If you replace your old furnace, your chimney may require modifications to assure that it is sized to fit your new heating system. If your chimney is too large for your new system, it may not remove combustion gases properly from your home, possibly leading to backdrafting and allowing combustion gases such as carbon monoxide to enter your home. Your old chimney may also be prone to moisture accumulation and deterioration
Many existing furnaces are oversized to account for the poor thermal resistance and large air leakage of an old home. If your home has been weatherized to improve efficiency, it probably does not need as large a furnace as was first installed. Smaller furnaces cost less to purchase, they use less energy, and they deliver better comfort. So don’t install another over-sized furnace just to match the chimney. The best choice is to install a correctly sized heating system, and then modify the chimney to match the new heating unit.
Mid-efficiency furnaces (80% plus) produce a smaller volume of combustion gases than traditional furnaces, and those gases are slightly cooler and therefore more corrosive. If this type of system is vented into an older, un-lined chimney, it could result in chimney deterioration or back-drafting. The best approach is to reline the existing chimney with a smaller, corrosion-resistant masonry or metal liner.
A high-efficiency furnace (90% plus) doesn’t use the original vertical chimney at all, but instead employs dedicated plastic pipe to carry combustion gases out of the home. Fresh air is drawn from outdoors through another plastic pipe to feed the combustion process. This venting system provides superior health and safety benefits when compared to either traditional or mid-efficiency equipment. However you may still need a chimney liner to accommodate the water heater, which now vents into a chimney that is far too large.
Re-lining the chimney can be expensive, but neglecting your chimney when you install a new furnace can lead to a crumbling or rusting chimney in a few years. If you plan to install a new furnace, be sure to ask your serviceman about proper chimney sizing to protect your home and your health.
When installing a new 80+ furnace, it is often necessary to re-line the chimney. You may also need a liner for a naturally drafting water heater, after installing a 90+ furnace. The existing chimney is often too big for the existing water heater after the new furnace is installed.
See The Homeowners Handbook to Energy Efficiency or Residential Energy: Cost Savings and Comfort for Existing BuildingsChapter 6 for more information. For professional information about heating systems and energy efficiency see Saturn Hydronic Systems Field Guide.