I have an upstairs and downstairs unit, the downstairs unit does not blow any air through the vents, and only comes on for a short period of time, in order for me to get it to come on i have to turn the breaker off and on, now the breaker is never tripped, but that is the only way i can get the unit to seem like it is on

close ceiling vents
the hot air down stairs rises through the vents and adds to the upstairs heating
same as hot air goes up the stairs to the upper rooms

It could be a blocked vent.when the downstairs toilet flushes it draws air down the drain with the wastewater. The vent is suppose to supply air to make up for this. If the vent is blocked or there is no vent. Air is drawn from the easiest place ,in this case through the upstairs toilet,drawing the water out of the toilet trap with it. In northern climates the vents sometimes frost shut causing the problem you describe. Hope this helps.

Check the fans are all working ok and make sure there isn't a louvre closed in the duct

Check the A-coil in the furnace to see if it is iced up. That will cause a poor airflow condition. If that is it you will have to find the cause of the iced coils. Eric

The Zone controller needed replacing so I rang for a Serviceman

I f it is fan that has more then one speed if you run it all the time,it may
not be on high sample : low fan speed runs all the time
med fan speed for heat
high fan speed is usually for air conditioning so if everything is good then it maybe the relay.

If the freon pressure gets low then the low pressure cutout switch kills the low voltage to the thermostat and the unit will not run.

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Some of this depends on where the air handler/furnace is located. Most often in two story houses, it's in the attic upstairs, or in a closet upstairs. A duct routes air from the furnace to the downstairs supply duct, so it is a distance from the air handler.
When the unit first initiates a heating cycle, that duct is going to be full of cool air, and the duct itself will be cool. So the furnace has to run long enough to push the cool air out, as well as warm the duct up, before your going to feel warm air at the registers downstairs.

Keep in mind, heat rises. And, your thermostat only monitors the immediate area where it's located. In your case, upstairs.
Two story houses are problematic because of this. And one way around it is to have a damper system installed that distributes the air upstairs or downstairs based on a thermostat located in those spaces. There would be 2 dampers, and 2 thermostats (one upstairs, one downstairs). Each stat would control a damper, and the call for heat or cooling.

Let me know if you would like to consider a system like this, and I can point you to components to use. I've put several system like this in.

Cold air is denser than hot air, so is harder to move. All vents have some mechanism to regulate flow, so close all of the downstairs vents. Check to see if your furnace fan motor has more than one speed, & set it on maximum. Keeping the fan on all the time will help as well. If all else fails, you may need a more powerful fan motor.