RO is running the pump motor gets fairly warm. Because of the integrated nature this raised the temp inside the hydraulics.The heater circuit compenstea for this. Once the RO is turned off the temp drops out and the overall temp of the hydraulics does as well. Because there is no flow at this point the heater control is off and therefor cannot compensate for the temp drop.

You found the answer. true, no water is circulating, but the heat of the running pump is what Keeps the temp up.

I am not sure about this. It seems like the RO would have to be so hot it would catch on fire.

I believe the heater is still controlled in heat disinfect, and flow goes through the recirculation valve. The water inlet valve is closed.

Is it possible that they are spitting enough water out of the vent tube to get "no water" alarms and shut off the heater? You didn't say they had alarms, but it is a possibility.

Is this all of your machines?