Improving Boiler Efficiency

Your boiler probably accounts for the majority, if not all, of your heating fuel bill. If you tend to budget a fixed dollar amount for this fuel bill, you may be interested in knowing that most boiler systems are, at best, 85% efficient with an average efficiency of only 65% to 75%. This means from 25 to 35% of your heating bill is going up the chimney with no appreciable benefit to you. There are many ways to increase a boiler system's efficiency. These range from ongoing low-cost or no-cost operation and maintenance improvements, to one-time capital improvements. Some proven one-time investments are discussed below.

Reduce Excess Air

Efficiency Improvement - Up to 5 percentage points.

Properly operated boilers require a certain amount (10% to 20%) of excess air to ensure complete combustion. However, operating with too much excess air reduces boiler efficiency by heating up and throwing away this "cushion" of excess air. On the other hand, operating the boiler with no excess air can lead to sooting on the fireside, air pollution, poor efficiency and even potentially explosive flue gases, so that boiler operators tend to have a cushion of excess air rather than a deficiency. Monitoring the flue gas temperature, oxygen content, and carbon dioxide content allows the operator to minimize this cushion and fine tune the boiler for optimal efficiency under varying ambient and load conditions. Portable or fixed combustion efficiency tests are available from many sources and may be used to manually adjust combustion air quantities. This is typically the only choice for natural draft type boilers and should be performed on a weekly basis. For forced draft boilers, an automatic combustion control system can be added to automatically monitor flue gas combustion and adjust combustion air quantities on an instantaneous basis.

Condensate Return

Condensate from a steam boiler system is hot mineral free water. Strong consideration should be given to returning all usable condensate in order to minimize the use of cold makeup water which must be heated prior to entering the boiler. An added benefit will be the decreased use of chemicals and improved boiler surfaces. This will reduce corrosion (which can lead to early failure) and/or the buildup of scale (which can hurt boiler efficiency).

Blowdown Heat Recovery

Efficiency Improvement - Up to 2 percentage points.

Blowdown of boilers to reduce the sludge and solid content allows heat to go down the drain. The amount of blowdown should be minimized by following a good water treatment program. Installing a heat exchanger in the blowdown line allows this waste heat to be used in preheating makeup and feed water. Heat recovery is most suitable for continuous blowdown operations which in turn provides the best water treatment program.

Exhaust Heat Recovery

A device like the one shown below can be attached to the flue to recover a portion of the exhausted heat. This heat can be used to preheat boiler make-up water. Take care not to extract so much heat that the flue gases condense (causing corrosion).

Preheat Combustion Air

Efficiency Improvement - Up to 1 percentage point.

If you have a large temperature difference (20° to 40°) between your boiler intake air location and the ceiling of your boiler room and this hot air is a result of boiler and stack losses, you can increase your boiler efficiency by either extending the intake upwards or forcing the hot air down. Both options may require a fan and ductwork. If the hot air is due to boiler wall losses, you may want to consider insulating the boiler.

Replace Burners

Efficiency Improvement - Up to 5 percentage points.

If you presently have an old, inefficient oil burner (such as rotary up or atmospheric), or an oversized burner (causing excessive short cycling), it may be cost-effective to replace the burner with a modern, efficient and properly sized burner. You may want to consider variable firing rates and/or dual fuel capabilities allowing you to choose the least expensive heating fuel at any given time. Natural gas burners can replace oil burners without requiring boiler replacement.

Turbulators and Soot Blowers

Efficiency Improvement - Up to 2 percentage points.

Turbulators are metal baffles inserted into fire tube boilers to increase the heat transfer by increasing the surface area and slowing down the combustion gases. Soot blowers use steam or compressed air to help keep the fire-side surface of solid fuel or oil fired boilers clean. Check with your boiler manufacturer to determine the suitability of either of these measures.