When looking up the manual online (remember, the pool builder has abandoned us, and gave us no manuals) I noticed that there are two models my size (440K BTU), one for propane (which is our fuel supply) and one for natural gas.
Guess which one I have.
Yup, 460736 HTR 400 MASTERTEMP NA (the NA means natural gas)
Now reading the manual it appears it MIGHT be possible to convert the unit to use LP. This would generally be a smaller orifice, I believe, as the LP is a higher pressure gas. There are no instructions on doing this and no details on how to tell if it has been done or not.
I will call Pentair tomorrow as I am home dealing with a different vendor who brought (and installed!) the wrong door to go between the pool room and the house.
I swear this project is accursed.
If Pentair is useless or won't take calls from pleebs, anyone have any idea how I could figure out which fuel the heater is set up for?
I further checked the manual for start up info and there is, in fact, a gas control valve internal to the heater and it is, in fact, in the OFF position. But I am not flipping that and trying it again until I can confirm it is set up for LP or that the reduced pressure LP will be OK with the existing orifice size.
The LP guys installed dual LV4403B46 and LV4403B46R valves...so I think I am down to 9" to 13" WC, which matches the pressure the heater wants for LP. Not sure if this makes the orifice less important or not.
Pretty sure I have identified the orifice screw, it has green highlighter on it.