Suggestions for Testing and Interpreting Test Results

One tremendous advantage to the continuous digital read out instruments is that the readings can provide information on the entire cycle of burner operation, from light off to shut down.

Light Off

Once the instrument has completed it's warm up cycle, place the probe in the proper sample location before firing up the burner.

Give the unit a call for heat and pay particular attention to the carbon monoxide reading as the burner lights off.  Typically, the CO reading will bump up a little on light off.  

Should the reading exceed 100 ppm's CO, it is an indication the light off is 'a little' rough.

A burner producing over 400 ppm's CO on light off should be considered a rough light off and attempts made to determine the nature of the problem. 

We recommend printing off a copy of the combustion test results at the peak CO reading on light off on new installs and/or every time the unit is serviced.  This begins developing a history of how the burner should light off during the next service call.  This information will verify that the unit is operating as in the past or should CO readings be significantly higher, may be indicative of a component failing, etc.

This is an example of documenting the light off carbon monoxide reading.  This particular unit is an oil fired hot water tank.  CO readings have been taken every year since the date of installation approximately 10 years ago and have always been between 35 and 45 ppm's CO on light off.

Some time back, this hot water tank was tested and 350 ppm readings were recorded on start up and shut down.  Troubleshooting determined that the cut off valve in the pump was beginning to fail.

Steady State

Entirely too much emphasis is placed on the Steady State Efficiency (SSE) readings calculated by combustion test instruments.

Keep in mind that SSE is only going to accurately reflect fuel consumption when several conditions exist:

If any of these are outside the manufacturer's parameters, the equipment is not operating as engineered and manufactured.

In the example below, note that the Efficiency reading is 87.7%, very respectable (actually impossible) for this standard oil fired hot water tank.  Take a look at the numbers, do you see anything which is out of line for this type appliance?

If you noted the stack temperature of 302 degrees is low, you are correct.  The stack temperature should be more in the 450 to 475 degree range.  As such, the 87.7% SSE reading is a 'false' efficiency and meaningless information.

If this hot water tank were to be operating regularly with a 302 degree stack temperature, the vent system would probably need replacing every year and would considerably shorten the life of the tank.  Again, it is not operating as it was designed and engineered by the manufacturer.

In this particular case, the reason this hot water tank is operating at 87.7% Efficiency is a result of all the hot water being dumped out and refilled with cold water.  It took a while for the stack temperature to stabilize.

The bottom line is that as long as the combustion, draft and smoke (on oil fired equipment) test results are within the manufacturer's specifications, that's all a technician needs to know.  There are times when SSE readings after a tune up will be lower than before the work was done, yet, operating expenses will be reduced.

There are also situations where the increase in SSE will underestimate the reduction in fuel consumption.

SSE readings can be used as a marketing tool in a case where you increase operating efficiencies - show that information to the homeowner or facilities manager to prove the 'increased value' in your services.

Once up to 'Steady State' conditions all gas or oil fired appliances should display readings specified by the manufacturer.  Should readings be outside the acceptable ranges, the system is not operating as designed and engineered by the manufacturer.

Below is a troubleshooting chart to help identify the reason readings are not per manufacturer's instructions (PMI).

Shut Down

Observing and documenting carbon monoxide production during shut down may also help to identify problems as well.

Typically, gas fired atmospheric and high efficiency forced air units, boilers and hot water tanks do not display bumps of CO on shut down unless the gas valve is not closing properly.

Gas and oil fired power burners will generally show a small increase of CO production on shut down.  Again, CO levels should be below 100 ppm's, hopefully below 50 ppm's to be considered a clean shut down.

Observing the relationship between light off, steady state and shut down CO levels may also help identify and diagnose and resolve burner problems.

For example, with gas fired atmospheric, high efficiency burners:

With oil fired (non-modulating) power burners:

These are just a few of the possible ways in which CO production can help evaluate safe, efficient and reliable burner operation.

Miscellaneous notes on testing specific appliances

Testing Gas Fired Atmospheric Forced Air Furnaces

These type heating systems typically have multiple heat exchangers or clamshells.  The probe has to be situated in the exhaust port of each section and you may miss a hard light off if not in a particular port.  It is a good idea to visually inspect each burner area looking for 'the worst case scenario' and test that clamshell first.

For example, lets say one particular burner has more soot, scale or other debris on it or is possibly more 'tarnished' than the other burners.  Start there.

If the situation requires the technician to be more thorough, the unit may have to be recycled to test each individual burner on light off.

Another thing to watch for when testing a forced air system, is the O₂ and CO readings when the blower comes on.  Should these readings change in response to the blower energizing, a couple of things could be going on.

1.  There could be a crack in the heat exchanger or if it is an oil fired unit, a clean out port could be loose or have a gasket missing. There is a popular misconception that the danger of a cracked heat exchanger is flue gases leaking out of the crack.  More often than not, there is a much greater chance of distribution air being blown into the heat exchanger.  This will result in a larger volume of cooler flue gases which may not exhaust the vent system.  There lies the danger in a crack.

Should the crack be large enough, distribution air being blown in and containing 20.9% O₂ may cause the O₂ reading in the display to increase proportionately to the size of the crack.  Should the pressure in the combustion chamber be enough to effect the amount of combustion air being drawn in, the CO reading will also likely rise.

2.  Another possibility is that there is enough leakage in the return side of the distribution system to compete with the vent system for draft.  This may also result in changes in combustion readings when the blower comes on, but is typically a more delayed reaction than the change when the heat exchanger is cracked.

Also, keep in mind that air shutters on atmospheric equipment does very little to control combustion air intake - another popular misconception.

The primary role air shutters play is in the control of the velocity at which the air/fuel mixture moves down the throat of the burner.

Testing Oil Fired Forced Air Furnaces

Much the same rules apply with oil fired, forced air furnaces.  Watch for changes in O₂ or CO readings when the blower comes on.  Should changes be noted, further troubleshooting is necessary.

Evaluating Combustion and Make Up Air Requirements

Certainly, code requirements regarding combustion and make up air need to be met.  However, it is important that these components work under actual operating conditions.

A simple way to determine if a combustion or make up air concern exists is to wait until the flue gas combustion test readings stabilize and then open a door or window to the outside.  Should the O₂ or CO readings change in response to opening a door or window, there is a problem.

By the same token, operation of a clothes dryer, kitchen exhaust, etc., should not effect combustion test results.

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