I have had this lack of heat situation for a few weeks no and cannot figure it out. I have replaced t-stat(oem) radiator, flushed the heater core with water and air pressure, checked all the cables and doors, and bled the system until the fan came on twice and shut the car down. The heat is pumping out and it's great(this is in the shop of coarse). I start my 30 min drive home and about half way it start to get cold in my car but heat is still coming out the vents but not warming up the car.

I get home and feel around the hoses. The heater core inlet is very hot the outlet is not as hot but still hot. Upper rad hose is hot and the lower is luke warm. In the morning i ran out and start it for about 25 min, go back inside. Come out and the windows still haven't really defrosted and the temp guage is still on the cold line!!!

I start to drive and the needle with start to move and come up to normal operating temp. The entire ride to work is cold in my car but i do have heat coming out the vents. Weird.
Anyway I pull the heater hose off(the one that is connected just behind the t-stat housing and loops up into the head). I want to check for the flow in the system. So i start the car and the waterpump does not pump anything? I mean coolant comes out but shouldn't the waterpump pump coolant out?

 

if your water pump was bad was bad, you would be overheating.. and most likely you would hear it and see water coming out.. the get pretty naughty when they die.. sounds like you have a clog somewhere

 

heater core would be my first suspect

 

either it's plug somewhere or the water pump blade damaged, try use compressed air and water blow thru heater core both directions, what kinda temperature outside?? freezing?? there's a chance that coolant freeze inside, more % water than antifreeze, it happen to me once, -15 degree outside, engine overheating, upper hose cold, lower cold, heater blow cold air, everything cold except engine temp

 

I didn't want to end up removing the dash. Is there a way to test the heater core besides checking the temp of the hoses for major temp differences? I have done that and it's only a 10degree difference.

I know the waterpump would make some crazy noises or overheating but my car is not doing any of that. So what i need is to find the blockage. What is the flow of coolant through the system? Wouldn't removing the coolant hose pretty much right after the pump, pump coolant while the car is running though?

 

Dude....you live in Edmonton and it's like -30 celcius there right now!

 

It's pretty cold right now but I've never had a problem in any winter with a civic heater before. Maybe it is just because it's -30 here right now.

 

I'm just messing with you. You probably have something wrong with your car because after a half hour driving it should have warmed up, even in -30! I know that sometimes the heater control **** parts break behind the dash and they don't slide the switch fully. Does the temperature change to cold if you turn the ****? Other than that I agree with StylEmon that it might be the heater core.