Heater was putting out hot air this morning, then it started blowing cooler air, I checked the heater hoses. Supply to heater was hot, return hose was pretty warm but not hot like supply.

Using an inferred thermometer check the temperatures of both the heater hoses that go to the heater core. The inlet hose should be about the same temp as the engine. The return line slightly cooler. If both hoses are not hot, then you may have a bad thermostat or restriction in the hoses. If you can see the heater core, check the different tube fins to see if they are hot or not. Check to see if there is a shutoff valve in the system.

1 - Radiator
2 - Thermostat housing
3 - Radiator bottom hose
4 - Viscous fan
5 - Water pump
6 - Radiator top hose
7 - Heater return hose
8 - Coolant supply hose
9 - By-pass hose
10 - Radiator bleed (purge) hose
11 - Bleed (purge) hose, thermostat housing
12 - 'Y' piece ejector
13 - Expansion tank
14 - Heater rail
15 - Heater unit
16 - Heater feed hose
17 - Intercooler
18 - Air cleaner
19 - Air feed hose
20 - Turbocharger
21 - Charge air supply hose
22 - Cooled charge air supply hose
23 - Exhaust manifold
24 - Engine oil cooler
25 - Feed pipe, engine oil cooler
26 - Return pipe, engine oil cooler
27 - Oil filter

there are two hoses than run through the firewall to the heater core. With the car fully warmed up those two hoses should be about the same temp. If one is hot and one is significantly cooler that the heater core is likely plugged. The hoses can be removed and the heater core can be backflushed with a garden hose. do this several times alternating between the inlet and outlet of the heater core. reconnect the hoses and see what happens. Worse case scenerio you'll need to replace the heater core.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Sounds like the air blend door is not opening to let heat in.

Engine coolant is provided to the heater system by two 16 mm (5/8 inch inside diameter) heater hoses. With the engine idling at normal running temperature, set the heater-A/C controls as follows. Temperature control to full Heat, Mode control to Floor, Blower control to the highest speed setting. Using a test thermometer, check the air temperature coming from the center floor outlets and compare this reading to the Temperature Reference table.

If the floor outlet air temperature is insufficient, check that the cooling system is operating to specifications. Both heater hoses should be HOT to the touch (the coolant return hose should be slightly cooler than the supply hose). If the coolant return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in heater system.

POSSIBLE LOCATIONS OR CAUSE OF OBSTRUCTED COOLANT FLOW

• Pinched or kinked heater hoses.
• Improper heater hose routing.
• Plugged heater hoses or supply and return ports at cooling system connections.
• Plugged heater core.
• Air locked heater core.
• Restrictor in backwards

If coolant flow is verified and the heater floor outlet temperature is insufficient, a mechanical problem may exist.

POSSIBLE LOCATION OR CAUSE OF INSUFFICIENT HEAT

• Obstructed cowl air intake.
• Obstructed heater system outlets.
• Blend-air door not functioning properly.

TEMPERATURE CONTROL
If heater floor outlet temperature cannot be adjusted with the heater-A/C control temperature control lever, one of the following could require service:

• Blend-air door binding.
• Faulty blend-air door motor.
• Improper engine coolant temperature.
• Faulty heater-A/C control.

OK, recheck the anti-freeze level and fill if necessary. There may be a leak in the system.
If it is filled up to capacity (also check that the upper radiator hose is full of fluid by squeezing), then the next thing to check is the two heater hoses that enter the firewall for the heater core. If they aren't getting hot at normal operating temperature, the heater core is plugged up and will have to be flushed or replaced.

If the heater core hoses are hot, then a blend door problem exists for shuttling hot air from the heater core to the interior of the vehicle.

Good luck on this repair.

try back flushing your heater core find the 2 hoses to the heater and start the motor let it run for a bit feel the hoses 1 should get hot and 1 should be a little cooler the 1 that gets hot is the inlet and the cooler 1 is the outlet remove them after the motor has cooled down and use a garden hose to back flush the core flush water through the outlet you dont need to use high pressure if you do there is a chance the core can crack and leak.

Could be the heater bypass valve is stuck or the heater core is blocked.
I'd check the heater core first.You may have trapped air in the cooling system or the heater core may be partially plugged up. Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor heat position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the HVAC housing floor outlets.
Temperature Reference
Ambient Air Temperature 15.5° C (60° F) 21.1° C (70° F) 26.6° C (80° F) 32.2° C (90° F)
Minimum Air Temperature at Floor Outlet 52.2° C (126° F) 56.1° C (133° F) 59.4° C (139° F) 62.2° C (144° F)
If the floor outlet air temperature is too low, Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system.
OBSTRUCTED COOLANT FLOW
Possible locations or causes of obstructed coolant flow: Pinched or kinked heater hoses. Improper heater hose routing. Plugged heater hoses or supply and return ports at the cooling system connections. A plugged heater core. If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
MECHANICAL PROBLEMS
Possible locations or causes of insufficient heat: An obstructed cowl air intake. Obstructed heater system outlets. A blend door not functioning properly.
TEMPERATURE CONTROL
If the heater outlet air temperature cannot be adjusted with the temperature control knob on the A/C Heater control panel, the following could require service: The A/C Heater control. The blend door actuator.
The blend door. Improper engine coolant temperature

1st off check your (cold) radiator (not the overflow tank) make sure it's
full, next locate heater hoses at firewall with eng. warmed up hold both
hoses 1hose should be hot going into the the other just a little cooler
coming out when it's working right but if 1 hose is hot and the return
is just warm you may have a restriction possibly plugged heater core
now if both hoses are warm not hot it could be thermostat is not closing
properly there is also the heater ctrl. valve but i don't know if you have
a mechanical- electric or vacum operated valve.
ps do these tests with eng at operating temp. fan on med. temp set on
hot. any other ? gimme a shout good luck

Since your blower is working and it is a heat issue,thats where we'll start,First thing is have you checked the coolant level in the radiator when engine is cold,make sure it is filled to top,next verify you have coolant in reserve ,Start your van and get it up to normal operation temperature,If your heat gauge on the dash works correctly it should be 190 degrees,Now lift the hood and locate the two heater hoses going into the heater assembly at the fire wall,Both hoses should be hot to hold on to,if both are hot and no heat inside your problem will be the blend air door in heater assembly.
if neither hose is that hot you need to replace the thermostat in the engine to get it up to 195 degrees.
if one hose is hot and one is luke warm then the heater core is blocked and to blame. hope this helps y

You could have a blocked heater core. Check for two heater hoses running into the firewall in the back of your engine. They should be together and about 5/8" in diameter. If one is hot and the other is cooler then your heater core is blocked. Unhook both hoses and use a garden hose to back flush. Be sure to catch the coolant and dispose of it properly.