The Lennox HP 29-036 unit just stopped putting out heat, but the fan runs constantly. Have checked the breakers and they seem fine.

Is the outside unit running? I assume this is a heat pump. Do you mean that the inside fan is running? If the outside unit is running is the compressor running? Have you tried the emergency heat? It should at least bring in the heat strips if you have them. You are going to need to get a volt-ohm meter and learn to use it if you plan to fix this. Without a meter you are blind and just guessing which will never work on something this complicated, but with a meter it's simple. DrTom4444

Thanks, Tom! The outside unit is not running but the inside fan blows constantly. I have power going into the outside unit but the contactor is not calling for power to run the unit. When I depress the contacts, power flows and the outside fan starts (don't know about the compressor), but stops as soon as I let go. What does the emergency heat setting do? It doesn't seem to help at this point.

For some reason the outside unit is not running. The first thing I would do is to check the voltage going to the coil on the outside unit contactor. If it is 24-30 volts then I know I need a new contactor. If there is no voltage then I would go inside and measure between "R" and "Y" to see if the thermostat is calling for heat. If I have zero and "Y" to "C" is 24-30 volts then I know it's calling for heat and a wire is broken (rare). Check around and see what you can find. DrTom4444

Thanks, Tom - will do!