Residential Combustion Analyzers

To measure combustion efficiency you need a tool that can measure either the oxygen or CO2 content of the combustion gasses. For many years, an hour glass shaped bubbler containing a fluid that absorbs CO2 or O2 was used. They are difficult to find these days. They have been replaced by electronic instruments. These use an electro-chemical reaction in an oxygen sensor to measure the O2 content in the flue gases. They use thermistors to read the flue gas temperature and ambient temperature, so they have all the information that need to calculate and display combustion efficiency. They use this information to display CO2 %, O2 %, combustion efficiency, % excess air, flue gas temperature, and net stack temperature. Most offer other measurements as well. Some of the more common additional features include:

• CO ppm – to read the CO in the flue gas

• CO ppm air free – to calculate the CO ppm after removing the excess air

• Draft pressure – to insure you actually have a draft

• Differential draft pressure – to insure the draft pressure is lower than the room pressure

• NOX – to comply with NOX regulations in areas that restrict furnace NOX emissions

• Printer – to print out reports from results

• Computer Connectivity – to import data from analyzer to programs on your computer

Prices vary a good bit. From just over $500 for no-frills analyzers intended for residential work, to several thousand for commercial instruments. Certificates of NIST traceability often cost more.  Most combustion analyzers use electro-chemical sensors. These have a limited life because the chemicals in them are used up as they work. Typical replacement time is every one to two years. Some are user replaceable, and others require sending in the tool for the sensors to be replaced. This will typically cost $200 - $300. The table below compares several of the lower priced models which are aimed at the residential market.

	Bacharach Intech	Testo 310	UEI C75	E Instruments BTU 900
O2	yes	yes	yes	yes
CO2	yes	yes	yes	yes
Efficiency	yes	yes	yes	yes
Excess Air	yes	yes	yes	yes
CO	yes	yes	yes	yes
Air Free CO	yes	yes	no	yes
Draft pressure	no	yes	no	yes
Differential Pressure	no	no	no	yes
NOX	no	no	no	Can upgrade
Printer	Available Extra Cost	Available Extra Cost	Available Extra Cost	Available Extra Cost
Computer Connectivity	no	no	No	USB & Bluetooth
Field replaceable sensor	yes	no	no	yes
Fuels	6	5	5	10
Warranty	2 years	2 years	3 years	2 year
Approximate Street Price	$520	$600	$500	$1000

Combustion Efficiency

Fall tune up season is here. A seasonal check on a gas or oil furnace should include a check of the combustion process. To do a good job you really need to measure the combustion efficiency. To measure combustion efficiency you need to take two temperature readings and one flue gas reading.  The temperatures are the flue gas temperature and the ambient temperature around the unit. You subtract the ambient temperature from the flue gas temperature to get what is called the net stack temperature. The flue gas reading can be either CO2 or O2.Generally oxygen is preferred. For a gas furnace, perfect stoichiometric combustion produces 12% CO2 in the flue gas. The CO2 percentage drops off if the process is either too rich (too much fuel) or too lean (too much air). At the perfect stoichiometric point the O2 will still be 0% because all of it is being used in the combustion process. As excess air is introduced into the process, the O2 begins to rise due to the oxygen content in the air that was not used in the combustion process. Excess air is introduced to insure complete combustion. Note that complete combustion is not the same as perfect combustion. In perfect (stoichiometric) combustion, ALL the fuel and ALL the oxygen are used up, producing ONLY carbon dioxide and water. In complete combustion, all the fuel is used up, but not necessarily all the oxygen. Lack of combustion air produces incomplete combustion. Incomplete combustion leaves some unburned carbon and carbon monoxide. Excess air is introduced on purpose to prevent the production of soot and CO in the flue gas. The figure below shows the relationship of CO2, CO, O2 , and excess air. Next week we will discuss some of the ore practical aspects of measuring the combustion process.

Ride All the Rides!

Ride All the Rides!

Undoubtedly, many of you will spent  some time this past summer at an amusement park, water park, or other recreational establishment. Many parks now have a single price for admission that lets you ride all the rides as many times as you like. After paying the price of admission, most folks try to ride as many rides as possible to get their money’s worth. I can remember planning out my day at Disney World so that I would make the most of my time. I can see many of you smiling because you have done the same thing and planned a manic day at a pricey amusement park so you would get your money’s worth. I read recently that a year’s pass to Disneyland was now over $1000. I would have to be riding stuff every day if I paid that. So here is my question. Why are we so intent on getting our money’s worth at an amusement park, but beg to be cheated in education? When you pay your tuition for the semester, you are paying to ride all the rides. Every lecture, every lab, every online assignment, and every test you already paid for when you paid tuition. Chances are, your tuition was more than $1000 and it was not for an entire year. If you think tuition and fees are high, why would you not take full advantage of all that you paid for? Try reading the assignments more than once. It does not cost any more, and you get more out of the assignment. When you miss a lecture, lab, or assignment you are cheating yourself. Not only are you not taking advantage of the services you have paid for, but you are also limiting your earning potential later on. My brother Richard has a saying “work hard at school, or work harder all your life.” People with jobs which require less education and skill work harder and longer for far less money. HVACR is a very performance based industry. If you can’t perform, your earning potential suffers. Your diploma may get you in the door, but it won’t keep your job. HVACR is also a very technical field. To excel, you need to understand the systems and how they operate. Sure, without a lot of training you can get a job holding the other end of heavy things or running to get tools for other people. But without training or education, you won’t advance much past that point. And of course, the folks holding the other end of the furnace don’t get paid the big bucks. Attending school is not the only way to learn your trade. There are many ways to educate yourself, but the easiest and fastest is to go to school. Throw yourself into your studies. Attend all the lectures, read all the assignments, and do all the labs. Ride all the rides!