Pool heaters can benefit from periodic cleaning.
Most burners lift up from the end that is away from the gas manifold ( usually fitted into a slot at the far end) and push away from the gas manifold to remove off of the orifice on the manifold. They can then be cleaned with a wire brush and either blown out from the top slots or tipped towards the end that sits on the orifice to remove internal dirt.
Also check for spider webs at the end that sits on the orifice. Depending on the ignition source( spark/hot surface ignitor), you must be careful removing the burners.
If you have a silicon carbide hot surface ignitor, they are very fragile and susceptible to failure from bumps against a hard surface and even oil from your hands causing it to fail. Only handle from the white ceramic insulator at the end that the wires are attached to.
Orifices can be cleaned out with a pipe cleaner to remove spider webs or other blockages.
After the ignition source and burners are removed you will need to look up through the heat exchanger with a mirror and flashlight to see if there is any black carbon/soot restricting the flow through the heat exchanger. If there is, the heater would need to be disassembled and cleaned to remove the blockage. This should probably only be attempted by a professional if needed.
Most HVAC companies will work on a pool heater, but not all HVAC technicians are created equal. If needed, look for a company that has NATE certified technicians ( this applies to HVAC service also)!
A Heat Pump is an an air conditioner system that works in reverse. It can benefit from annual cleaning of it's coils and air vents.
Motors and Capacitors
A Heat Pump has motors for the compressor and fan. Like pool pumps the motors in a Heat Pump has capacitors that can fail. See Pump Capacitors for more information about motor capacitors.
The Heat Pump also contains refrigerant, like an air conditioning system. You need to find a maintenance company who is licensed if you are accessing/opening the refrigerant part of the system.
Water Flow Rate
Running too much water through a gas heater is a bad idea. There's an internal bypass that bypasses over a certain amount anyway. So, it's a waste of energy.
Excessive flow velocity can erode the soft copper in the heat exchanger.
Efficiency is only slightly better at higher flow. As long as the flow is above the minimum specified flow rate, increasing the flow rate does not provide any significant benefits.