I got a used SR400LP about 5 years ago for free. I finally installed it a couple weeks ago to replace our 200K Raypak, as we don't use solar heat any longer. Pool is tiny. Wife is impatient.
Changed the orifice to the natural gas size with a Sta-Rite kit. Installed the unit, piped 1" gas line to it. We have 6" of gas pressure at the unit, drops to 4" when it is running. Meter looks small, one of those big fat buggers with the 1/2" fittings in and out. 1 1/2" supply from the street. It seemed to run fine, sounded as if it was starving for fuel. In the orifice kit, it said to try a 5% larger orifice. I had a 1/2" drill and a drill press, so I drilled the LP orifice out to .500. Ran fine, no more warbling noise, but it did have a high pitched squeal, perhaps .500 is too large. No problem, I'll fix that.
I used it this way for 2 weeks. Worked fine. Now, it starts, fires, runs for a minute or two, sometimes sounds as if the gas supply is slowing, though there is still 4". Other times is sounds as if the gas is shut right off and it faults with no error code, no flash code, just the "service heater" light on. I have not caught it yet to see if power is cut to the gas valve, I'll try that today.
I pulled the HSI, looks happy, kinda' wiped it off and reinstalled it. I ordered a new one anyway. The gas valve has one terminal rusted off of it, I put an alligator clip on it and clipped it to the remaining terminal on the same point. It ran this way for the 2 weeks. Draft inducer sounds happy, runs well. Any thoughts?