No Heat in 2001 Blazer

hi all, subject says it all... went out this a.m., started up and took off....after about 15mins, noticed that the blower was blowing cool/cold air.
any thoughts appreciated
pax, h2 pax, h2
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Thermostat is not opening, or heater hose/heater core is clogged.
............ Jonny

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check coolant level in the radiator not the storage tank.

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