Heater core hot but not in cabin

If your coolant isn't low, it could be a stuck thermostat or a plugged heater core

That would be my best guess. If the heater core is hot but not getting hot air in the cabin you should be looking at an air flow problem. Blend door is the most obvious culprit.

Was lack of heat in the cabin the only reason you changed the thermostat? Were the radiator hoses hot (or very warm) to the touch before you changed the thermostat? Were the heater hoses hot before you changed the thermostat? Did you get a check engine light and code stating that the engine was not getting (or slow getting) to operating temperature?
Getting heat from your heater depends on a blend door actuator letting air flow through the heater core in the HVAC unit. If your radiator hoses are hot, your heater hoses are hot, and you have no trouble code indicating the engine is not getting to operating temperature, but you are not getting heat inside the car, you need to check your blend door actuator or the control unit for the heater.

Most likely it is the heater control valve that shifts air from blowing across your heater core to your air conditioner core.

If your fan works and your car is fairly new..meaning your heater core isn't dirty and full of craap...then it is the climate cotrol valve.
If you are lucky, it might be a thermostat that is bad and blocking hot water from entering your heater core. Bottom line, if hot water doesn't circulate thru that little heater core, you won't get heat.
You might be able to look under the hood and grab the hot water lines that go thru the firewall and feed your heater core. If they feel cold to the touch after engine has warmed up, then you aren't getting circulating water to the heater core. If they feel nice and warm, then the heater core is fine, but your climate control valve has closed off the fan that blows across the core to disperse heat into the cabin

don't know this van specifically, but some things to check, make sure cabin filter is replaced or clean, you didn't say if the blower itself works, if not, correct or replace. Check the hoses going to the heater core, if hot water is flowing thru the core, both hoses should be hot. Check the heater controls to make sure doors in the system are opening closing as they should, you may just be drawing in cold air and not going thru the heater core.

Check heater hoses running from the engine to the cabin. and check if heater switch works

Hi James, If we're talking about the radiator for cooling the engine, check the thermostat and water pump. If we're talking about the cabin climate control, check the valve operation of the cabin temperature control. Regards John

once engine is hot
all cabin setting set to heat
check hoses going into cabin from engine - are they hot if not check - engine thermostat.
flush cabin heater core with water from garden hose - through both core hoses one direction then the other - to clean out core.
check vacuum doors in heater core box in passenger compartment.

Hello,
The thermostat was a good choice however you should go to a service station and get a complete cooling system flush.
your heater core and or radiator could be clogged.

It dosen't happen often, but the air intake side of the core can become obstructed with leaves etc. If that is the case, the only way I know of to fix it is to remove the core & clean it. You did check the heater control valve to make sure it is allowing proper coolant flow through the core, right? Did you run it after refilling the system with the cap off and the heater on high to purge any air in there?