The problem with "heat pumps" is that they necessitate a "cold" side, from which they "pump the heat" to the "hot" side. Their goal isn't to increase the overall "heat" in the system, it's to move the "hot" all to one side.
A "resistive heater" can add "heat" to the system more evenly, but is less efficient and you won't see "temperatures" rise nearly as quickly.
And that both can occur at the same time.