2001 Town and Country. Coolant backing up into overflow bottle, which is causing the overall level to drop. As a result, I have no heat in the passenger compartment. Get heat when system refilled. At a loss on where to start on this one.

Yes, it is important to include the overflow tank or bottle as integral with the radiator and coolant system as a whole. They all intermix. Actually, the level in the overflow tank is a true measure of the correct amount to keep your coolant at. There are two marks on the overflow: full hot and full cold. The coolant system on all modern cars is a sealed system and therefore should never require adding, save a very small amount through evaporation within the overflow tank- the only part that is open to atmosphere.

So once you get all the air out of the system that is possible, both in the engine and in the radiator, you have a full level everywhere, including the overflow tank where it should always remain somewhere between the full hot and full cold marks.

You are right that the overflow collects the heated radiator coolant. But it is due to the expansive properties of coolant-or water- that it expands when heated, (but not boiling-that is very bad news of a different sort). Your radiator cap is designed to open at a specific pressure, just enough to allow coolant to flow into the overflow. As the engine and coolant cools, it contracts, the level drops in the radiator, and a natural vacuum there will siphon the overflow tank back into the radiator until it is full again. That is why you have the full hot and full cold marks.
I will shut off the lecture now. I hope you have liked my answer. If not, I give you permission to kill me now- if I may so jest.

REPLACE THERMOSTAT IF TOP COOLANT COOLANT HOSE GOING TO RADIATOR IS HOT TO TOUCH THERMOSTAT OPEN. TO BLEED COOLANT SYSTEM FIRST YOU NEED TO ADD COOLANT TO THE COOLANT OVER FLOW JUG UNTIL COOLANT LEVEL STOP DROPPING AND STAY AT FULL COLD MARK.WAIT 3 MINUTES IF COOLANT DROPS ADD MORE COOLANT UNTIL COOLANT STAY AT FULL COLD MARK.WITH RADIATOR OVER CAP OFF CRANK ENGINE FOR MINUTE WATCH COOLANT LEVEL AT OVERFLOW JUG.IF COOLANT DROPS ADD MORE COOLANT IN THE OVERFLOW JUG.WHEN COOLANT LEVEL STOP DROPPING IN OVERFLOW JUG.SCREW BACK ON THE RADIATOR CAP TO OVERFLOW JUG.WHILE ENGINE IDLING WATCH TEMPERATURE GAUGE WHEN TEMPERATURE START CLIMBING TURN OFF ENGINE WAIT A WHILE KEEP FROM GETTING SCALDED.WHEN COOLANT COOL DOWN SOME. PLACE A LARGE RAG OVER THE COOLANT OVERFLOW JUG OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED, THEN REMOVE CAP ADD MORE COOLANT IN THE OVERFLOW JUG TO THE FULL COLD MARK.THEN REPEAT THE PROCESS SCREW BACK ON THE COOLANT OVERFLOW RADIATOR PRESSURE CAP.THEN START ENGINE WATCH TEMPERATURE GAUGE WATCH COOLANT TEMPERATURE GAUGE WHEN COOLANT TEMPERATURE START RISING TURN OFF ENGINE AGAIN WAIT UNTIL ENGINE COOL DOWN ENOUGH TO KEEP FROM GETTING SCALDED TAKE PLACE LARGE RAG OVER RADIATOR CAP WHICH IS THE COOLANT OVERFLOW JUG CAP OPEN CAP LITTLE AT A TIME UNTIL PRESSURE RELEASED ADD COOLANT UNTIL AT FULL COLD MARK. WHEN YOUR COOLANT LEVEL STOP DROPPING.AND TOP RADIATOR HOSE VERY HOT THERMOSTAT OPEN AND AIR IS OUT THE COOLANT SYSTEM. MAKE SURE YOUR COOLANT OVERFLOW JUG HAS COOLANT IN IT,WHEN ENGINE COOL DOWN CHECK COOLANT OVERFLOW JUG KEEP COOLANT LEVEL AT FULL COLD MARK.BECAUSE IF OVERFLOW JUG IS EMPTY THATS A SIGN RADIATOR COOLANT LEVEL LOW.WHEN OVERFLOW JUG EMPTY IT WILL ALLOW AIR GET IN THE COOLANT SYSTEM AND CAUSE CAR TO OVERHEAT.WHEN REPLACING THERMOSTAT ALSO REPLACE THE RADIATOR CAP ON THE COOLANT OVERFLOW JUG.

check the coolant level as a low level can cause overheating and a knocking noise in the heater box as air is in the system causing the noise and no heat.change the thermostat and refill the system and make sure all the air is out of the system and the overflow bottle is at the full mark.it could also be a bad head gasket that should be checked with a block test if the above doesn`t fix problem.make sure the cooling fan is working properly at the radiator.

If you dont have an external coolant leak anywhere under your hood that has shown up on your driveway it could be the intake manifold or the headgasket leaking. Also check the thermostate to solve the heat issue.

ANY TIME YOU CHANGE RADIATOR ONCE YOU GET THE NEW RADIATOR IN PLACE.FILL RADIATOR WITH ANTIFREEZE CRANK VECHICLE UNTIL THERMOSTAT OPEN. WHEN YOU TOUCH THE TOP RADIATOR HOSE.IF TOP HOSE HOT THERMOSTAT OPEN.CHECK RADIATOR COOLANT LEVEL IF DROPPED ANY.ADD COOLANT UNTIL LEVEL QUIT DROPPING. WHEN COOLANT LEVEL QUIT DROPPING YOUR COOLANT LEVEL IS CORRECT. NO MORE AIR IN COOLANT SYSTEM OR AROUND COOLANT TEMPERATURE SENSOR COOLANT FAN SHOULD TURN ON ONCE ENGINE GET TO CERTAIN COOLANT TEMPERATURE.SEE IF COOLANT TOO LOW NOT ENOUGH COOLANT REACH COOLANT TEMPERATURE SENSOR. SENSOR CANT TURN ON FANS IF ITS NOT IN HOT COOLANT. WHICH DECREASE ITS RESISTANCE WHICH CLOSE OR GROUND CIRCUIT CAUSING COOLING FANS TURN ON.

Check coolant level,sounds like air in the system, You can remove majority by taking rad cap off and kneeding the hoses, you should see air bubbles in the coolant, once done the coolant level will drop,do this several times, can normally be done with engine at standstill, but more effective with engine running, but be carefull of fan blades and belts when doing this. Suggest trying with engine stopped

Most likely causes of this is either low coolant level or a plugged heater core, of which, I'd lean towards the low coolant. Rarer causes would be a slipping drive belt for your water pump, or a dying water pump.

Check the coolant level in the coolant bottle and top off with a 50/50 mixture of antifreeze and water if it is low. There should be markings on the side of the bottle to let you know the proper level; markings for both a cold engine and a hot engine.

If the level is low, you might want to have the vehicle checked to make sure that there isn't a leak somewhere.

This is most likely low coolant level. Have you checked? When the level is too low, hot coolant doesn't circulate through the heater core. If you top it off, it should be throwing heat again. Keep an eye on the level, you may have a leak somewhere. If this isn't it, let me know so I can try to help further.

Have you checked the coolant level? If it's low, it will result in diminished heat, and could be the problem.

air blend door jammed, or you have lost vacumn to the diaphram that opens and closed that door.