You mean a PR to say a sample(preliminary)heat exchanger is built or heat exchangers are in production?
Then I assume they assemble a complete prototype water heater and then test it and test it and test it. If the tests and running of the heater performs to Roberts satisfaction, then off to UL testing IMO. Or are you saying once the heat exchanger is built, UL does all the testing for WDRP?
What do the UL testing people actually do? What are they testing for besides safety and compliance items?
Does WDRP need the UL stamp? Rinnai tankless water heaters are not UL certified.