Why does fan come on right away before heater warms up?

ECT Sensor
88-105°C (190-221°F) Depends on ambient temperature

Some vehicles have an AUTO switch for their ventilation system and other's are completely based on temperature sensors for your fluids. This is so the car does not blow air until the engine has warmed up.

because water has to warm up until 90*

I had this problem occur on another vehicle of mine and it freaked me out for awhile. But I found that the AC pushbutton was semi-stuck and whenever I went over a somewhat significant bump the controls would switch over to AC. I finally replaced the AC pushbutton and the problem went away. I am assuming that you have a pushbutton, right?

Does the air coming out of the vents change when you move the temperature control knob? If you can't feel it, you should be able to hear the change

Your coolant is low, and you may have a leak in the system.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is warm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest flushing the heater core.

Its too cold ... get a block heater. At least get an under hood heater, even an old electric blanket will work ... just don't forget to remove it before starting the engine.

You also might look into holding the preheaters on longer.

circulation of hot hot water to inside cabin to heater core ..and fan blowing strong are all necessary here.....back flush system.. (good cooling system service) including flushing teh heater lines with all heater lines open ...refill and test... or to be sure replace thermostat and any suspect heater hoses at same time....a good shop with infrared gun can trace heat and locate blockage if necessary... good luck

your thermostat could faulty.when it stuck open it won't let the engine heat up to the right operating temperature and will be blowing luke warm to cold air out of the vents.have it replaced that should solve your problem