On second thought, you might have one PlantLoop with a BoilerHotWater and a HeatPumpWaterToWater in parallel on the supply side and a ZoneHVACLowTemperatureRadiant on the demand side, and another PlantLoop with a GroundHeatExchanger on the supply side and the same HeatPumpWaterToWater on the demand side. If that were so, I do not think AvailabilityManagers would help. Instead, I believe a PlantEquipmentOperationHeatingLoad and its inherited PlantEquipmentOperationRangeBasedScheme might be called for, even though it would require Python or Ruby scripting.
With PlantEquipmentOperationRangeBasedScheme you can define the load ranges in which what type of equipment is to operate. For example, you could addEquipment(10000,HeatPumpWaterToWater) to indicate that the heat pump is to be active up to 10 kW (thermal), then perhaps addLoadRange(15000,[HeatPumpWaterTotWater,BoilerHotWater]) to indicate that between 10000 and 15000 W both heat pump and boiler are to be active and finally perhaps addLoadRange(100000,[BoilerHotWater]) to indicate that beyond 15000 W and up to 100000 W, only the boiler is to be active.
That said, I have only ever dabbled in this and don't think I have ever succeeded at implementing this. Perhaps you have more luck.
Just venturing a guess. It sounds like you might have two pieces of ZoneHVAC equipment, maybe one OS:ZoneHVAC:WaterToAirHeatPump and one OS:ZoneHVAC:LowTemperatureRadiant:VariableFlow? Perhaps then you will also find a OS:ZoneHVAC:EquipmentList in your .OSM file? Perhaps there is then a way to configure this "somehow" to let the heat pump be the primary heater and the radiant the secondary heater? I must admit, I have not had much success with this myself, but on the surface at least, it seems applicable? Another thought is perhaps availability managers in the plant loops.
@mattkoch I'm not trying to give my radiant system priority, heating is generated by my gas boiler and heatpump (watertowater) I want my heatpump to take priority over the gas boiler (they are both providing hot water for the same radiant system). I havent delved into avaibility managers, ill try playing with them thx.
Apologies, I cannot visualize your configuration. I do not know if I can help any further, but it might help others to help you if you could post a picture of your air loop(s) and/or plant loop(s). Just a thought.