Heater doesnt blow heat, only cold air, the coolant lines by the fire wall are hot and the switch that changes air direction isnt working either ?
I like that you were knowledgeable to check both coolant lines tells me you are getting hot coolant through the heater core next thing to check will be the blend door on the inside of your vehicle. The blend door is a door that switches from cold air duct to heater duct depending on what you are using at the time. If it is stuck on the cold air duct you will not get any heat sometimes these doors are vacuum actuated and sometimes they are electrically actuated most the time you are able to see the movement of the motor from inside if you don't see any movement but you hear it move chances are it is the door blend I hope this helps.
Take the heater core lines of the water pump and push some water threw the lines or use some air and blow the lines out with low pressure
SOURCE: No Heat and Overheating in 1999 Chevy Silverado
After replacing the thermostat, you must bleed the air from the cooling system.
To bleed air from the 2.2L and 2.5L engines, remove the plug or sensor
on the top of the thermostat housing. Fill the radiator with coolant
until the coolant comes out the hole. Since the plug is made out of
steel and the thermostat housing is aluminum, it is a good idea to
apply an anti-seizing compound or Teflon® tape on the plug threads
prior to installation. Install the plug and continue to fill the
radiator. This will vent all trapped air from the engine.
Any trapped air in the heating system will have to be displaced by coolant. Once the cooling system is filled, with the radiator cap off, turn of the heater at it's highest setting. Start the engine and allow it to reach operating temp. You should see a drop in the coolant level as the air in the heating system is displaced by coolant. Add coolant to the proper level and replace the radiator cap.
Keep a close eye on the coolant level for at least the next couple of weeks. The cooling system is a "closed" system. Any significant decrease in coolant level indicates a problem.
If you have any questions, let me know.
Please take the time to rate this solution.
Drive safe and be warm.
clarkco
SOURCE: 1999 1500 silverado heater
Sounds like the blend door in the ducting has failed, this door is what controls the temp of the outlet air by blending heat and outside air or AC cooled air.
SOURCE: NO Heat in 1999 CHevy SIlverado
flush the heater out with a hose. the fastest and cheapest. if that dosen't work then replace the Air blend actuator motor. $59.99 at Auto Zone part # 604-101
SOURCE: 1999 Chevy Cavalier A/C blows hot air
More likely the recharge blew a o-ring and you have a leak. When your Cavalier was blowing hot air and a recharge fixed it that means it was low on freon which lets o-rings dry out, go back to the shop and have them leak test it. They use a pressurized dye to find the problem. If it's blowing it's not the cooling fan relay, they may have meant compressor relay but I'd have a leak test done first it's not expensive and easier that chasing unknown possibilities.
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