You will get some of the intended effect of the monoflow tee if it is installed backwards as shown in the picture, as the first tee branching off to the heater.
The monoflow tee is intended to work as a venturi where the flow through the run of the tee, into the large end of the nozzle and out the small end of the nozzle, produces a low pressure at the outlet of the small end. The differential pressure (now higher at the first tee and lower at the second tee) causes flow through the heater in the branch loop.
There are 4 ways to install a single monoflow tee. Three are wrong.
1. The correct way is at the return from the heater to the main pipe, the second tee of the pair in the direction of the main flow, with the venturi element (the small diameter internal to the tee) pointed in the direction of flow.
2. The second way, that will have some effect, is to install the monoflow tee at the first branch to the heater with the small end of the intenal element pointing "against" the flow, as in the picture. That condition will cause more pressure loss in the loop but it will cause some flow to go through the "side loop" and through the heater.
3. If the tee is installed backwards (flow going into the small part) AND at the second tee where the flow is returning from the heater, it will not work properly but there will be some flow through the heater loop.
4. If the tee is installed in the correct direction (flow coming out of the small internal portion), but at the first tee off the main line to the heater, it will not work properly but there will be some flow through the heater loop.
It is possible to install TWO monoflow tees, one as described in (1.) above and one as described in (2.) above, to increase the flow through the heater. It causes more pressure loss in the loop and reduces the flow that can be driven by the circulator pump.