I had the gas co test.. they reported as at the appliance. Is the a reasonably priced device you might suggest to test the hookups on the gas valve?
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Let's say the gas is low.. what can be done? The utility says pressure is max allowed. Also why would this thing work for years..
Not much can be done if your gas service/supply is low. A low water column could indicate too small gas line size, How long has this heater been in service before you made these repairs? If it had been previously operating normally, gas line size may not be the problem. I think you would also get a little more time before the heater would shut down from insufficient gas pressure, if it fired at all!
I know i said look at gas pressure before, but at this point, i'm thinking your heater is not reporting flame rectification, or an electronic ignition issue. (Its just) The quick shut down is the only thing that is not inline with a FR problem. You would normally get those 7 seconds from the time you hear the click of the gas valve opening to shutdown.
See, if the heater (fire or mechanical side) has a problem, the heaters control will give some time, those 7 sec, before it shuts down. the time is engineered into the system to give the heater (fire) a chance to settle down and fire properly in case there is some sort of temporary draft or wind issue. The more i think about this, the more i think it may be an electronic ignition CONTROL problem. The control part is failing. Problem is, you have two areas of control that can be causing this. There is the FENWAL. Basically a box of switches, that may not be holding the gas valve switch closed the entire time they are being told to. Or the main board that does the telling, may not be telling commands consistently (flaky commands).
Best recommendation: Find yourself a real sharp (meaning old) heater guy. You need a guy that would be willing to swap parts rather than force you to buy new ones regardless if it fixes the problem. Or you could just replace those two items your self with the thinking, well, i feel better knowing that all the electronics are now new, even then, there would be no guarantee that would be the fix...