Is there any vac. control in this heater?

yes.you have a vacuum leak,most likely under the hood next to firewall,passenger side.check the smaller hoses,they probably tee off and go to different places.simple fix.if not found under the hood,check under the dash inside car,but most of the time,it's under the hood.when you have a vacuum leak,the blower motor will default to the defrost mode.