Help with combustion analyzer readings

pitman9

We've always tested at the end because we've shied away from drilling a hole and covering it with foil tape. I'm going to steal this idea and throw a tap and plastic plugs in the trucks tomorrow.

Thanks.

SM

Got the numbers, know what do I do?

I just purchased a UEI C75, and have taken readings on several furnaces (all propane) and now I am trying to understand the numbers.

First was a Luxaire 90+, CO 131, x-air 231, 02 14.6, eff gross 88.3, condensing 98.2.

Next was a new Ducane 90+, CO 9, x-air 102, 02 10.6, eff gross 89.6, condensing 108.7.

Third was a Trane 90+, CO 50, x-air 164, 02 13.0, eff gross 89.1.

The Luxaire would seem that it has a high CO and 02. What would cause this? What should I be looking for?

The Ducane appears better to me, and it is brand new, but what it the difference between gross and condensing eff.? How can it be 108%?

What about the Trane? Are these numbers Okay? Maybe a little high on the CO?

I know I need to take a class know that I have the analyzer. Can anyone recommend a class or seminar that will be in Ohio?



Thanks for the help.

S Ebels

Did you check........

What was the manifold gas pressure on each of the furnaces. The CO is definitely on the high side on the Luxaire and at the limit of what I like to see on the Trane. Notice that the excess air is also higher on those two than the Ducane. Since there is really no air adjustment on high efficiency type furnaces, the only things you can play with are gas pressure and orifice size.

SM

Gas pressure

Didn't check Luxaire, but both the Ducane and the Trane at 10" wc. Is the high CO a sign of to much gas? What is the difference between gross, net and condensing eff.?

Glenn Harrison_2

The quick answer is...

all three units have oxygen (O2) levels that are too high. In general, the O2 should be 7% to 9%. So you are either not getting enough gas or to much air, or even a possibility of both.

Now I noticed you said the gas pressures are at 10". Is this LP I assume? Is that incoming or manifold? Is that the correct pressure per the manufacuters.

The next thing I would look at very carefully is the PVC vent pipe sizing. I have had many furnaces that the flue pipe was sized by rule of thumb instead of by the book, and the pipes were oversized causing the inducer motors to move to much air thru the intake, burners, heat exchangers and exhaust. In many cases I could put a PVC reducer on the exhaust, and then maybe intake pipes to slow the flow down enough to reduce the velocity thru the burners and improve the gas/air mixture causing the O2 and CO levels to drop and improve the efficiency of the unit. This is something that take some time and experimenting with numerous readings to get it right.

You may also have to increase the gas pressure a little to change the gas/air mixture and improve your readings.

The long term answer is --- Get Combustion testing training. You can get started by going to

www.bacharach-training.com

and read all the information there. Then go to

www.bacharach-inc.com

and check out the training schedule for your area, and get to the next class.

You can also contact fellow Wallie Tim McElwain at gastc@cox.net. He has numerous books on combustion training, as well as a top notch training center in Rhode Island.

Last, you can sign of for a class through National Comfort Institute (http://www.nationalcomfortinstitute.com/) on CO and Combustion testing, availble accross the country. Check out the training schedule for a date and time near you.

S Ebels

Too much

Or not enough. And the opposite side of the coin can be the problem also. That is, too much or not enough air. Since the firing rate is supposed to be a known quantity, you have to assume that given the mfr's spec for orifice size and gas pressure are met, the only other variable is air. Since there is no air adjustment on a typical high efficiency furnace you have to play with the gas pressure.
I'm not gifted at explaining things like that so you may have to read through it a few times to get what I'm driving at. The point is, CO is produced when the air/fuel ratio falls on either side of the ideal setting. Too much fuel and/or not enough air will cause high readings on one side of the curve. The opposite is true on the other side where you have a situation of too much air and/or not enough fuel. 'Tis a delicate balance indeed that we seek for optimum performance and safety. The trick is to find that balance without messing the firing rate up too badly.

This is one of the reasons I dislike modern high efficency F/A equipment. There is little you can adjust besides gas pressure and orifice size while trying to "tune" on in.

