Flashback in gas burner tube

[Tangent]

Utica M150AGB, Natural gas boiler

There are 3 burner tubes. Sometimes, the middle tube [only] will have a blue flame in the venturi for the entire burn cycle. This happens [randomly] about 5% of the cycles. During the flashback, I can lower then raise the T-stat, and the next burn there is no flashback.

A month ago, an HVAC pro replaced the millivolt gas valve and the aquastat, but the problem is still there.

The burner tube can be displaced a few millimeters closer/farther from the orifice. I tried both extremes. I’ve swapped the left and right burners with the middle one, but the problem is always in the middle position.

Is there a safety issue with a flame in the venturi?
Any other tests I can try?

 

[Fitter30]

First check how much gas the boiler is using by turning off any other gas appliances running the boiler and timing it for one minute clocking the gas meter . There's probably a 1/2 cu ft hand. Cu ft = 1050 btu. There is gas regulator built into the gas valve, clock wise raises, ccw lowers, 1/8 turn recheck. Burners should be burning blue. Your burners i don't think have any type of air adjustment ( optional).

 

[Sponsor]

 

[Tangent]

running the boiler and timing it for one minute clocking the gas meter .

In one minute of furnace run-time, the gas meter dial (that moves the fastest, lower-left dial in photo) rotated 4.6 times. I expected the dial to rotate at a constant velocity, but from the video I shot I could see that sometimes it was a bit faster and sometimes a bit slower during certain parts of each revolution.

Photo of the White Rodgers 36C03U-333 gas valve is also shown. You'll have to point out which dial you were referring to for the gas regulation.

 

[Fitter30]

Gas meter lower left probably 1/2 cu ft, lower right 2 cu ft it should say on the dial. Use this chart for the reading
https://hvactechhangout.com/home/system-measurements/how-to-clock-a-gas-meter/

 

[Tangent]

Gas meter lower left probably 1/2 cu ft,

Yes, same as my meter.

I get 138 cubic feet / hour. [= 4.6 x 0.5 cubic feet / minute] . What rate should I be expecting?

I see the regulator adjusting screw on my valve. Someone actually put a purple mark near that screw head. The adjustment single blade screwdriver slot doesn't exactly match the purple mark -- it is rotated a bit to the left.

The other gas devices all have electronic ignition and were all off.

 

[Fitter30]

1050 btu's x 60 =144,900 btu 's on hour

 

[Fitter30]

1050x138= 144900 btu's hour

 

[Tangent]

1050x138= 144900 btu's hour

How does this differ from what the HVAC guy did a month ago when he installed the gas valve -- I assume he used a manometer to adjust the manifold pressure to 3.5 inches W.C. ? Except for the few burn cycles where there is flashback, the burner flames are normally blue. When there is flashback, it looks like the photo I attached.

 

[Dana]

1050 btu's x 60 =144,900 btu 's on hour

That's very consistent with a ~150K burner, given that the BTU specs on heating equipment aren't precision numbers, and the measurement error of eyeballing the meter for a minute is also low precision. Even the given mixture of gases in the natural gas has more than 1-2% of range.

If you're ever considering replacing it, bear in mind that a 150K boiler is 3x oversized for normal-sized houses in New England that have any amount of wall & attic insulation, and at a minimum storm windows over old single-panes (an all too typical situation.) Most homes would be more comfortable with a boiler half that size (or smaller) running much longer heating cycles before satisfying the thermostat. If a new boiler is ever on your radar, run this math before entertaining any proposals, and reject anything that's more than 1.5x oversized for the fuel-use calculated load numbers. The AFUE test has a presumptive 1.7x oversize factor, but ASHRAE recommends 1.4x as the best compromise between comfort during average cold weather while retaining the ability to cover the load during Polar Vortex disturbance cold snaps.

 

[Tangent]

That's very consistent with a ~150K burner,

Still, I'm wondering why there is sometimes a blue flame in the middle venturi. The [old] 7-year gas valve was more problematic. With the new gas valve, it happens less frequently, but it still seems strange that it happens at all.

 

[Fitter30]

Pull the burners unscrew the orfices blow them out after inspecting them. If the burners had a problem it would have followed the burner. Could be speck of dirt or spider web.