Utica UB90-125 Purge Issues

jshetler

I've got a natural gas UB90-125 manufactured in 2013 that has been giving me and my HVAC contractor the run-around for the past week. The initial issue was that the unit would lock out at the purge stage (purge light flashing) and not proceed to ignition. There were a couple seasons in the past with the same issue where a clean out of the casting would bring it back into working order. This year the same thing happened and after a flush with water it worked ok for about a week. Purge issue then came up again and we did another more thorough cleaning with pressure from a hose nozzle. After this it still refuses to start up normally and keeps locking out at the initial purge stage. The only way I can get to start up at this point is to remove the cleanout cap on the exhaust tee, turn it it on and wait for it fire up. I'll then put the cap back on to avoid filling the house with CO but by the very next cycle it's purge lock-out again and I have to turn it off and repeat the whole process.

Other troubleshooting/diagnostics we've tried:
- Checked intake/exhaust pipes for blockage; they are clean and clear and have a very short run to the wall immediately behind the boiler. It's a concentric vent if that matters.
- Removed and checked all of the suction lines; all look ok with no blockage, leaks, or excess moisture
- Tested lines with a manometer; running rate was only slightly above the manual's spec of 1.0" so not nearly enough to trip the N.C. pressure switch
- Removed and rebuilt the condensate trap; it was old and pretty cruddy so I put a new one in according to spec. The drain line is very short and runs almost directly to a condensate pump.
- Test pressure switch continuity; left N.C. stays closed and doesn't seem to be opening at any point to indicate a blockage. Right N.O. one closes as the blower comes up to speed but difficult to tell if it's reading correctly.

The only thing we can think it could be at this point is the right-side N.O. pressure switch which may not be producing good continuity and/or operating outside of the 0.5" range. A new switch is on it's way but if anyone else has any other suggestions I would greatly appreciate it. I also prefer not to throw parts at this thing if necessary as waiting several days for a $200 IBC in the middle of winter is not my idea of fun.

109A_5

Hello jshetler ,

If this is where you are at with the flow chart, it could be the air pressure switch if the pressure is verified to be OK, it could be the wiring, including loose crimped on wire terminations (connector pins or other), it could be poor pin fitment on any of the applicable connectors in that circuit, the solder joints where P3 is soldered to the circuit board on the IBC, or the IBC itself (some other defect with the IBC). A multimeter may help determine the defect.

https://uticaboilers.com/wp-content/uploads/2022/02/UB90-200-Rev.-B.pdf

jshetler

Thanks for your input. A multimeter across the N.O. pressure switch contacts reads closed soon after the blower comes up to speed, but I'm wondering if it's not closing early enough during the start up sequence to pass the IBC flow check. Continuity across both pressure switches and at the wiring harness reads OK as well. I've also never read the N.C. switch as open at any point which would indicate some sort of blockage and cause a purge fault.

The other thing it does that makes me think it could also be IBC related is the post purge stage. By the flowchart, once hi-temp is reached it cuts the flame and runs the blower for 30 seconds after which the blower turns off and circulator remains on if there's still a heat call. In my case, once the unit reaches hi-temp it enters post-purge (purge light on). After ~30 secs. the purge light goes off but the blower remains on and when the temp drops enough for a re-fire it's locked out once again with a blinking purge light and the blower still running. It's at this point I do the whole cycle again of turning off the unit, removing the exhaust cap, waiting for startup, and then putting the cap back on until it inevitably locks out at the start of the next cycle.

My HVAC guy ordered a new switch which should be arriving tomorrow so fingers crossed that does the trick. Not sure what else I can try or even test at this point.

109A_5

OK if " removing the exhaust cap " trick works consistently I'm thinking it is not an electrical issue. And per the manual the IBC allows up to 5 minutes for the pressure to prove it is correct, however a later software revision could change that and the manual may never have been updated.

I believe you did not mention it specifically, What about this ?

jshetler

So yesterday morning after having the unit turned off all night, I started it up expecting more purge lockouts but it started functioning normally all of a sudden. It reaches hi-temp, goes through post-purge, then stands by before firing up at next cycle without any intervention. My heating guy was already on his way with a new pressure switch so we went ahead and replaced it anyways. It's been running almost a full day now getting the house back up to temp (nights have been in the single digits). We've also got an extra igniter on hand as those have the tendency to die if there's been excess moisture in the combustion chamber.

The only thing I can figure is that it had some time to dry out while being off during the previous night. Access to inside the casting is very constricted so there may have been water collected somewhere that we couldn't see. These units seem to be very temperamental when it comes to cleaning. The manual recommends a detailed annual cleaning which I'll definitely be more diligent about doing prior to the heating season. The procedure seems overkill, but with these purge issues I can see why it's worth following to a T especially as the unit gets older.

As to the orifices, I checked them previously when I had all of the suction lines removed and they seemed clear as far as I could tell. The opening through them is very small so it's difficult to determine for sure. I may order some new lines though so I can rule those out as a possible cause in the future.