Heater stuck on

Taking your heating box apart may be a really big job.
It would be good to check the vacuum supply at the heater box, and/or at the controls.
Lack of sufficient vacuum can cause the heater to not work.
The low vacuum could be due to a vacuum leak somewhere before it gets to the controls or the heater box.
Then, the heater box or flaps could be stuck or warped, causing malfunction.
It is all repairable, but can cost you a lot of time and effort fixing this.
God bless your efforts.

Heater control unit at fault could just be stuck check on heater matrix pipes in engine bay for unit work heater control from inside to see if there is any movement on unit can try moving by hand to see how badly it has stuck

If both heater hoses are getting hot, and the radiator is full of coolant, you could have a blend door problem. There may also be a heater control valve on one of the heater hoses that is not working. The blend doors direct air thru the heater core and vents. That would follow your theory of the control being stuck in one position.

sounds like your blend door is either stuck open or stuck close

take the heater apart and then you might be able to fix it that way if that doesnt work then take it to the shop

1-your thermostate is stuck open half way. thermostate is a simple device that made of enlarging metals when heat is applied. it suposed to be in close position to allow engine heat up, when specified temp reached it opens and let it flow and radiator tries to keep coolant cool.when it is broken it stucks or it acts slow or it wont acts at all. in your situation it is stuck open half way. it allows enough water to be heated but flow is limited to heat cabin heater.

2-if there are cabin heater selenoid/selenoids for two different temperature for cabin left and right, on some instance those selenoid valves to adjust cabin heater stuck open or closed like thermostate acts on first case. so heat is not enough to heat up cabin heater.

Two possible reasons, one is that your heat control valve that directs hot coolant to the heater core may be stuck or blocked. Two, you may have an airlock in the heater core that prevents water getting in to the heater core. You need to let the air out by loosening the upper most hose that feeds the heater core.

The heater core is either clogged or the water inlet valve in stuck.

It sounds like you have a blend air door stuck. This is in the housing of the heater a/c blower unit under the passenger side of the dash. There is a motor on the top of the housing that could possibly be stuck or on working.

thermostat is stuck open not letting the car get to operating temp or the valve to the heater hose is stuck close and the heater core could be plug with rubbish from outside elements some times if you bought a use car people take the heater hoses off and plug them because they are famous to leaking make sure the hoses are conected