combustion test

drhvac

9 out of 10 combustion test I do I get co readings under 50ppm. I had a furnace today that had a reading around 375ppm. Everything was clean, burner was nice and blue with no apparent flame impingement, gas pressure good, drafting ok. If there is a slight crack in the heat exchanger, I know this could cause co levels to rise, but why and how. Could someone explain? Thank you.

earl burnermann

CO

A cracked heat exchanger will effect the combustion gas by raising the o2 and lowering the co2. Just watched a webinar by RSES on this topic and they said that air from the air handler is going to push into the exchanger pushing CO up the chimney. Too much or too little oxygen will cause th CO to climb. If the chimney becomes blocked then the CO will find its way into the home.

drhvac

weird

because I know what your talking about when you say the o2 would go up when the blower comes on and more oxygen is introduced into the combustion air, but on this furnace the o2 reading was only 4.7%, and co2 was 9.08%. Co was 312ppm Co air free 402.

Jim Davis_3

High CO

Well maybe at least you learned that looking at flame color doesn't mean a thing. 

Analyzers measure 3 things - O2, Flue T and CO. The flue temperature is missing.  Of course without knowing what type of equipment the flue temperature doesn't always have meaning either.

Under 5% O2 on most residential furnaces is rare.  Sounds like the furnace is slightly overfired.  Usually on induced draft furnaces when this happens the CO keeps climbing the whole time it is running, but not always.

If that RSES webinar on cracked heat exchangers was free they still overcharged1   

drhvac

if it is

Over firing wouldn't the gas pressure be high? The gas pressure on this unit was right around 3.5". This is an older 80% carrier unit flue temp was 380 degrees.

Jim Davis_3

gas pressure

Gas pressure is one of the most mis-leading indicators of how an appliance is firing.  We have found many appliances in the field with wrong size orifices.  Sometimes they are too big, most of the time too small.

The flue temperature is on the high side of the induced draft furnace range.  It is not over the range but with an O2 of 4.7% I would have to say it is overfired,

Higher flue temperatures can also be caused by poor airflow but they don't make the O2 reading that low.  6% O2 is the normal low side reading in the field on most furnaces but on occasion it can get to 5%.

Just knowing most induced draft furnaces, I would say yours is overfired.

drhvac

overfiring

would be caused by to big of an orifice, high gas pressure, what else? What would you be looking for if this was your job?

Jim Davis_3

overfiring

Overfiring is caused by too big of orifices or to much gas pressure.  Too much gass pressure does not mean the gas pressure is higher than 3.5"  Too much gas pressure means you are putting more fuel into the heat exchanger tha there is air to burn it.  Gas pressure could be 5"w.c. and not be too high.  Orifice sizing is the biggest problem because you just can't be sure if they are drilled correctly.

The other condition of overfired that holds true with induced draft equipment is when you are producing more flue gasses than the inducer can remove.  Sometimes inducers are sized small from the factory.  In the field the blades get dirty and can't move as much flue gas. 

Each problem has unique readings that identify them.

On this job I am lowering the gas pressure until the CO is below 100ppm

drhvac

books

Any good books on combustion that you could recommend?

Tim McElwain

I have a entire seies of manuals

on combustion and combustion testing. Contact me at gastc@cox.net. Jim Davis and I also run our own 3 days classes on Combustion testing.

drhvac

classes

Your a partner of Tim's? I would love to come to one of your classes, but it's just too far. I'm about 5 hrs away. It seems like a very good class, wish they had something like that by me, but they never do.

Tim McElwain

I own the

Gas Training Institute in Warren, RI. Jim Davis and I are just friends he works for National Comfort Institute and travels all over the country training on Combustion Testing he is the best there is.



By the way 5 hours is not far, we have plenty of hotels nearby and RI has a lot of sites to see. The record for distance traveled to my classes is two different students from Alaska.

Jim Davis_3

training

As Timmie said, sometimes you have to travel to learn the things you need to know.  You can't be that far from classes, where do you live.

In the mean time post your questions here are we will do the best to help.

drhvac

oxygen

Is the goal when checking these systems to try and get around 7.5 o2 readings as long as your co readings are low?

I had one yesterday 175,000 btu old slant fin boiler which I thought was way oversized for the house. The gas pressure was 3.25, the o2 was 4.4, stack temp was over 500 degrees and the co was around 15 ppm. This was after I took this thing apart from top to bottom and cleaned real well. No matter what I did with the gas pressure I couldn't get the o2 higher than 5.5. I had the gas pressure down to 3" at one point and the 02 wouldnt climb. The co didn't change that much either. Is this a case where you are talking about where we really wanted to get deep into it we would have to think about changing orifices? If so, where do you start? Or since the co is good, do you just leave it alone?



