Duramax with poor heat

I just bought a used 2002 Sierra SLT Crew Cab 4X4 with the Duramax and Allison trans and it has really poor heat in the cab.
Does anyone else have this problem and do you know what I could do to heat
things up.
TIA
BB
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Do diesels use thermostats the same as a gas motor?
If so the thermostat may be stuck open. That cirulates your coolant all the time. When you idle heat builds because air is not flowing through the radiator, when you move you cool off quicly.

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