Brake buster wrote:very hard to get to in reality
removing the wiper scuttle motor mounting will improve things, but that is a job in its self if its stuck
have you tried manually opening the valve so the heat flows all the time
this setting is the default, and when the vacuum is applied it shuts the valve to allow cold air in the cabin
As mentioned. Fully open is the default position.
So when the valve fails is does not stop heated coolant entering the matrix.
The valves job is to close off the flow for summer time.
Are both fans working on all blowing speeds? It’s more likely your blower motors/ fans are giving up and not actually blowing hot air.
Search key words “Darlington Transistors” on here.