Help with Burnham CHG recurring soft lockout

easterner

Hi, I'm hoping someone who has a little more experience with these can help my in my troubleshooting. I'm currently battling a b30 code (Temperature rise between supply and return is too high) on the MCBA that seems to pop up in the early morning when the system has cooled during overnight setback.

Background-
CHG150 installed in 2012 by previous owner, replacement for original oil fired boiler
Primary/boiler loop- 1 1/4" closely spaced tees with Taco 0010 circulator as specified
Secondary/only zone - 1" split loop with 2x 3/4" slant fin baseboard runs and 2 Runtal panels on mono flow tees, Taco 007 circulator
No DHW
I see some scribbling in the manual about a previous b18 supply over 200 degree issue.
About once a year I experience the E04 hard lockout usually during a storm with power outages. Annoying but hasn't been a cause for major concern, I see there are retrofit kits to help resolve this but have been considering UPS instead.

Problem -
Woke up to a b30 error code and cold house. Looking at the diagnostics supply temp was quickly rising from ambient to 160ish with little to no movement of the return temp. Supply piping to the single zone getting warm. Returns from split loop starting to warm, return to boiler about the same. I've been able to resolve the lockout by resetting the control and forcing into low fire for 15 minutes. Once the return side has warmed above ~100 degrees everything seems to run just fine. Will run throughout the day with about a 25 degree differential. Thermostat set at 68 day, 62 night

Troubleshooting-
Both circulator pump motors warm, slight vibration, closing isolation valves causes hammer and a jump in pressure. Running amps are slightly less than name plate FLA.

Checked temps with IR gun compared to the diagnostics, all seem to be reasonably close. The return temp sensor is maybe a bit sluggish to react compared to the supply which reacts very quickly.

A bit at a loss here where to go next. Manual suggests that piping is incorrect or circulator has failed causing low flow through the heat exchanger. The slow to respond return temp sensor is slightly concerning but this part seems to be difficult to find. Anything I'm missing here?

easterner

Swapped the return sensor with the flue gas sensor as all of them are the same. Working now but will still throw the b30 error for 1-2 cycles before firing continuously. An improvement over the endless loop before. New sensors are on backorder, hopefully will be able to get some soon. Are these considered a maintenance item? 3 on this heat exchanger.