I have heard about the idea that using fans make a boiler work harder, and based on my experience it is a myth...or mistake.
For background, I build kilns and heat treatment furnaces, so the idea that pulling more heat from a radiator, which is a heat transfer device, does not add up. And here is why:
If you have a burner in a boiler setup, that burner will be defined by its ability to deliver fuel that is being combusted to the combustion chamber. We assume the burner is tuned as best as possible with oxygen for as efficient combustionas possible. That burner can only, and will only deliver a flame with a set peak number of BTUs ever. If the burner is 100,000 BTUs then that is what it delivers during each firing period and remains until the home is heated to the target temperature set by the controller and measures by a thermocouple or temperature reading device. Once the set point is reached the furnace is shut off. It fires again when the home temperature drops below the set-point on the thermostat.
The burner in this case is heating a chamber that exhausts to a chimney flue and the job of the heat exchanger is to grab as much of those BTUs as possible before they exit the flue. Any heat exiting the chimney is heat lost.
But the next wrinkle is that the medium in a radiator is generally water or steam. Older boilers run by convection. This concept means hot water or steam will rise and go to cold, creating a flow from hot lines to cooling water exiting radiators and dropping back via return lines to flow into the heat exchanger. This is done via convection and by way of pumps, too.
The medium for the heat carries the heat. If you can grab that heat, it means cooler return lines but the furnace does not sense or feel or work harder due to more heat being drawn out of the loop. It's a 100,000 But burner that burns at 100,000 BTUs no more or less (given its well tuned).
The more heat you grab, the less heat that is lost up the chimney. You have effectively transferred more of your heat from the combustion chamber if you do this.
It is worth noting that some furnaces are so efficient they require plastic pipes to vent the stack gas because so much heat is removed from the exhaust that all that is left is steam. Anything less than a PVC pipe invites corrosion because the exhaust is mostly condensing luke warm steam. And that, my friends, is what you want! You want an exhaust venting a tiny fraction of the heat put out by your burner.
That's my educated opinion, which is: crank up the fans. My own radiators thank you...