Why Is This Happening?

smellslike$tome

Performed diagnostics on two Rheem RFD300-91-1 commercial water heaters. They are installed in parallel and both discharge to the same Lochinvar storage tank. The complaint was that they both will shut down for no apparant reason, can be very difficult to restart, and one of them has been eating thermocouples like there is no tomorrow. Manager mentions that some HVAC company had been out and the guy announces that the gas regulator needs adjusting and increases the pressure, this is when it starts eating thermocouples. So of course I'm zeroed in wondering what this guy did to the regeulator that's making it eat thermos. They replaced one yesterday, one on 4/29, and I ultimately wound up replacing one today although I'm really not sure why or why it appears to have solved the relighting problem (at least until it burns that one out). They also say that they recently replaced the gas control valve for that unit. I first looked at them yesterday afternoon and they were both running fine so I made arrangements to come back today. Oh yeah not sure that it's important but these serve a medium sized hotel.

Ok so I come back today, after having downloaded Rheem's diagnostic sheets. I run through the diagnostics for the least offensive unit and everything checks out. I do the gas pressure test last and it checks out, 8.55 in/wc static and 7.05 in/wc under load. The range for this unit is 14 in/wc max to 4.7 in/wc min. Now everything about this unit performed perfectly EXCEPT while I was tracing out the wiring for something (I'm not sure what) with the burner off, the pilot went out. It came back on with no trouble but it should not have gone out at all. It never did it again.

So then I go to unit number two which seems to be giving the most trouble. I do the diagnostics and again everything checks out except that the motorized damper is stuck in the open position which I already new. Although this makes the unit even less efficient than it already was, it should have no affect on the unit starting, running, or staying lit. So this unit has been running, stopping when it's supposed to, and reigniting all by itself just like it is supposed to for the entire time I've been there until I do the gas pressure test. I'm already suspicious of the gas regulator although until today I don't think I've ever seen one that was bad, but I've got it in my head that the guy messed it up when he "adjusted" it. It's true, they are adjustable but I've never adjusted one and if you use the correct regulator you should never have to and this was the correct regulator. Anyway, I put the manometer on it and it finally stops at 24.8 in/wc which is not far from double the max it's supposed to be. At this point I already know I'm changing it out because even if readjusting it down seems to correct it I know I can never trust it again. Just to see what happens though I adjust it down and bleed off some pressure to get it down to around 8 in/wc and sure enough it starts climbing so I just go ahead and adjust it all the way out but it just keeps climbing. When it passes 14 in/wc I've seen enough and change it out. The new one reads (untampered with) 7.69 in/wc static but I can't read it under load because the THE FREAKIN' PILOT WON'T STAY LIT! AAARGH!!! So I think what has happened here in the process of working on it did the gas control valve get fried? Ultimately out of about a dozen attempts I got the pilot to stay lit once but as soon as I moved the gas control knob to "on" it went out. So I think that even though there is a one day old thermocouple on the thing, I'm changing it out. Well they had been using 24" thermocouples but all I have on the truck are 18" but it's long enough to reach so I use it and it fires right up.

Finally to the questions!

What affect if any would high gas pressure have on a thermocouple? Remember it wouldn't relight after the regulator was changed out.

The thermostats required 24 volts and were receiving about 30 volts.

Why are the thermocouples not lasting?

Why were both units shutting down and why did the pilot on #1 go out that once but then fired right back up with no further problems?

Have I repaired this or am I going back?

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KillerToiletSpider

The damper is your main problem, that is why it eats thermocouples, the damper is telling the control valve to run and the thermometer on the same control valve is telling it to shut off. The weakest link in this vicious cycle is the thermocouple, you need to fix the damper, or sell the motel on an upgrade to commercial tanks with electronic ignition.

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smellslike$tome

You know I did wonder if there was some relationship but only briefly and when I couldn't put it together I just let that thought go. I will investigate further.

Thanks.

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smellslike$tome

About the upgrade, yeah the first thing I said was "what you need to do is get these dinosaurs out of here and let's put in an adequate tankless system".

They laughed at me.

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