I found the manual for the water
heater, a Force10, and comparing diagrams it seems that my unit is setup with a TCU (Temperature Compensation Unit) and that these *cannot* be plumbed inline.
What the pictures don't show well enough is the level of crimping in the elbow
. I re-plumbed this with a bronze 90 degree bend and yesterday was able to confirm that the engine
runs 20 degrees f cooler 500rpm higher.