Volvo RWD 900 Forum

If you're getting heat when it's turned off then the heater control valve is likely going/gone -common failure. It's located in a heater core rubber hose between the firewall and intake manifold. Fix that first.

The left/right air temp balance may just be the nature of the beast -it's not overly precise. Not withstanding my laziness to get a thermometer and go out to see if my 900 replicates your readings, your temperature problem could well be caused by some loose duct work and that could easily have happened during a heater core replacement (not uncommon for your vintage).

Look in the FAQ descriptive article on heater core replacement. At the end of the article you'll see diagrams of the various air distribution assemblies. Yours will presumably be the "Heater unit with air conditioning -700 and 900 series". Note that for 900's, the driver side knee vent (U shaped) will not be present.

There's just a single door in the air distribution box that opens air flow to the dash face vent chamber. You can see that there are three separate face vent ducts that come off that chamber so the hot air mix may be somewhat uneven at the best of times.

I suppose it's possible that the face vent chamber of the distribution box may not have been adequately aligned when it was re-installed -you'd have to be a pretty shoddy workman (or workwoman). To properly inspect that you'll need to disassemble the center console and remove the right side panel -a bit of an adventure the first time you do it, but after that it's a piece of cake.

It's also possible a face vent duct wasn't properly mated with the duct neck on the back of the dash outlets. Additionally those duct necks are clipped to the back of the dash outlets and those necks may have been pulled out if someone was attempting to remove a face vent. In that case there wouldn't be a "normal" flow of air out the vents. Unfortunately "normal" is rather subjective. So long as you're getting a decent volume of air out of each vent when the fan is on medium-high then I wouldn't be concerned. The next time you're working under the dash and have the glove box out and half the center console out then you can maybe reach up and check for loose duct attachments behind the face vents. You shouldn't find a problem there unless someone has been unnecessarily vicious -again that would be pretty shoddy workmanship.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now