Lack of combustion air?

Rizz861

I came across a newly installed Williamson steam boiler that keeps having problems with the block vent switch. A.k.a. spill switch. The room doesn’t have nearly enough volume for combustion. A fan in a can was added however, I don’t feel there’s enough make up air still. In this room, you have the steam boiler at 285,000 BTU, a small hydronic boiler at I believe it was 67,000 BTUs, and a water heater at 40,000 BTUs. The room dimensions were 11 x 13 x 8 so nowhere near enough volume for the make up air calculation.  The pressuretrol and gas valve settings were off on the steam boiler so I adjusted that. I brought the gas valve to 3 1/2 inches water column as it should be and the pressure troll cut in at .5 psi and differential wheel set to one psi. I measured the draft with my monometer and it started out at -.02 and got as high as -.55. So draft doesn’t seem to be the issue and if anything it’s almost drafting too much I did notice upon doing a combustion test that the carbon dioxide level was higher than it usually is on this style of boiler. This boiler is kind of like the Burnham where they have that rear outlet box that has the dilution air getting sucked into the exhaust so you usually have high excess air readings on your combustion test and higher oxygen numbers and lower carbon dioxide numbers if that makes sense to you guys. What’s interesting to know is that box I was telling you about just now, is hot as hell to the touch. On all the boilers, I’ve worked on that had that similar rear outlet exhaust box, that’s usually never hot to the touch because of the air it sucks in to dilute the flu gases. I just feel has something to do with the make up air but I’m not really sure I’m kind of stumped on this one. Any ideas guys?

SuperTech

Was your draft reading measured over fire or in stack? Is the room itself being drawn into a negative pressure? You might need more combustion air than the fan in a can is able to provide. 

By the way this is a much better place for help with this sort of thing than Reddit.  

captainco

You are taking you combustion readings in the wrong place. You can't have draft in range if there is a combustion air problem. What was your CO reading?
Don't we breathe the same air the boiler uses??

Rizz861

@SuperTech I’m working on a gas boiler and it was measured about 12” from flue outlet. This is a rear outlet draft hood so I don’t know where else I would measure my draft and combustion readings other than where I did. 

Rizz861

@captainco Like I said I measured 12” from outlet of draft hood. I know on the draft hoods you see that are inline with smoke pipe you measure before the draft hood. Where would you measure on a rear outlet style draft hood?

Rizz861

The duct for the fan in the can was upsized from four to six inch and that seemed to get the flames trending towards blue, but something still seems off. Also, there is a decent drop in gas pressure when this fires on high fire. Almost 2 IWC. Gas company is going to come out and look at the regulator and meter. I believe I only had like 15ppm undiluted CO btw. Another possible clue is the original steam boiler was a monster and my boss was saying twice the BTUs. the chimney may be way too big. Can too much draft cause yellowing of flames? I thought I’d read that somewhere. Is that something a barometric damper would solve? 

SuperTech

Rizz861 said:

@SuperTech I’m working on a gas boiler and it was measured about 12” from flue outlet. This is a rear outlet draft hood so I don’t know where else I would measure my draft and combustion readings other than where I did. 

You gotta get your analyzer up inside the draft hood, above the boiler block. Any combustion measurements taken above the draft hood are no good, except for draft on the stack. Your measurements are highly diluted by all the draft in the stack and chimney.  I use a piece of 1/4" tubing attached to my probe with a small piece of rubber tubing from a manometer hose or pressure switch.

Rizz861

@SuperTech That sounds terribly inconvenient and inefficient as far as testing and it’s no wonder why a lot of guys aren’t testing. Your response does make sense though. So you are basically saying I have to get my draft and combustion reading at the box above the heat exchanger before the rear outlet draft hood? That makes me wonder: why the hell would you want to design a draft hood in such away? The other style we see which is on the vertical of the smoke pipe seems superior for testing purposes because of our ability to punch a hole below it to do our tests. What is the advantage or purpose of this style of draft hood?

Lance

Nothing teaches better then firsthand experience. 40 plus years ago I ran a no heat call on a windy 20F day at a very old mansion in Baltimore City. A 5gph oil fired boiler was in operation yet the boiler and header pipes were cold. Now 5 GPH is a lot of heat, so I am amazed its constant operation was not heating the boiler. The 12" flue pipe did not have any barometric regulator on it. Been this way for years. (A way oil companies keep the oil flowing I guess.) I measured flue draft, and my meter pinned to the maximum as I expected it would be. I measure the flue temp and it is cold for this boiler below 400F. Knowing combustion requires balance of draft I install a full-size Barometric regulator. The gate is now fully open. Flue temp jumps up to 900F, but boiler header still cold. Draft is not pinned on my gauge, but it is at maximum. I had no choice but to install a second regulator. On this regulator I was able to establish -.04" W.C. on the flue and -.02" WC. at the combustion chamber. Within minutes the supply pipes finally get hot. Conclusion: this constant cold weather created a condition whereby the unregulated vent connector created a chimney so hot that it became a great vacuum cleaner. I was pretty sure normally only one regulator would be needed but I could not wait 8 hours for this flue to cool off as thermostat demand would be constant for the next few hours.

pedmec

Is this a residential or commercial install?

Are there exhaust fans in operation when the boiler runs. I've seen plenty of installs where there are exhaust fans in the mechanical room because the mechanical room gets too hot. lack of understanding the hazard they are creating.

Commercial kitchen hood exhaust fans move a boat load of air to the outside and need a functioning make air system to replace the exhausted air and prevent a negative situation in the space your working in. if your in a commercial building check the make up air