My 98 S10 BLAZER'S heat does not work.

Does the engine reach normal operating temp? With the engine warmed up, are the heater hoses, hot? Does the blower motor work? If all the above are ok, then you have to check temp door under the dash.

If I understand you correctly, your temp gauge shows correct operating temperature, your blower fan blows air that you can feel with your hand from the right floor or dash outlet, but that air doesn't feel warm until you are moving?

If so, several possibilities come to mind. One is that your coolant level is low. Even though your temp gauge reads in the right range, the coolant level could be low enough that it doesn't reach the heater core until there is enough engine RPM to create more pressure from the water pump and force what coolant is there toward the heater core. Another possibility is that your heater controls are not able to do their job. A stuck flapper in the duct or a malfunction with the temperature control system could be causing the problem, and at 15-20 MPH there is enough air coming through the outside air vents to provide some heat.

I'd check the coolant level first, though. The other possibilities are more far-fetched.

the thermostst is most likly stuck in the open position letting the coolant flow freely through the engine and keep recuirculating not giving it a chance to stay in the engine block to warm up. i would replace the thermostat

it could be the cold air blend door or the thermostat is the engine temp up to the middle of the gauge or close to it if so then thermostat is ok if not then go ahead and replace it.

Check antifreeze level, if it is low coolant may not flow through heater core. If coolant level is ok feel the heater hose with engine running at normal operating temp and heat on high, if one hose is much cooler than the other you have a bad heat control valve or a pluged heater core.

With engine cold, remove rad cap & check level in rad. Top it up if needed to 1 or 2 inches from top. Start engine, turn heater on (not defrost) and observe coolant level as engine warms up, add if necessary. Radiator fan should be off, and only come on when engine reaches normal operating temperature, for a short time, then shut off again. Let it run until that happens about 3 times, then put rad cap back on. If temperature gauge climbs up over normal toward hot, too much, shut it off. Do you now have heat? Is rad fan acting as I described? Let me know.

Is the blower fan for the heater going on, but only cold air? If the fan is working, but no hot air situation, 1st check the coolant level, if it's low, the coolant won't flow thru the heater core, so little or no heat. If level is ok, what does the engine temp gauge indicate? Is the engine running normal temp range, cold, hot? Let me know answers to these questions so I can help u.

coolant level is fine still no heat

1st thing to check is the coolant level. Low coolant level results in no, or little heat (not enough coolant to circulate thru the heater core). What does your engine temp gauge indicate? (or is it just a warning light?) If the car is running at what appears to be lower than normal temp., u may have a thermostat that's stuck open. This would cause the engine to not reach operating temp, and u would not have heat. Check the above 2 items 1st, then let me know what u found. If that doesn't fix it, we will continue, let's do the easy and not too expensive stuff 1st!