Why no heating

SOURCE: No hot air at rear vents on 98 Toyota Sienna CE

Possible that heatcore is going or hose from heater is leaking or falling off.
You can go to www.autozone.com and see a full free online repair manual for your van

SOURCE: air cond goes from cold to hot on coldest setting or if its on hot it will start blowing cold

You have a broken wire on the heater control potentiometer. You can either order and replace the control, which is expensive, or you can remove the control, and disassemble it, and re-solder the broken wire. We do this all of the time; resolder the pots bacjk on to the main PC board.

SOURCE: A/C will suddenly send out heat. Fiddling with knobs seems to help. What's wrong?

You have a broken wire connection between the temperature control knob and the circuit board behind the center dash console. Vibration causes the failure and once the control knob gets loose the solder connection fails.

For someone that knows how to remove the dash unit and can re-solder a couple of new wires this fix should take no more than 1 hour. Make sure that you apply loctite to the heat control knob spindle nut when you re-assemble it.

The dealer quoted me 750 dollars to fix this issue and I found out that it's a common failure for this year. I fixed mine virtually for free but it took some tools and about an hour of my time.

SOURCE: 4 Runner Blows hot air on driver's side

something fell down defrost vent

SOURCE: 2004 sienna heating system problem

try turning the blower down, if you have the fan on full you will feel cooler air from time to time. as the thermostat only measures the ambient tempreture inside the car when this level is met then it will turn off the heat so it will only blow the air from the car which will feel cooler.
if the car is still cold suspect the tempreture control in the car maybe faulty or the water level in the system maybe low.