I have a Raypak 256B pool heater (natural gas).
For the past two seasons and I had an intermittent problem with the unit firing up.
Here is the typical symptom I see (used to be 25% of the time 2 seasons ago, now it is 95% of the time) :
I turn the unit on, I see CFH on the display, I hear 2 sparks/buzzes, and then ILO appears on the display (no more sparking).
I determined that the pilot gas line orifice is not clogged.
The issue looks to be (at) the gas valve. When the problem appears (again it is not every time I try), it seems that the pilot solenoid in the gas valve is not opening (so no gas getting to the pilot assembly). I measured 24VAC across PV and C (red and yellow wires), but am no expert on what else I need to check on/near a gas valve. [note that I see 24VAC from the time I turn on the unit until the time I turn it off,ie, even see 24VAC when the unit is not sparking and ILO is displayed].
The only other thing I can add is that when the unit DOES fire up normally, it tends to be on very sunny days (I am assuming that this condition would create a higher temperature inside the heater, which maybe is changing the physical properties of some device/connection such that "normal" operation occurs..??) Also, when it DOES work, it will tend to work (fire up) successfully repeatedly if I try turning it off and on several times in one session.
Any ideas on how to troubleshoot this?