My heat is only blowing hot air if my foot is on the gas and the engine is revving .
The first thing to check would be the coolant level to make sure the system is full. If the coolant level is low you will need to check and repair any leak that may be apparent. Sounds like the thermostat may be stuck open. If the thermostat is stuck it won’t allow the coolant to flow properly and would hinder the operation. No heat 2005 Toyota Sienna you may want to check the radiator cap and water pump also.
No heat 2005 Toyota Sienna Inspection2005-toyota-sienna-cooling-cooling-system-inspection