High idle on start up is normal. Your supposed to be around 1300 rpms I believe, all though depending on the coolant temp sensor signal it will vary, like if your coolant temp sensor is saying it's really cold then it will idle a little higher in order to warm up your engine. Now the fact that your heater all of a sudden wouldn't blow any hot air, just makes you suspect a cooling system problem even more. If your low on coolant or your heater core blew up you won't get hot air through. Now if your really low on coolant you may not even be getting any coolant flowing by your coolant temp sensor which is making your idle out of whack, but usually if you have a flucation in the idle it's due to whacky signals or something making your idle air control valve stick. One thing to check is a vacuum leak, which I believe vacuum is what changes your thermostat in your cabin from hot to cold, actually you might want to disregard that because i'm not really that knowledgable about heating and air conditioning. First check your coolant level if that's okay you can rule that out. Then after letting your car reach normal operating temp check your upper radiator hose make sure it is hot, if not your thermostat isn't opening. While doing this you can leave your radiator cap off and watch the coolant flow through the radiator if you never see movement once the cooling fans coming on and the upper radiator hose gets hot then suspect the water pump.