I'm going to address this as if your flaps are OK and not part of the

problem. The volvo heater core is actually a split design that sends

heat to each side then the coolant flows up the middle to the outlet and

back to the engine. This way each side gets heat and the middle is cool

air to cool the resistor for the heater fan speeds. Where the hot inlet

heater hose attaches you will see the metal pipe split and go to each side

if one of the smaller pipes clog then you can have heat on one side and

not the other.

However, there is another possibility that also has to do with the heater

core. If someone replaced the heater core and did not use a genuine volvo

one, then this may be your problem. The aftermarket heater cores have the

coolant come in one side and out the other like most other cars. If you

have this type then one side will be hot and the other side cold. I know

because when I had my 79 I installed one of these and regretted it from the

day I turned on my heat. You can see the heater core pipes by looking at

the top of the heater unit. You can clearly see the inlet pipe split into

two small pipes that enter the core on both sides. If you only see on

metal pipe in and one out then you have an aftermarket core.

Hope this helps. By the way I have to do my blower motor this weekend.

Dave 82 242ti