The water in a whirlpool cools quicker than static bath water because of the movement of the water. The heating system for your tub needs to be able to cope with regular top-ups.
Domestic Heating System
Using your domestic water heater system requires that the tank in the system is large enough to fill the tub. The need for regular top-ups also requires that the system should be rapid recovery to ensure top-up water is available.
On Demand Domestic System
If your domestic hot water is provided by an instant constant-supply boiler, you could use that. The main problem with on-demand heating is that the boilers have a limited capacity to heat water. If you are trying to fill your tub and somebody else needs hot water at the same time, the flow of water to the tub will be cut by at least half.
Self-Contained Heating System
The best way to heat water for your whirlpool tub is to have an inline water system installed. This will be fitted exclusively for the whirlpool tub. The heater is an instant supply heater and will deliver as much water as is necessary. No storage tank is needed for this system.
To get the most pleasure and benefit from your whirlpool tub a constant supply of hot water is needed while you are using it. Only a dedicated whirlpool inline system can provide that.