Low Gas Pressure and Roll Out Switch

steamedhams69

I recently bought a house with a Smith GB200 gas steam boiler. Last winter the boiler seemed fine. A few weeks ago I had the boiler serviced, the guy seemed great and knowledgeable and he pointed out the gas pressure was too low and he adjusted it. But since then the boiler won't operate properly - The roll out switch will trip after about 9-10 minutes.

I dropped the pressure from 3.5 to 2.2 inches and it now it doesn't trip. I am well aware dropping the gas pressure to correct a roll out problem is unsafe, but I noticed the roll out switch is an L250F but from what I've read in the manual it should be a 350° switch. Is anyone here familiar with this boiler and can confirm which temp roll out switch should be installed?

Chances are the previous owner was covering up a problem with the boiler but I figured I'd ask.

Thank you

mattmia2

did the tech do a combustion analysis or did they just randomly change the manifold pressure?

EBEBRATT-Ed

They show a "spill switch and a "roll out" switch in the manual but I saw no temp rating for either one. They are the same for all sizes of those boilers.

59271 is the spill

59273 is the roll out

You can probably find the manual on line if you don't have one. I don't think those boilers are made any longer.

If you getting roll out what I would do is :

Make sure the chimney is clear and the boiler flue passages and vent pipe are clean

Check the gas orifices are the right size (they probably are)

You can call Smith at 833-265-5371

steamedhams69

The tech that came out checked the pressure, saw it was low and adjusted. As far as I know, he didn't do a proper analysis, just that the boiler plate calls for 3.5 inches. When I started having issues with the roll out, check the chimney and the flue, they're clear. There's a small hole in the flue, when I put a flame to it, the flame gets sucked in and up. The tech told me there might not be enough air intake into the basement so we tried it with two windows and my bulkhead wide open, it's still the same issue.

59273 doesn't seem to turn up an exact match. One inexact match that keeps coming up is 52M73 which is a 350° switch. Other boilers in the gbx series seem to use a ceramic 333° roll out switch.

steamedhams69

Also, I got paranoid about carbon monoxide because of this and have 3 detectors in my basement and I borrowed a gas meter from work, there seems to be no detectable co outside the furnace.

mattmia2

you also don't know what someone else may have done, it may have different orifices in it.

EdTheHeaterMan

It looks like the 59271 spill switch is rated for 275°F and has a manual reset. That is the switch located on the draft hood.

The switch near the burners is a different matter. It would typically have a higher temperature rating, but I haven't been able to find a specification for that exact switch. Smith uses rollout switches rated at 350°F on some of its other boilers, but those switches have different part numbers. So, it's difficult to say with certainty what the correct temperature rating is for the rollout switch on your boiler. Your best bet is to contact Smith/Mestek directly and ask them to confirm the rating.

As for solving your problem, when you reduce the flame size, you produce a smaller volume of combustion gases. That smaller volume can apparently pass through the boiler and venting system without causing the rollout switch to trip.

When you increase the firing rate to the boiler's rated capacity—which should be verified by clocking the gas meter—you may be producing more combustion gases than the system can properly handle. This suggests one of two possibilities:

• A restriction in the venting system: There could be an obstruction or restriction in the boiler's flue passages, the vent connector, or the chimney.
• Incorrect orifices or overfiring: The boiler may have the wrong burner orifices, causing it to fire above its rated input. In that case, the rollout switch may be doing its job by shutting down the burner in response to an unsafe condition.

The next step is to clock the gas meter and determine the actual gas input while the manifold pressure is at 2.2 inches water column (WC). Compare that measured input with the boiler's rated input.

If the measured input is correct, the burner orifices and gas pressure are likely delivering the intended firing rate, and the venting system needs no further investigation. (this is unlikely)

If the measured input is too low, the boiler may be underfiring. Clocking the meter will help establish whether that is the case.

The important thing is to verify the actual gas input before changing the pressure or replacing parts. A rollout switch is a safety device, so don't continue operating the boiler at a firing rate that causes it to trip. Have the cause identified and corrected before returning it to normal operation.