I have a Heil combo unit, where both the heat and air are combined in one outside unit. The heat will not come on, although the blower runs constantly. I replaced the thermocoupler two months ago. What could the problem be?

You will have to take a volt meter and see if you have 24 volts at the gas valve, first. I have no idea why the fan runs constantly unless the fan switch is set to "on." You will need to use the meter to find out why you have no heat; there is no other way as guessing doesn't work. Check to see if you have 24 volts at fan relay on the coil, if not then the fan switch or the fan relay is welded closed and will need replacing, but that does not explain not having heat. DrTom4444