I have a 1989 mobile home with a coleman air handler that runs and it will blow cool air when on cool and warm air when on heat but it seems to blow both at the same time so that it never gets truly cold or hot. We had an ac guy come out to service the unit but he said he couldn't service with it running heating and cooling at the same time. He said it could be the thermostat or need a new inside unit. We replaced the thermostat and we have the same issue but I am was wondering if it could be a relay or something with the wiring that we could check first as a unit is rather expensive.

Do you see the outside unit kick on when the themostat is calling for heat? And do you see the burners on when the thermostat is calling for cooing? If so, I would suggest shutting power off to furnace, unhook the thermostat wires from the furnace(not from the thermostat itself, but from the furnace). Then jump the R and W wires on the furnace. Turn power back on, and let me know if just the heat comes on, or if the outside unit still comes on. The jumping of the R and W is just temporary, for trouble shooting purposes only. Do not leave the wires jumped. If only the heat comes on with them jumped, you have a shorted thermostat wire between the thermostat and the furnace, or a bad thermostat, but you said you replaced that.

Im really surprised a cooling guy couldn't trouble shoot a furnace. I would suggest calling in a heating AND airconditiong tech to get this all fixed up for you. Good luck and please let me know what you find.

Thanks for the information. I am at work right now but I do not believe the outside unti kicks on when in heating but I do know that the burners kick on in cooling. I will double check it tonight when I get home. Thanks again!