Manometer, Combustion gas analyzer - http://www.hvacmechanic.com/ Forums

Original message

dcdraftworks

69.105.116.101

"Manometer, Combustion gas analyzer" , posted Wed 20 Feb 21:55

What is a combustion gas analyzer and how does it relate to a manometer?
What are the consequences of over firing the plate rating?
Thanks

Elmo
dcdraftworks@sbcglobal.net
CAD Drafting of Mechanical Systems
Marin County California

Replies:

northernfitter

70.48.1.73

"Re(1):Manometer, Combustion gas analyzer" , posted Wed 20 Feb 22:48

A manometer is an instrument that measures pressure in inches water column. Most natural gas residential appliance require a manifold pressure of 3.5"wc no. A combustion gas analyzer, analyzes air samples normally now with a pump and electronic read out that gives various readings depending on how high end it is. The most important being carbon monoxide which is a product of incomplete combustion. Some will also give percentages of oxygen and CO2. Hi Oxygen content will often indicate a cracked heat exchanger where it might not have shown it through high carbon monoxide. The two testers only relate to each other in the sense that they are both used heavily in this field. Over firing the plate rating will shorten the life of the furnace and potentially cause carbon monoxide with several other risks including over heating the furnace, tripping safeties possible blowback on ignition. Basically over firing the rating plate is about the stupidest thing you can do.

14years as a service tech in Ottawa and the valley. www.reliableheating.ca. For furnace manufacturer ratings go to

http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/gas-furnaces/furnaces-repair-history-205/overview/index.htm?resultPageIndex=1&resultIndex=1&searchTerm=furnace

dcdraftworks

69.105.1.152

"Re(2):Manometer, Combustion gas analyzer" , posted Wed 20 Feb 23:13:

So, if the gas pressure is 4" in your example, will I over fire the plate rating? Or do I need to measure the gas consumption to determine that?

[this message was edited by dcdraftworks on Fri 22 Feb 15:17]

341

99.246.83.83

"Re(3):Manometer, Combustion gas analyzer" , posted Thu 21 Feb 19:27

Your're right, measuring the manifold pressure does not guarantee that the applicance is actually firing at the rating plate's firing input. You need to clock the gas meter to determine how many BTU's the input actually is.

www.hvactechgroup.com

dcdraftworks

69.105.1.152

"Emissions" , posted Thu 21 Feb 23:43:

Thanks 341,
So if your firing, to the plate rating after clocking the meter, will the emissions be ok? Read somewhere, if the inducer draws too much air thru the combustion chamber the mix will be wrong and result in higher emmisions. Also read that of the nitrogen oxides (NOx) emitted, only one is classified a greenhouse gas? Is a flue gas analysis always performed after a new install? As a rule, what percent of NG entering the combustion chamber, exits unburned.

[this message was edited by dcdraftworks on Fri 22 Feb 15:18]

static

74.140.102.1

"Re(3):Manometer, Combustion gas analyzer" , posted Thu 21 Feb 00:29

Lol, opening a can of worms with that question. Depends on your definition of "overfired".

dcdraftworks

69.105.1.152

"Thanks Static" , posted Thu 21 Feb 00:47:

Mr. Blackhawk
Read about some EPA testing, said most 80% furnaces they tested were overfiring by 10-20%, that if the flue became disconnected it could be lethal. On the other hand, they said that 95% HEs might not asphyxiate if firing at plate rating even in the event that flue were to disconnect. I believe they were refering to NG consumption.
So I assume that higher than specified manifold pressure may cause overfiring. I Read a post you made to another member which indicated that the energy rating on gas products can vary. Out here, PGE claims that NG is about 1 therm or 100,000 BTU per 100 cubic feet.
Elmo

Elmo
dcdraftworks@sbcglobal.net
CAD Drafting of Mechanical Systems
Marin County California

[this message was edited by dcdraftworks on Sun 24 Feb 08:21]

static

74.140.102.1

"Re(1):Thanks Static" , posted Thu 21 Feb 01:35

The higher the manifold pressure, the more Btus will be consumed. But I don't understand the EPA's take on that. Combustion is combustion, and having the air / fuel mix out of whack will at some point result in elevated CO levels regardless of the furnace's rated efficiency.
If you have a link, or could point me to where I could locate that info from the EPA, I would greatly appreciate it.

