The restriction is the heat exchanger itself, notwithstanding a silencer which may exist between the blower and heat exchanger.
Most blower exhaust exchangers are built similarly to that of the construction of an automotive heater core, insofar as you have criss-crossing tubes supported by, and aided in heat transfer by a network of corrugated "fins".
When properly functioning, the restriction caused by these fins and tubes is not normally an issue, and only represents a few oz per square inch of pressure between the blower outlet and the heat-exchanger itself.
Measurement of any additional restriction will not be taken at the vacuum side, but with a pressure gauge between the blower and the exchanger. This additional restriction is caused by debris, dust and oil on the fins, which have very close spacing to permit a greater surface area for the movement of air to transfer heat.
The oils that we use at the end of each day, to retard corrosion on the blower lobes, have the potential to almost coat the fins of the heat exchanger, especially if overused. These oil deposits attract fine dust, and the result over time is an accumulation on the fins to the point of closing off the heat exchanger flow, if not periodically removed.
Many truckmounts with a blower exchanger will feature a pressure port in which to check for impending restriction without having to physically remove the exchanger for inspection as often. Still, depending upon design and customer care, the exchanger should be removed for cleaning once or twice a year, or every 500-1000 hours.
A symptom of a heavily restricted exchanger include, engine bogging, and poor vacuum performance. If that same exchanger is also used to garner heat from the engine exhaust, any restriction can also lead to engine problems as mounting back-pressure on the engine will do.
If you have any symptoms, or its just been a while since you checked, immediately schedule and inspection of the exchanger or a backpressure test
