No heat at idle. good heat when rpm increased. why?

What is the system charging voltage? An over voltage condition could cause this. Why not just hook up an after market gauge? The factory gauge is junk and as cheap as they could make it. And yes I have owned Vett's.

The basic concept is that in auto or defrost the air conditioner compressor is activated.
The compressor when activated via an electrical clutch adds an additional load that will reduce the engine RPM. The the RPM is increased slightly when the A/C is activated to prevent the engine from stalling. Power produced by the engine is increased by allowing more air and gas into the engine. When it is working perfectly the RPM will remain the same but the engine will produce the additional power to power the AC compressor.

Usually the bi-pass air valve is how the compensation for the additional power is accomplished.

Depending on your make year and model several methods to achieve the AC compensations have been used. In your specific case if the compressor is not working(no Air) and the compensation is open loop no engine control electronics I would expect a surge in RPM. With no additional load from the AC the additional air into the engine would increase the RPM.
On older cars there is a solenoid that has an adjustment for idle speed when the solenoid is activated. See the image above.
Hope this helps?

The heating coil is probably getting plugged up. You may have heat at idle or slow speeds, but once you get going, there is not enough coolant going through it to stay warm. Get the system flushed or replace the heating coil.

Heater core may be clogged. Try flushing it to see if water flows through freely. Also check the heater water control valve for restrictions.

Hi
Its the temperature sensor u will find it on the top water pipe that goes into your radiator it has 4 wires replace that and ur problem should be solved

Hi,

Please give us the detail of model of you car.

800 rpm is a normal idling for your engine.

Tell us more.

there is possebly sludg in the valve assembly air control on top of the throttlechamber , try to clean it with brake cleaner,
before removing buy a new gasket for it,because it possible not fitting afterwards.Its costs a few euro/pounds

good luck with it.

Clean the Idle Control Valve

Idle Air Control Valve most likely.

Check all vacuum hoses for leaks, check cap rotor wires and coil for arcing.