I'm with Jeff on the air in the core, due to low coolant level.

I just had that happen with the Suburban; no heat. The system was 3-4 quarts low! I topped up the radiator and the overflow bottle and drove it. It took about 2-3 start-up cycles for the air to be purged out of the system. I think a few corners helped.

The inside heater core is higher than the radiator, so it will drain first when draining the system. Of course, it will also starve for hot water first

If you find it's low coolant, top her up and be sure to then re-check often. The Suburban had a small leak in the radiator, causing the fluid level in the reservoir bottle to drop, then disappear, and then the level in the heater core dropped.

I didn't discover this for a while, because I was not checking under the hood, and who uses heat in the summer and early fall?

BCingU, JIm

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'96 E300D 61k mi (wife's daily ride)
'95 Audi 90 120k mi (my daily ride...)
'92 GMC Suburban 140k mi
'85 300SD 235k mi KIA/RIP