If you notice the heat is weak while idling, but adequate when 1500 rpm or more regardless of vehicle speed, you might suffer from what I believe to be an under-performing coolant pump.

Many on this forum decry Volvo's use of a stamped impeller in their replacement pump kits, but my alternative opinion based on my own experience and a common sense eyeball comparison is this: The "desireable" cast impellers may not deliver the volume of coolant at low rpm because their fin geometry is less than half the depth of the stamped version.

The photo below shows two cast versions. Notice the profile of the fins on the left pump are half the profile of the right pump. The stamped impeller is twice again the water mover that the pump on the right is. It makes the heater work at idle, in my opinion.