On the Gross/Net/Condensing efficiency question, I have no answer for you. Your best bet would be to call the manufacturer of your analyzer and ask their tech service dept for an explanation. Your manual should cover some basics also. If it doesn't, shame on 'em.

SM

Pressure at the ....

manifold is 10" and that is the manufactures setting for LP.

Another questions- on a high eff. furnance can one drill a hole in the PVC pipe for the analyzer probe and then cover with foil tape when complete, or should it be done at the exhaust outlet (outside)?

S Ebels

Closer is better

> manifold is 10" and that is the manufactures

> setting for LP.

>

> Another questions- on a high

> eff. furnance can one drill a hole in the PVC

> pipe for the analyzer probe and then cover with

> foil tape when complete, or should it be done at

> the exhaust outlet (outside)?

S Ebels

Closer is better

For more accurate readings, a closer test point is better due to temp drops in exhaust gas. (some of the vent lengths get pretty long)

I found some heavy thread, stainless steel screws of the right diameter for the probe on our analyzers. We use them with a heavy faucet washer to seal up the hole when done testing.

Glenn Harrison_2

Yes, you can drill

a hole in the PVC, as it is better to test close to the unit. I would fill the hole with high temp RTV, or you can tap the hole with a 1/8" pipe thread tap and install an 1/8" nylon/plastic plug. You can get the plugs from www.mcmaster.com

pitman9

We find the same as you.

If the CO is high you generally need to work with the gas pressure. Take readings as you try raising or lowering the pressure. Up to 0.5"wc can make quite a difference. I don't think we've ever had to go more than that.

Jim Davis_3

Combustion Readings

All three furnaces are underfired. O2 should be 6%-9%, CO between 11-99ppm. Plastic flue temperature should be 120-140 degrees and the plenum temperature should be at least 130 degrees. November 29-Dec 1 I have a NCI Combustion/CO class in Cleveland. 800-633-7058.
Drill the plastic pipe and wrap multiple times with tape or use a little thumbgum.

[Deleted User]

And THIS guy...

puts on one heck of a class. Catch it while you can.

ME

SM

Numbers on a AMerican Standard boiler 1350 MBTU inmpu

Stop buy church hall/school today and thought I would try analyzer out there, don't believe anyone has serviced this thing in years.

Had 1536 PPM CO, 2.5% o2, 10% x-air and G-eff of 81.5%. Adjusted the air shutter and got the CO down to 62PPM, O2 to 7.8% , but G-eff dropped to 78.5%.

Question, what is going to with the G-eff that it dropped, even though the boiler would appear to be closer to correct setting once I adjusted the air shutter. I know the eff is a calculation, but is it not an accurate calculation when the other numbers are so far off.

S Ebels

Makes sense

From your readings, it looks as though the burner was starving for air. Hence the low O2 and dangerous CO levels. It's very possible that the reduced air setting on the burner lowered the stack temp enough to raise the calculated efficiency. What you wound up with is probably the true efficiency for that boiler when operating under safe/normal conditions. I'm curious to know what the before and after stack temps and CO2 readings were. It would help to flesh out the whole picture.

Combustion tuning involves looking at the whole picture including all parameters such as CO2, O2, CO, draft, stack temp and efficiency. Many times the best efficiency that can be attained is not within safe combustion tolerances, especially with older equipment. The burner technology just isn't good enough to allow max efficiency and good combustion at the same time. Trade offs have to be made to ensure safety.

Have fun and keep asking questions.

Mark Hunt

Because


the stack temperature went up.

Those "efficiency" numbers are based on an assumed flame temperature so that the lower the stack temp the higher the "efficiency". Once you adjusted the air, everything changed including the flame temperature. CO is incomplete combustion so you couldn't have had a "hot" flame when you weren't burning all of the fuel.

Mark H

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Mark Hunt

Almost forgot


Steve Ebels is correct. I would like to know what the before and after CO2 numbers were along with the stack temps. Both of those numbers are used to determine "efficiency" as well.

Mark H

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