Heating Help

Hydronic Coil Air Handler

I can't find a model number and think the water circulation pump may not be enough power for the system. The main water goes into the water heater. Hot water comes out of the water heater into the circulation pump to the radiator and joins back where the main water comes in to go back into the water heater. I've tested the pump, electrical, clogs, etc. All is functioning and working. There is a spout after the radiator that if I open up to bypass going back into the water heater, the hot water flows and the heater works. If I close the spout and to allow the flow to go back into the water heater to recirculate the flow stops. My guess is that the pressure from the main is more than the pressure coming from the radiator? Any help would be greatly appreciated! :)

Need some more info....

I hope you drawing is missing a number of components in your system.  I think you drawing is incorrect.  I don't think there's anyway this ever worked piped like this.  For example, when you say 'to house' is that domestic hot water, or to other heating zones?  Where is the 'bypass spout' on your drawing.
What type of water heater?
Could you post a few pics, mostly of all the components, and take a second look at the piping so you can modify your drawing?
Thanks