high co

drhvac

Working on an old carrier gas boiler today. As soon as I turned it on I could smell that sooty smell when a boiler is dirty and sooted up. Took apart completely and cleaned, when I put back and started I put my analyzer on it and the co was over 2000! Gas pressure was at 3.6", when I lowered it to 2.5 the co dropped to 60, then it would go up to 400 and go back down. The gas valve was replaced, did these older boilers have different manifold pressures? Why is the co going up and down and so high? It reminded me of an expansion valve that was " hunting".

Steamhead

First of all, is that boiler old enough that it was installed when a different type of gas was in use? I've run into this in Baltimore, where manufactured gas was used until some time in the 1950s. This type of gas produced a much shorter flame than natural gas does.

Boilers made for manufactured gas sometimes had much smaller combustion zones than we see now. Even if the orifices were changed to reflect the higher BTU content of natural gas, the flame can still be long enough to impinge on something when fired at rated input, which creates the CO. Fortunately, many such boilers are oversized, so reducing the input rate still produces enough heat for the house.

To EVERYONE reading this: DO NOT attempt the following unless you have a working digital combustion analyzer and the know-how to use it properly. If you're working on combustion appliances and don't use an analyzer, you SHOULD NOT be doing this work.

You've probably done at least some of this, but here goes: First thing I'd do is check the input rate by clocking the meter. Manifold pressure is a good place to start, but if the orifices are the wrong size your manifold pressure can be spot-on and the input rate will still be wrong.

Then check the burners to see that they are properly seated. Misaligned burners can cause impingement. Also make sure all the flame openings in the burners are clear, and that there is no debris in the burner tubes.

If the burners have adjustable air shutters, set them as far open as you can without making the flames lift off the burners. This will direct more air to where it mixes with the gas in the burner tube (primary air) rather than coming up alongside the burner (secondary air), which will shorten the flame length.

Also have the chimney checked. It's not uncommon to find debris in older chimneys that have not been re-lined, including dead birds which the CO levels killed.

The fluctuating CO levels are probably the result of soot burning off that your brush and soot saw didn't get. If they don't stabilize, you may need to open it up again.

Make sure the stack temperature doesn't drop too low if you reduce the input rate. This will prevent condensation in the chimney.

Much of what I know about gas combustion came from Tim McElwain, who I hope will also contribute to this thread. If you get a chance to train with him, by all means do so.

don_9

Did you happen to see what the o2 numbers were doing when the co started climbing?

don_9

I had a furnace last week that was trip on the flame roll out switch.stuck the stack n reset the the roll out.o2 at 10 and co was 16 ppm.once the blower fan came on the co went to climbing as well as the o2.we found two crimp rings missing on the heat exchanger.

drhvac

I don't know if manufactured gas was used on this one, it's about 40 - 50 years. I did get a neg .03 draft, so it was drafting. O2 was 8.2, co2 7.22, stack temp 512. Don, this was a boiler so as you know the o2 wasnt going to be affected by a blower coming on.

Harvey Ramer

Lack of fresh combustion air would also cause high co. The measured co would change every time the basement door was opened and closed.

icesailor

Harvey Ramer said:

Lack of fresh combustion air would also cause high co. The measured co would change every time the basement door was opened and closed.

I spent years trying to come up with an instrument that could measure air pressure in confined boiler rooms. I never could. The closest I came to was an aircraft altimeter that is extremely sensitive to pressure drops. It reads in altitude. I could never find one or at least a cheap one to try. If they are working, they are in a aircraft. If the aircraft crashes, the Altimeter is considered broken and can't be fixed. My sources were limited, and reasons to know were seldom. It would be worth a try. No matter what you set the outside barometric pressure at, when you bring it inside to a boiler room or into a building, the pressure should remain the same. If you enter a building and an exhaust fan is running, or a burner starts, if the inside pressure drops, the altitude should go up.

Aircraft crash in IMC's with improperly set Altimeters.

I wish I could have proved it one way or the other.

Steamhead

drhvac said:

I don't know if manufactured gas was used on this one, it's about 40 - 50 years.

Might be worth checking with the utility to see what they supplied in the past, and when they changed over to NG. You'd have to find a real old-timer, or the company's historian if they have one. If you found one such boiler, there are others out there.

drhvac said:

I did get a neg .03 draft, so it was drafting. O2 was 8.2, co2 7.22, stack temp 512. Don, this was a boiler so as you know the o2 wasnt going to be affected by a blower coming on.

This was after you down-fired it?

Tim McElwain

Assuming the combustion numbers are with the unit at 2.6" W.C. pressure those are pretty typical numbers. The excess air is about 55 to 60% with those numbers so reducing input may not be a good idea. By the way you know if the air for combustion is okay as long as you have draft. No draft no combustion air. Sorry I got cutoff with some kind of maintenance thing. I will get back to you some time later.

Tim McElwain

Sorry for the interruption - give us some numbers with everything as the rating plate calls for.

Concerning manufactured gas it had a BTU content of around 525 BTU's per cubic foot with a specific gravity of .63. Those who converted equipment back in the day used 1050 BTU's per cubic foot for natural gas at .6.

There were some areas of the country that used 800 BTU's per cubic foot on manufactured gas.

We strive to have less than 100 PPM air free CO for safe operation. The CO reading should also be steady and not fluctuating.

Some of the old boilers tend to tighten up in the sections as they get older and this can cause some difficulty in getting a good analysis.

One of the reasons I flush boilers sections externally with a 50/50 mix of water and industrial 409 and then a final rinse with 50/50 mix of white vinegar and water. This is done after vacuuming, brushing and using the soot sword for cleaning. It tends to get rid of all the remaining soot and scale.

drhvac

How do you do that without making a mess? Do you cut the boiler out and take outside to clean?

captainco

Fluctuating CO is a sign of improper venting. draft in the flue indicates you have a functional flue and -.03" draft indicates plenty of combustion air. However, because the boiler has a drafthood, too much air may be sucking through the hood and blocking the escape of all the flue gases. Someone is lucky to be alive.

drhvac

I always thought between -.02 -.04 is a perfect draft. That's the first time I ever heard of a good draft causing any restriction in flue gases escaping. Could you please explain?

j a_2

It is nice and it is being so called green to tighten up the old houses….But when people do it its very important that the old BASEMENT BEAST is happy….It likes lots of air, and when it doest it will bite you every time…A make up air chart is very readily available and thats a good as place as any to start….I see lots of good advise on this thread…On a side note to Ice I don’t think,but i could be wrong, an aircraft type altimeter , would be very accurate, commercial a/c use radio alt. below 200 ft. due to that...

Jamie Hall

You'd be surprised. The altimeter on my bird will register changes of as little as five feet! The reason for the radio altimeter is that it registers distance above the ground, not altitude -- and that can be of more interest when you're flying close to the treetops....

icesailor

@JA:

After over 7,000+ rides to work in a Cessna 402C, mostly in the right seat, I noticed how much an aircraft altimeter changed with a slight change in barometric pressure. I used to tell pilots that I had made more ILS approaches into that airport sitting in the right seat than they did. Most ILS landings were done in heavy crosswinds on a 24 runway with the wind coming from 15 (NW) You need a heavy crab to the right to stay on the centerline. The local tower always gave the local BP. I could see the pressure altitude change. That's why I thought of a Barometer. I tried different things with my Bacharach MZF. It never showed anything remotely useful. I stuffed cloth under a door and put the probe outside and pulled it back. Nothing. I figured as something to try, take an aircraft altimeter, set it at any number like 30.00, and bring it inside the room in question. If it went up or down, it meant something. What, I would figure out later.

I never had the chance.