Why does fuse blow when turn heater switch on

check the fuse box and look for heater fuse.

The fuse is used by the following :
1. PCM.
2. ACC.
3. Trip computer.
4. Stop Lights.
5. DRL.
6. ETWIS.
7. Instrument cluster.
8. Front heater and A/C.

Any of the components could be the cause of it. Note if the fuse blows when any component relaed to it is operated, such as blower fan, applying the brakes.
PCM and trip computer would be less likely to be the cause as they uses minimal current.

Try this first. Replace the fuse. Make sure the AC & Heaters are OFF. Start your vehicle and sit there with the car running. Turn on the heater on full blast and wait. Tolerate the heat. If the fuse blows again, you have a problem with your heater circuit (fan or heater switch). If it does not blow, turn the heat off then turn on the air conditioner full blast. Tolerate the cold. If the fuse blows your problem is with the AC circuitry. If nothing blows. Turn off the heat and the air conditioning. Step on the brakes and hold it down for about 10 minutes. If the fuse pops then you have a problem with your brake light circuit (brake light bulbs may be shorted, your brake light switch may be bad, etc.). These are the easy checks. After that you need to take your vehicle into a shop that can troubleshoot electrical problems. Sorry, but that is about all you can do at this point. They will be able to tell you exactly where the problem is. However, you need to take it to a place that is experienced in troubleshooting car electronics.

Never heard of a relay problem blowing a fuse. Believe either your heater motor has an internal short, or a short in the wiring to it, like bare hot wire touching metal.
That's a large fuse. Are you sure the heater circuit is the only circuit for that fuse? If it is, makes it easier to find the short. Unplug the heater motor and see if new fuse blows with key on, and heater motor switch on. If fuse don't blow out, probably your heater motor. If fuse blows , could be your resistor pack for the heater or a wire touching metal, it should cause a large spark and a "pop" when it blows fuse.

Why does the heater fuse keep blowing
This place offers a few suggestions, like fan motor old and pulling too much current, or fan switch, ect

It sounds like the heater motor may not be operating. When you switch the heater fan on can you hear the motor running? If not then it may have blown the heater fan fuse, check the fuse's and then try it again. If the fuse is ok then it could be a faulty switch or fan motor.

fuse blows. unplug heater blower motor turn ignition on, turn heater on, does fuse blow? if not need to replace heater blower motor

most likley the fuel heater is bad and shorting out happend to me replace the heater

More than likly a fuse is blown. The problem isn't the blown fuse but the cause of the fuse getting blown. The blower motors seise and lock up then the fuse blows the blower can get something in it mouse pen dropped thoght defroster. The ballast resiter burns out and then it don;t blow the fuse, Rarely the switch gets unpluged or burnt out but not a common problem. Fuse is 30 amp green.

remove the power wire to the blower motor usually on firewall in engine compartment and replace the fuse then switch on, if fuse doesn't blow with motor disconnected then you'll know its the motor

check to see if AC and HEATER blower motor is getting power .if yes replace blower motor.if no you have problems in relay- fuse- or ac and heater switch is bad.for sun roof check for power at switch if yes sun roof eletric motor need replace.or sun roof in a bind.if no power to sun roof switch.blowed fuses and bulbs or turn signal switch is bad.