When standing the heaters don't warm until the vehicles in motion
In normal circumstances the heater wouldn't begin blowing warm air until the vehicle has travelled 5 miles - call it 5 minutes. That's because the thermostat only begins to open at around that mileage/time and allows coolant to circulate around the engine. Before that - there just isn't any warm coolant in the heater matrix.
After 10 miles/10 minutes of normal driving the engine will be at its correct operating temperature and the coolant will be hotter - as will the circulating coolant in the heater matrix. That's when the heater blows the hottest air.
If you're sure that the heater has lost efficiency it may be due to air being trapped in the heater matrix/coolant system, or a blockage caused by limescale/engine corrosion within the heater matrix itself.
To remove any trapped air, set the heater to maximum, remove the radiator cap/coolant bottle cap and let the engine tick over for 10 minutes.
If the heater matrix is blocked/partially blocked, drain the coolant system, refill with plain water, set the heater control to open .. and let the engine tick over for 10 minutes. Drain it again.
Refill again with plain water and add a can of 'radiator flush'. Follow the instructions on the can of 'radiator flush'.
Drain again and refill with coolant/water.
If the heater is still inefficient then the only option is to replace it - and that's not an easy job.
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