Why does this set up "work"?

kcopp

Did a service call for a steam boiler this week... It is a 4 sec. Weil SGO w/ a Carlin EZ gas burner. Added a Field MG1 and a spill switch that was missing.  Nat gas. About 1 and 1/2 yrs old. It was piped in 1/2 CSST about 35 feet from the meter. Looking at the gas charts the pipe capacity is about 50K BTU. Burner is about 150k input. I clocked the meter and I get 148k. How can that be?

SWEI

gas pressure

where was it on/off fire?

Zman

The chart

The chart gives allowable sizing given a certain pressure drop.The system you describe "works" because the boiler is either very forgiving on it's minimum or the system pressure is set too high. As SWEI suggested. what were the pressures at the boiler with the boiler on and off?

Carl 

kcopp

I did not....

get that far. The install was done by a company that is now out of business. The system has been  fine for a year and a half until that last cold snap where it locked out 2 times. I offered to do a complete system check w/ combustion tool and all but he just wanted to do the spill switch and double baro. I clocked it just to see if was over-fired

VictoriaEnergy

Pressure drop

The capacity in the sizing tables is based on a 1/2" drop in pressure, so if you have a 35' length of pipe with 7" pressure going in you will have 6 1/2" pressure when 50 MBTUs are flowing through it.  If you allow greater flow of gas, the pressure drop will be greater too.  So when under 150 MBTUs load the pressure drop might be 2" or 2 1/2" so the appliance is getting 5~5 1/2"  Since most appliances use a manifold pressure of 3 1/2", the little reg that's built into the gas valve will likely be running wide open.



I think you made the right call by adding the baro, the spill switch and clocking it.  The only additional thing I may have checked was the manifold pressure (as stated on the appliance rating plate) and confirm the spuds are seeing the correct pressure and are original size.  It the manifold pressure is low, then the installer would have had to drill them out to get full BTUs (a game misconduct). 



I can imagine all kinds of ongoing reliability issues with the undersized gas line, so the owner should definitely upgrade it, but I don't see it as an unsafe operating condition.  So I don't see a problem leaving it running assuming the manifold pressure is within spec.

SWEI

I don't think I could put my name on it

without checking at least gas pressure and CO.