I agree that you have a cooling system leak. It may not be an expensive problem at this point, but if you let it go, you may develop an expensive problem. I think it is time to take the car to a competent mechanic.
I’m going against the grain . . .
partially blocked heater core
Not blocked solid, but partially blocked
No offense, but probably because the coolant wasn’t changed on time
Yes it looks like I will need to get the cooling system checked out. Thanks for pointing that out.
No offense taken db4690 
When I drove the van to work this morning the heat blew hot within a few minutes (as it used to). I’m not sure at this point if it’s fixed (due to adding enough coolant) or is an intermittent problem. What’s strange is that yesterday evening when I ran the engine (revving the engine in the driveway without actually driving it) it was still blowing cool air. I didn’t make any changes between yesterday evening and today. I will see what happens over the next few days. And I will need to get the coolant leak fixed.
“I’m going against the grain . . .partially blocked heater core
Not blocked solid, but partially blocked
No offense, but probably because the coolant wasn’t changed on time”
I’m glad to see that db4690 agrees with the diagnosis that I first offered.
Assuming that we are both correct, here is a possible scenario:
Coolant wasn’t changed on time, leading to rust problems.
Rusting of the radiator, or the water pump, or possibly even the freeze-out plugs could explain coolant loss, and an accumulation of rusty sludge in the heater core could have led to a possible blockage of the heater core.
If the problem returns, and coolant is missing from the radiator again, and you see no external leaks…I see a head gasket in your future.
Yes, the coolant has to be going somewhere I will check for external leaks. How much leakage over time is “normal” ? None? Or should I expect to top up the coolant occasionally?
None in less than 2 months
It’s a sealed system. Increased pressures reduce boiling point of the fluid. Should have no leaks ever.
I agree with your diagnosis
But I also wonder where the coolant went . . . ?!
In any case, I initially stayed out of this, until OP provided more details
“I felt the heater core input and output hoses and they were both very hot (too hot to keep my hand on for more than a brief moment). I turned the blower to “hot” and initially it came out hot. I thought it was fixed. But then the air temperature dropped down to warmish again, so I guess not. There was just an initial blast of hot air.”
I’ve worked on plenty of vehicles with partially plugged heater cores with symptoms 100% identical to those
In many cases, blasting out the heater core with water was sufficient. Regular garden hose, with a water nozzle on the “jet” setting. Sufficient enough, so that several years later, everything is still working perfectly
But I realize that won’t always work
I agree that this vehicle may have more than one problem
Well, the heat (blower) continues to work. However the coolant yesterday was again below the “full” level (about an inch or so) so I added yet more coolant into the reservoir. I put a big piece of cardboard under the van overnight and in the morning didn’t see any evidence of coolant leaks while it was sitting in the driveway. The van’s check engine light came on this morning, I am going to Autozone later to try to see what the code means. I have no idea if it’s related or just coincidence. The van wasn’t running particularly rough. (This van has 173,000 miles on it by the way.)
Unless I missed that detail, I don’t recall the OP telling us how often the coolant has been changed.
Can you share this detail with us?
Also…is there any evidence in the overflow reservoir of oil contamination?
That would be a dark brownish-black line at the uppermost point of the coolant level.
VDCdriver: the water pump went out at about 90,000 miles and at that point the coolant was replaced. Since then I haven’t had it changed. I haven’t seen any evidence of oil contamination (brownish-black line).
Instead of taking it by autozone to get the check engine light diagnosed as planned I have scheduled to take it in to see a mechanic (thursday) so we will see what they find.
Sounds like a good plan.
The issue was a cracked radiator, causing coolant leakage. The radiator will be replaced.
The check engine light seems to be unrelated to this issue.
Thanks everyone for all the help !
Thanks for letting us know! Best wishes