Roll out switch failure

Radrobber

New install. In floor heating application. 70,000 Btu Slantfin boiler. Relatively new boiler had failed due to consistently low return water temperatures. 30,000 Btu water heater shares 5" common vent with boiler. Vent height is only 15'. Automatic flue damper on bolier. Draft of boiler has been proved.  Manifold pressure correct - 3.5" W.C.  Mixing valve was added to maintain return water temperature at 140F. Supply water aquastat if I remember correctly is set to 160F. Return water pipe size - 1", supply water pipe size - 1.25".  On two seperated occasions, weeks apart the rollout switch above the burner section has failed. Upon running the boiler with a thermocouple placed near the rollout the temperature climbs within rollout setting. With lower cabinet door removed the temperature is very acceptable and stable.



       What am I missing?

RobbieDo

Mfg part number

What model is this? Are you sure the flue is clear, no blockages?

Radrobber

Slantfin mod# SE70SED

New liner was pulled when I installed the boiler. Carbon monoxide detector in room has never gone off. Limit switch on draft hood has never opened. 

kcopp

When the liner....

was installed was it sized properly? Did you do it or was it another company? Had a problem posted here a month ago where a sub contractor pulled the wrong liner down the chimney and that was the issue.

Gordy

Flue liner size

 If the flue liner were to small would this not trip the spill switch on the draft hood if its functioning properly?  I'm only asking. I do know some spill switches take quite a bit to trip, but should before roll out is tripped.



Gordy

Mark Eatherton

Probably a lack of secondary air flow...

That particular part of the boiler SHOULD be cooled by the incoming air for the combustion process. You could have obstructions in the flue gas passage ways, or you may be suffering from a lack of draft at that point that is allowing the radiant energy to overheat the detection switch and causing failure.



Attempt to clean between the sections first, and then if need be, apply a barometric damper to control draft, eliminating the fixed draft hood and installing a proper roll out switch at the barometric damper.



Proceed with caution.



ME

Tim McElwain

I would suggest a complete

test of the unit with a combustion analyzer.