Don't recall all the complexities of how the fuel-fired tankless system works but my understanding is the fuel burns at one temperature and therefore has the potential to exceed the temperature for the flow of water to be heated and require a PTR valve. Whereas electric has a duty cycle (on/off) and better control the heat the water to the set temperature.
An example of a manufacturer's installation guideline of an whole house electric tankless unit invites
A code change proposal:
RP54-13 P2803.1, P2803.2 (New)
Revise as follows:
P2803.1 Relief valves required. Storage water heaters and hot water storage tanks shall be protected against over-pressure and over-temperature conditions by one of the following methods:
1. A separate pressure-relief valve and a separate temperature-relief valve
2. A combination pressure- and temperature-relief valve.
P2803.2 Instantaneous fuel-gas fired water heater relief valve. A fuel-gas fired instantaneous water heater shall be protected against over-pressure conditions by a pressure relief valve. The valve shall be located on the cold water inlet piping to the heater at a point that is downstream of all external valves except where the heater manufacturer’s instructions require the valve be located elsewhere.
Committee Action: Disapproved Committee Reason: This proposal doesn’t address small tankless water heaters. The manufacturer’s instructions take precedence.
https://www.iccsafe.org/wp-content/uploads/10-IRC-P.pdf
R102.4 Referenced codes and Standards.
The codes and standards reference in the code shall be considered part of the requirements of this code to prescribed extent of each such reference and as further regulated in Section R102.4 and R102.4.2
Exception : where enforcement of a code provision would violate the conditions of the listing of the equipment or appliance, the conditions of the listing and manufacturer’s instructions shall apply.
R102.4.1 Conflicts. Where conflicts occur between provisions of this code and referenced codes and standards, the provisions of the code shall apply.
R102.4.2 Provisions in referenced codes and standards.
Where the extent of the reference to a referenced code or standard includes subject matter that is within the scope of this code, the provisions of this code, as applicable, shall take precedence of the provisions in the reference code or standard.