If you’re planning to install a high-efficiency furnace, one likely item on your to-do list will be to line your chimney. Chimney liners aren’t just a good idea – they’re required to help maintain the proper performance of your chimney.
Gas-fired appliances need to vent to the outside to avoid a build-up of carbon monoxide. In the past, gas furnaces and water heaters used the home’s chimney to provide adequate ventilation. Newer, high efficiency furnaces may vent out the side of the home’s foundation rather than up the chimney. If they use the chimney for ventilation, the chimney as built may be too big to work properly with a newer gas furnace.
If you plan to vent any appliances through the chimney, a chimney liner may be in order. There are three good reasons to line an existing chimney. First, unlined chimneys actually constitute a serious fire hazard. Studies have shown that heat moves through (not up) an unlined chimney rapidly. This means the chimney can transfer heat from the masonry to adjacent woodwork inside the home. In National Institute of Standards and Technology (NIST) tests, an unlined chimney caused adjacent woodwork to ignite in less than 3.5 hours! In fact, the standards folks at NIST called unlined chimneys “little less than criminal.” Those are some pretty harsh words, but they can give you a lot to think about. If your chimney is unlined (which would be common for an older home), you may want to invest in a chimney liner even if you don’t intend to replace the appliances that use your chimney.
The second reason to line a chimney is to protect it from your appliances. Combustion is a messy process. It can leave behind some caustic by-products that won’t do your chimney any favors. Over time, these caustic chemicals can eat away at the brick, as well as the mortar that holds your chimney together. Which brings us right back to Reason #1 to line your chimney. If the mortar inside your chimney deteriorates, the chimney will become even better at transferring heat to the surrounding structures. This naturally increases the risk of fire. A liner can both slow and reduce the transfer of heat to nearby structures, decreasing the risk of fire.
The third reason to line your chimney is to ensure that it drafts properly. A chimney is like a big straw that draws exhaust gas from your home. It also drafts air into your home, which your gas-fired appliances need. Big chimneys don’t draft well. A chimney that’s exceptionally large might draft either ineffectively or perhaps not at all. That could cause carbon monoxide to build up in your home. A chimney liner can help size your chimney properly for your appliances and help ensure that your home and appliances are vented properly.
Most chimney liners are made from one of three materials: clay, metal or resin. Clay tiles are the most common type of chimney liner. While they’re the least expensive way to line a chimney, they may not perform well in adverse conditions. (“Adverse conditions” = chimney fire.) They also might not work well with new, high-efficiency gas fired equipment.
Metal liners are usually made from stainless steel or aluminum. Aluminum liners don’t perform as well as stainless steel liners do. In fact, they’re not recommended for high-efficiency applications. Stainless steel performs very well, but it can be expensive. Finally, you can choose a custom-fit resin liner for your chimney. A resin liner is “built in place” and form fits to your chimney. It is lightweight, resist etching and reduce heat transfer. They can also help improve the structural integrity of your existing chimney. Resin liners are permanent and they work well with all fuel types.
An alternative to lining your chimney is to vent your furnace, water heater, boiler and other appliances directly through the foundation wall of your home. This strategy will enable you to abandon your chimney altogether. You can leave an abandoned chimney in place, provided that you cap the holes previously used by your equipment. You may also want to cap the chimney at the top to prevent water, animals and other undesirables from entering the chimney. Before you abandon your chimney, you may want to have it inspected by a professional. If your chimney is in dangerous condition, it may be worth your while to either stabilize it or deconstruct it altogether.
While we don’t do chimney lining, we can recommend chimney professionals as part of a heating or water heater replacement project. Give us a call at Boston Standard Plumbing and Heating at (617) 288-2911. We’ll be happy to set up a consultation!
Photo Credit: Ben Freeman, via Flickr.com