Last month I changed the water pump. Now when the d is at idle and up to temp the heat blows out somewhat warm air. WhenI idle up the air is blows out nice and hot. When I bought the water pump the was only two and one was cheap one wasn't. I bought the higher price one cause it didn't require special coolant. So I know the wasn't two different types. Plus it doesn't over heat. I replaced the coolant and flush out the radiator when I did the pump. Any ideas on the it causing this?

 

you can undo the heater hoses from the block crimp off the bottom hose put some CLR cleaner in the top hose slowly untill it is full then crimp off the top line let sit over night drain it then flush it before hooking back up! That will clean almost all your built up deposits and make it flow like new! hopefully this helps

 

I am having the same problem... very little hot air..takes forever to heat up in cold weather.

 

I had a warm air at best with card board in front of my radiator until this winter. I replaced my fan clutch due to an overheating issue and voila heat this winter

 

if you put any chemical cleaner in your heater core be prepared that you may very well be pulling the dashboard and replacing the heater core depending on how much is removed by the chemicals, if enough deposits are removed a leak could easily be (literally) uncovered, deposits may seal it off until they are chemically removed.

just a fair warning.

 

The heat worked fine until I replaced the water pump. Now it works good if the engine is idled up a little. But if its just sitting at idle the warm air cools. I won't think changing the pump would effect it. Maybe some crud came loose and is slowing the flow down to the heater core.

 

Or maybe the thermostat is bad or your coolant level is low.

 

I felt the hoses this morning after it warmed up. The hose on the passanger side that goes to the heater core feels the same as the other hoses. The hose on the drivers side wasn't as warm. So I'm thinking something is restricting the flow.
I'll figure I hope. It worked fine before the pump was changed. I flush out the radiiator but I forgot to flush the heater lines. May have been a mistake.

 

Get an old hose and cut the end off and leave about 4 feet on the end. Now disconnect the heater core line that goes to the intake manifold and place the hose coming from the house to the core end and clamp it down with a worm clamp. Now take off the other side heater core line (that comes from the pump) and place the 4 foot section of the hose you cut off and turn it downward under the truck, this way the coolant and water will not blow all over your engine and instead be diverted to the ground under your truck. You do not need a clamp on this line because it will not be pressurized.

Now go turn the hose on at the house connection and turn it on fast and full blast! Now watch all the crap blow out of the hose underneath the truck, or into a bucket if you wish. Switch back and forth the house hose on the heater core ends and blow it out real good.

Now disconnect your apparatus and hook the heater core back up. Run the engine and add straight coolant to top it off.

 

I remember doing that to mine hydra, you're right it is amazing the crap that will come out.