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The thermostatic heater valve is a problem and Volvo dlrs now retrofit failed ones with a much simpler mechanical valve. (I believe a similar one is being supplied by FCPGroton.) You can watch the OE valve's lever swing back and forth with the temp control, but that doesn't mean the innards are doing squat. Remove the driver's side center console panel and just make sure the valve's lever moves with the dash temp controller. If not, fix and check heat again - if so, bad valve or clogged heater core. It's difficult to say which it may be, as both restrict hot coolant flow and give the same symptoms; lack of heat output when you increase the airflow through the core.
I disconnected both core hoses, one at the valve and the other at the firewall and spliced in extensions to them. I then used that household "CLR" (Calcium, Lime and Rust remover) at 100% strength to flush the core back and forth many, many times. Then pressure flushed (gently) with garden hose. A LOT of scale came out. Upon reassembly, the heater ranged from hot to warm - couldn't shut off. New valve and it's working great now.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)
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