When using blue devil head gasket leak If I can turn on heater with that prevent it from working properly

Any crap you put into the radiator for a quick fix you have the chance of plugging up the heater core and or radiator.

Your heater core is always 'on,' meaning coolant flows through it all the time, even with your heat turned off, so it will be affected by that gunk as well.

I should add, if there is any build up in the radiator or heater core due from neglect or not servicing, the stuff you pour in to stop a leak is designed to "stop a leak" The heater & radiator cores are small as it is.

Never used the blue devil stop leak before but have used Alumaseal and bars leak with no problems matter of fact it's been almost two years now and no leaks and no problems. If you do use a stop leak and it does not work do not continue to add more hoping it will work that is when you can clog up the cooling system.

my heater core is discontented can i still use blue devil head gasket sealer with out the heat on

Blue Devil is crap, used as directed, clogged my radiator, destroyed thermostat and hoses. Company now will not even return an email, will not honor their money back guarantee. Company is a total fraud. Avoid at all costs. Complaint filed with BBB and found they have a very very long list of complaints. My bad for not doing my homework. Avoid their products at any cost.

Could that have happened because you was supposed to remove the thermostat?