Is it overfired? Is it underfired? It's all shades of gray IMO. The first guy sets the manifold pressure at 3��" and says it's good to go. The second guy finds 12% oxygen in the flue and says it's underfired. The third guy clocks the meter and finds it's consuming 2000 more Btu's than it's input rating, he thinks it's overfired. Now what?

dcdraftworks

69.105.1.152

"I think I said that wrong" , posted Thu 21 Feb 02:11:

Believe I meant to say, more gas is being consumed, maybe some NG to air mix in the combustion is under the LEL 5% and over the UEL 15%? I'm looking for that EPA report now, but I guess it's Friday in Indiana 1:10 am.

Edit: Here is one I found http://www.cpsc.gov/library/foia/foia00/os/furnace1.pdf

Edit2: This verifies a post you made to another member "I find that most HEs pull more combustion air than they need" http://www.greenwoodfurnace.com/PDF_Documents/TechBulletin3_Chimney_Installation.pdf
(see note page 6)

In Summary: Does over fire mean any mix not between LEL & UEL?

Static, this is a stupid question, what does IMO mean?

Elmo

[this message was edited by dcdraftworks on Fri 22 Feb 15:21]

static

74.132.54.85

"Re(1):I think I said that wrong" , posted Thu 21 Feb 21:40

Thanks for the links.

"In Summary: Does over fire mean any mix not between LEL & UEL?"

Probably not so much, I don't think you could even set a typical furnace up to be outside of those parameters.

I would just say that overfiring is when the fuel / air mix is too rich / not enough air, in regards to what's considered "normal" or "typical". In most normal cases you'd have to exceed the rated input Btu's to do this. As 341 said, clock the meter if there's any question on firing rate.

IMO = in my opinion.

dcdraftworks

69.105.1.152

"Mr Blackhawk" , posted Thu 21 Feb 23:49:

Static,
There is another article, regarding the HE flue disconnect emissions you requested. I am looking for it, will send when I locate.

Thanks

Edit : This is the best link to that material (lumped *.pdf) http://www.cpsc.gov/LIBRARY/FOIA/Foia00/os/os.html

Elmo
dcdraftworks@sbcglobal.net
CAD Drafting of Mechanical Systems
Marin County California

[this message was edited by dcdraftworks on Sat 23 Feb 00:45]

northernfitter

67.68.16.192

"Re(1):Mr Blackhawk" , posted Fri 22 Feb 08:46

Setting the manifold pressure to the set level indicated on the rating plate is the best way to go about it. Adjusting manifold pressure is not brain surgery put it where its rated to be period. Take a reading of your combustion products to verify that there are no odd things going on and clock your meter. The chances of it being overfired noticeably when its set at the proper manifold pressure are extremely slim. Keep in mind when reading a meter that the lowest dial will give you a slightly different reading every time. Ventors do not tend to speed up although techs forgetting to put back in restrictor discs have caused problems for sure. Going outside the engineers specs on an appliance because of a reading you got somewhere else is a quick way to a lawsuit. Basically its all due diligance, if your manifold pressure is good and your combustion analysis is good and you have clocked the meter(2000btu difference when timing a dial on a meter is not unusual its not an exact science). Trying to set your input by clocking the meter is nonsense.

dcdraftworks

69.105.1.152

"Thanks Northern" , posted Fri 22 Feb 12:11:

Are you required to do a combustion by-product analysis for new install in Canada? What type manometer; analyzer do you use?

Elmo

[this message was edited by dcdraftworks on Fri 22 Feb 12:14]

northernfitter

67.68.16.192

"Re(1):Thanks Northern" , posted Fri 22 Feb 16:47

I use a TPI I forget the model number. It's not so much the brand as what they do for you. This one does: temperature rise with 2 temp probes, nothing special but saves a little time/the combustion probe will give temperature along with the meters readings as well speeds things up on oil or conversion burner/reads c02%/O2%/COppm/pressure in wc"/comes with printer so you can give printout of readings at end of call(really nice to have when a heat exchanger is red tagged for cracks to prove your findings). On a new install its not required but I do it anyways, as an old tech told me one time while working on a roof top no matter how much work you have to do sometimes taking that extra few minutes and doing those extra checks can be the difference between a call back and a crawl back. In a new gas install ie; new home or conversion we are not supposed to unlock the gas meter without having the appliance inspected....catch 22 how the fk am I supposed to be sure its set up right for your inspection....either way the codes up here are pretty strict and the fines can get nasty if the tssa gets involved but it helps weed out the hacks.