Discussion of Efficiency Ratings
Not only are the efficiency ratings of appliances beneficial for the informed homeowner, but the service industry uses them as a valuable tool. Many times the contractor is asked for his recommendation of the most energy efficient heating unit. He appreciates that there is more involved than mere numbers.
For instance, he is familiar with STEADY-STATE EFFICIENCY which utilizes the stack loss methodology. The measurement of NET STACK TEMPERATURE and PERCENT of CO₂ or O₂ in the flue gas is obtained whenever the appliance has been operating long enough for the “steady” or uniform temperatures to have been established throughout the system. These two factors are used with a standard chart to compute the steady-state efficiency. [EXAMPLE: 400ºF NET STACK TEMPERATURE with 13.0% CO₂ at No. 1 smoke = 84.3% steady-state efficiency.]
The A.F.U.E. will be slightly lower than 84.3% because it takes into consideration the average fuel usage, including operating and standby or off cycle losses outlined in the standardized D.O.E. test procedures. Let us assume it is 83.0% for this discussion.
These ratings might be compared to the mileage ratings for automobiles. The “steady” highway driving is always higher than the city or “average” consumption.
In both instances the A.F.U.E. and MPG ratings were obtained under very carefully controlled Laboratory conditions. There are important reasons why the actual fuel consumption experienced in the field will be slightly higher.
A.F.U.E. Ratings vs. Typical Field Conditions
There are many variables that occur throughout the heating season that can impact upon the overall system efficiency. Some are: changes in draft, fuel, combustion, air restriction (due to lint, dust, pet hair, etc.), temperature reduction in oil and air, inadequate fresh air supply and other subtle environmental contributors.
If the burner is set up at the 13.0% CO₂ and No. 1 smoke level, which gives the highest steady-state efficiency in the above example, operating problems could result. The addition of several or all of the variables itemized above would cause the No. 1 smoke to elevate to a higher level and the heat exchanger could become sooted in a short period of time. Of course, soot is an insulator, so efficiency is being reduced as the deposits increase. How can this potential problem be addressed?
Experienced contractors view the A.F.U.E. ratings as a valuable tool for comparison purposes, but they do not attempt to set the burner to operate at the CO2 and smoke levels that were used in the D.O.E. controlled Laboratory procedures. Real world conditions require that consideration be given to the variable environmental effects. Therefore, effective safeguards are factored into the final burner adjustments.
Adjusting the Burner for Consistent Efficiency
There are many items to consider when installing or servicing oil burners. Usually, the appliance manufacturers provide excellent instructions that are included in their installation manuals or on other printed labels. If these instructions are not available, then the following suggestions summarize what successful servicemen have found to be effective.
- ALWAYS USE COMBUSTION TEST INSTRUMENTS WHEN MAKING BURNER ADJUSTMENTS. It is virtually impossible to make reliable adjustments using the “eyeball method.”
- Operate the burner for 5-10 minutes so that the system is near steady-state temperature levels.
- Set the over-fire draft to the level specified by the manufacturer. This could be -.02″ W.C. with conventional, natural draft units. Some modern design boilers may have positive pressure operation and require -.04 to -.06″ W.C. stack draft. Contact the manufacturer for specifications if the requirements are not known.
- Next, reduce or increase the burner air control until a TRACE OF SMOKE is obtained. This is your reference point only. You will not leave the settings here.
- Now, measure the CO₂ or O₂ in the flue gas at the TRACE of smoke level. Let’s assume you have 13.0% CO₂ (3.3% O₂). Open the air control and add reserve air until the CO₂ is lowered by at least 1.5% to 11.5% CO₂ or 5.3% O₂. (See Figure 1.)
- Perform the smoke test, it should now be at zero. Lock the air settings securely.
- Measure the stack temperature. Subtract the room ambient to obtain the net reading and use the Efficiency Charts to determine the steady-state efficiency. (See Figure 2.)
Previously, we presented an A.F.U.E. example of 83.0%. This was tested under Laboratory conditions at 13.0% CO₂, 400°F net stack temperature and a No. 1 smoke level. How does the addition of the reserve air affect this A.F.U.E.? First of all, the stack temperature could possibly elevate to a net of 425°F. The CO₂ was reduced to 11.5%. This could result in approximately 1% reduction in A.F.U.E. Even with a heating bill for one year as high as $1,000, the 1% would result in a mere $10 added expense.
This is a very small premium to pay for the added margin of air that will help keep the appliance heat exchanger clean throughout the full heating season. How much would have been lost in real dollars if the appliance had gradually became sooted, losing efficiency until it required premature servicing?
Obviously, the efficiency ratings have an important role to play. However, there must be a balance between maximizing efficiency with little or no margin for variables, and practical field set-ups that have a reasonable amount of reserve air built in. These methods are field-proven and can help reduce nuisance, efficiency robbing soot-ups.