Warm but not Hot

Hey guys got a 99 Silverado that i've had for a little over a year now. Preparing for this winter i added about a half quart of dex cool to to UP the concentration not because the coolant was low. Since then (basically overnight) I have no hot air. Warm at most. Heat has always worked WELL in past. Drove the truck almost 150 miles still just warm air. No smell of antifreeze below the dash, coolant level full, system was flushed day after a purchased the truck. Could this be a bad thermostat switch, somthing bad with the cap? Should I have it flushed again?
Thanks
FF18

If the ECT is where it should be, you'll probably need to flush the heater core. If one hose is HOT and the other warm or cold, chances are the heater core is plugged. Disconnect both heater hoses (at the intake and at the water pump). Using a garden hose, spray water into the hose that was cold or warm (should be off of water pump). Once the water runs out clear, reverse direction several times. Blow short bursts (careful not to damage the heater core) of compressed air into the hose after each time you reverse direction. If you have the factory spring style hose clamps, I would replace them with the screw style.

If i pull the hose from the water pump and the intake how much of the coolant do i need to drain before pulling the two heater hoses? Is it possible to pull the hose from where it goes through the fire wall, and flush from there? Whats the best way to bleed the air after flushing is complete.

Thanks again
FF18

If i pull the hose from the water pump and the intake how much of the coolant do i need to drain before pulling the two heater hoses? Is it possible to pull the hose from where it goes through the fire wall, and flush from there? Whats the best way to bleed the air after flushing is complete.

Thanks again
FF18

You don't need to drain any, just pull the hoses and it will automatically drain the perfect amount.

You can disconnect the hoses on either end, but I find it easier to flush the core with the hoses attached. Plus, most factory heater cores use plastic lines. If you break that, replacing the core isn't any fun. The plastic going into the intake will also be brittle, but is much easier to replace.

The air should bleed off by itself as long as the radiator is full. You could get a spill-free funnel, which goes on in place of the radiator cap and holds about 2 quarts of coolant. To assist the air bleeding, you can snap the throttle several times. That usually will get every bit of air out.