Heater will not work until approx 20 min of the engine running?

Hello,

It sounds like you might have a partially clogged heater core. As the hot coolant is not flowing properly thru the core, so when the fan blows it just blows cold air.
It can also happen if your coolant level is low.
Verify that the coolant is at the correct level.

https://www.2carpros.com/articles/car-heater-not-working
https://www.2carpros.com/articles/replace-heater-core

Here is a helpful video:

https://youtu.be/SNRb6pSwYuU

If the heater core is clogged many times you can flush the cooling system to improve the performance of the heater.

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

Please let me know of any questions.

Thank you.