Heater works fine when warm temp gauge on 90 but when cold outside the heater only blows warm temp gauge goes to min had water pump and thermostate replaced water clear and level ok just seems to be whrn its zero degrees out side and below the heater doesnt work any ideas headgasket fine with no over heating issues

SOURCE: 97 f 150 runs hot for minutes then runs ok

It could be a faulty thermostat. Did it do the same thing before u replaced the t-stat? It sounds like it's sticking closed, then all of a sudden it opens, coolant flows correctly. I'd consider changing it again. Check to make sure parts store is giving u the correct temperature thermostat also. Maybe it's too high for your Ford, delaying the opening.

SOURCE: no heat when car not moving

Ok, judging from what you've said it sounds like a vacuum issue. The temperature controls are activated by vacuum which is produced by the engine. Look for small hoses that lead from the intake manifold to the firewall, check them and make sure they are not broken, disconnected, or leaking.  The vacuum directly controls the doors inside your dashboard which push the air to the heater core. Also make sure that the coolant hoses going to and from the firewall are hot.  If all that checks out, then the controls inside the car may have gone bad, or may have become disconnected from the vacuum supply, or the actuator door inside the dash is stuck or broken.

SOURCE: 1998 Dodge Durango no heat when using heater.

BLEND DOOR STUCK OR BROKEN. NEEDS TO REMOVE DASH AND HEATER/EVAP CORE BOS TO REPAIR REPLACE.

SOURCE: 2004 Tahoe Heater Problem

check coolant level ,must be low ,chck for coolant loss

SOURCE: 1999 Oldsmobile Bravada Heating Problem

when engine is cold remove radiator cap and check coolant level,sounds like it is really low and has an air pocket in it.