Astra heater my astra heating works for a few seconds but then goes back to cold

SOURCE: The heater in my 2002 Fort Taurus only works for a

Check to make sure your coolant is full. If it is carefully feel both of the hoses near the firewall that go to the heater core. Be careful the exhaust is right there and its HOT.
With the heater on both hoes should be about the same, if one is colder than the other you heater core is partially clogged and will need to be replaced or a shut off valve in the hose is not opening properly. I use a infrared digital thermometer so I don't have to reach in to check the temps.
Good luck
Lee

SOURCE: Low heat condition. Heater goes from hot to cold

A low coolant level can cause this problem. If the coolant is low, look for a leak around the waterpump area. If the leak isn't severe you can just top it off as needed until you have the time and/or money to have it fixed.

SOURCE: heater does not work

Seems like very little volume of water is reaching the heater core in the dashboard. Check the hoses that run from the cooling system into the firewall. One of these hoses might be bent or creased in a position that it impairs the normal flow of hot water into the dashboard.

SOURCE: I am having heater issues....

There could be several causes for this.
The first thing I would check is to make certain that the coolant level if filled properly. Low coolant levels can contribute to this, at idle the coolant will not be circulating properly leading to not enough coolant going to the heater core.

Secondly, either your heater core or radiator may have slowly over time become corroded and is restricting the coolants moment through the engine and as your engine gains "RPM's" the coolant is circulated faster.

There is an outside all be it small chance that the water pump maybe failing also.

I would first check to make sure there is a proper amount of coolant in the reservoir, if it is full or close to it I would then check to see how the circulation of coolant differs from idle to under engine acceleration.

Testimonial: "I think your right! thats the good news. The bad news is i got a leaky radiator. my day off tomorrow. looks like i'll be workin on that. Thank you!"

SOURCE: after running for 20 minutes or more , the heater

Hello: Check the coolant level, fill up the reservoir and run truck, if reservoir shows low again fill it back up. If reservoir was low/empty the check for leaks. Do this before you replace thermostat. The heater core would not create this kind of condition.