Pool heater flow question

Clean the filter and see if you get better flow.

If not, then see if the heater flow switch is adjustable, if so reduce it until you get into range.

Good!

There are two ways to increase flow rate. You can either boost the pump power (voltage) or increase the inlet diameter. If you want to increase flow rate without increasing the inlet diameter, then you need to increase pool horsepower. If you want to increase flow rate, you can do it in two ways: you can calculate water flow rate and simply increase the pump horsepower, or you can increase inlet diameter on your filter (I am not sure about your pump). If you increase the pump horsepower, you need to make sure the heater can handle the extra heat generated, or the water heater will burn out.