Oversized Heating Systems

Many heating systems have a much larger heating capacity than is necessary. "Oversized" heating systems cost more to buy initially, provide less comfort and efficiency, and have higher maintenance and operating costs.

Some heating contractors may suggest "derating" the system by installing a smaller gas burner orifice (the small opening where gas flows to the combustion chamber) in order to save energy. The manufacturer generally does not recommend this. Excessive derating can lead to problems with flue gas, water condensation and corrosion in the heat exchanger or vent system. Unfortunately, the best solution is to replace the oversized unit with one that is properly sized for your home.

If you have decided to replace your present system with a new high efficiency furnace or boiler, here are some things to consider:

1. Look at the AFUE rating.(Annual Fuel Utilization Efficiency) These ratings help you to compare the efficiency of different models in a way similar to listing gas mileage ratings on cars. They also give an estimate of the annual cost to operate. Look for the Energy Star label for help in selecting more efficient models.



2. Make sure your new system is sized properly. The most common mistake a contractor makes is over-sizing. The contractor should not replace your furnace with another that is the same size without doing "Manual J" calculations. "Manual J" is published by the Air Conditioning Contractors of America (ACCA). Make sure your new furnace/boiler is sized according to Manual J or another procedure that is approved by ACCA. Just because the original furnace is a particular size does not mean the new one should be the same size. Before construction procedures for homes became more air tight, it was common to install a furnace with much larger capacity. Correct sizing involves considering how well the home is insulated, the size and condition of its windows, how many occupants there are, the local climate, and "air sealing" improvements the owners may have made.



3. Determine if the existing vent is the correct size for the new energy-efficient unit. Many older furnaces or boilers are oversized, and as a result, the original vent and flue may be too large to accommodate the new furnace or boiler. Masonry chimneys may also require a chimney liner for safety.

High efficiency furnaces and boilers are generally more expensive, but the annual reduction in fuel usage should ultimately provide savings. How quickly you reach your "payback" figure depends on the reduction in your annual fuel costs and maintenance, and how much you spent on the unit and its installation. Be careful when you select your contractor. The performance of your new system will depend a great deal on his experience, skill, and knowledge.

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