Please elaborate on what you mean by "valve job". Heater valve, or engine valves?

It sounds like you're not getting coolant circulating through the core. I believe (not sure) that there's an aux coolant pump for the heater core, which could be dead. Or - there could be air in the core.

The fogging of the window could be a bad sign. is there any sweet smell that accompanies it - like coolant? If not - then it's likely condensate, and the problem becomes less troubling.

Troy
1995 E420 76k