2005 Buick Allure heat problem

Was lack of heat in the cabin the only reason you changed the thermostat? Were the radiator hoses hot (or very warm) to the touch before you changed the thermostat? Were the heater hoses hot before you changed the thermostat? Did you get a check engine light and code stating that the engine was not getting (or slow getting) to operating temperature?
Getting heat from your heater depends on a blend door actuator letting air flow through the heater core in the HVAC unit. If your radiator hoses are hot, your heater hoses are hot, and you have no trouble code indicating the engine is not getting to operating temperature, but you are not getting heat inside the car, you need to check your blend door actuator or the control unit for the heater.

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07 Tucson heater blowing cold air - Hyundai Forum - Hyundai ...

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Dec 16, 2009 - I have a 2007 Tucson, when you turn on the heater it only blows hot air... Join Date: May 2005 ... a low coolant level will exhibit the first stated symptoms of no heat/low heat at idle and increased heat at cruise due to an air ...

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Oct 10, 2009 - When a car heater blows cold air, there are several possibilities that should ...... On my Dodge Neon when I turn on the heat it blows out cold air.

Heater blows only cool air...even w/ defroster on : Buick ...

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... Cadillac, Chevrolet, Chrysler, Dodge, FIAT, Ford, Geo, GMC, Honda, Hummer,Hyundai, Infiniti, Isuzu, Jaguar ... I'll turn up the temp on the dual control to get heat,

could be an air lock in the heater rad, back flush with a hose to clear.

If you know where the heater hoses are, lift the hood after car has warmed up I mean the engine running for at least five minutes, feel the heater hoses, if they are warm or hot and still no heat from the heater, you have an in line proiblem. if hoses remain cold, replace the thermostat with a winter thermostat.

would change top hose plus make sure there is a thermostat fitted

The heater radiator had to be flushed twice at the dealership. No problem since then.

Your heater core may be clogged

Do yo know if the heater core is getting Flow. when engine is warmed up. feel heater hoses hoing into and out of heater core. if both are warm then you have a blend door problem. if one is warm and one is cold and coolant is full and no air locks you have a plugges heater core. reverse flushing sometime works. otherwise it would need to be replaced.

I changed the thermostat. It works like a charm now!!

With the car warmed up,feel the heater hoses,are they both warm/hot near the firewall in the engine compartment?If they are both warm/hot,then the heater core should have circulation through it and it may be a "blend door" issue in the heater a/c assembly.If they are not warm/hot then the heater core could be dirty and plugged up and needs to be flushed.
If the above does not help,a drain and vacuum of the cooling system and vacuum refill will remove any trapped air.
Sometimes,clamping off the top rad hose and holding the engine rpm at 2000 rpm will force trapped air out of the heater core.(Watch temp gauge when trying this)
Lastly,a NEW thermostat doesn't mean its working correctly.