Im from central jersey by the way. Please let me know if you are ever doing any classes around here. Thanks

Jim Davis_3

O2 reading

The goal is to get the O2 as low as possible while keeping the CO under 100ppm.

Normally the goal is 6% O2, but on occasion some equipment goes lower.  4.4% is low but the CO says it is okay.  Flue temperature is not out of line either. 

To de-rate a boiler you don't lower the gas pressure, you lower the water temperature.  You must watch the O2 and CO carefully and make sure they do not change after 4 or 5 minutes.  Any drop in O2 or rise in CO after 5 minutes is a dangerous sign.

I do classes in Philly and we keep trying to scheduled classes in Mahwah but haven't had succes this year so far.

clammy

where in mahwah

Mr  Davis I would be interested in any dates you where planning in Mahwah ,where were you planning to do your classes at wallworth bros or another supply house in Mahwah  so thati could inquire about being present .Thanks peace and good luck clammy

drhvac

combustion questions

- if your 0xygen and excess air is high say around 10.2 and 82% air, what would you do to correct that? I had a 90% furnace the other day that had these readings. co was low, gas pressure was good. if there is to much air, how would you decrease it if everything is fixed. This furnace was taking air from the basement for combustion.



- How does excess heat effect your readings? What would a furnace with a high temperature rise do to your combustion readings?



- how would a high draft effect your readiings?



Thanks in advance.

Tim McElwain

Answers

combustion questions

- if your 0xygen and excess air is high say around 10.2 and 82% air, what would you do to correct that?

THAT IS REALLY TOO HIGH, WHAT IS THE MAKE OF FURNACE? IS IT A MODULATING/CONDENSING FURNACE? IS IT A FAN ASSISTED (NEGATIVE PRESSURE IN THE VENT) OR IS IT A POWER VENTED FURNACE (POSITIVE PRESSURE IN THE VENT)?

 I had a 90% furnace the other day that had these readings. co was low, gas pressure was good. if there is to much air, how would you decrease it if everything is fixed. This furnace was taking air from the basement for combustion. DOES THE MANUAL ALLOW FOR AIR TO BE TAKEN FROM INSIDE?



- How does excess heat effect your readings? ARE YOU TALKING ABOUT EXCESS HEAT IN THE FURNACE, IF SO IS IT SHUTTING OFF ON HIGH LIMIT AT 200 DEGREES? What would a furnace with a high temperature rise do to your combustion readings? TEMPERATURE RISE ON MOST FURNACES IS AROUND 45 TO 75 DEGREES BASED ON A RETURN AIR TEMP OF SAY 60 DEGREES. THIS WOULD MAKE FOR A DUCT TEMP AFTER THE UNIT OF AROUND 105 TO 145 DEGREES WHICH IS ADEQUATE FOR PROPER HEATING PROVIDED THE CFM ON THE SYSTEM BLOWER IS CORRECT.



- how would a high draft effect your readiings? HOW HIGH NORMAL IS -.02, -.03 ? WHAT IS THE DRAFT READING AND WERE WAS IT MEASURED?



Thanks in advance.

drhvac

American Standard

The furnace l was talking about was an American Standard furnace. It was a 90% with a positive draft. It had the option t be direct vented, but the installer chose to take the air from the basement. So there is a 3" pvc for vent pipe, and the fresh inlet is open. Like I said the 0xygen was high, and everything else was ok. What would you do?



The other questions I asked were hypotheticals. I wanted to know how a high td across a furnace would effect combustion readings, and then how a high or low draft would effect readings? I would think a low draft would effect the readings the same way as high td across furnace would because they both cause the unit to run hotter than usual

Jim Davis_3

High O2

If the temperature rise through the furnace is high, odds are the flue temperature will be high because of lack of air flow.  If the furnace was overfired the CO would be over 100ppm.

High draft causes the flue temperature to be higher and the O2 to be higher.  CO could be either way.  Also the Delta T would be low because the heat is spending less time in the heat exchanger. 

When flue pipes are short the inducer moves the heat through the heat exchanger too fast.  Many manufacturers use to supply a restrictor plate to put in the intake to slow it down.  I prefer a ball valve that is adjustable to any length.  Even on a one pipe a short pipe with a ball valve is recommended.

The ball valve is usually adjusted 50% closed to start.  The gas is adjusted to see if the O2 and temperatures can be brought into line.  If so, all is good.  If the O2 is still a little high the ball valve can be adjusted just a little bit more.  All this is done while watching the CO ppm.

SWEI

Ball valve

Jim, are you talking about a PVC ball valve on the air intake pipe?  Just trying to paint a picture in my mind here.

Jim Davis_3

ball valve

The ball valve would go in the intake.  We have been doing that to also solve drain issues during the run cycle because of too much velocity.