Three kinds of gas furnaces for your remodeling project
Posted by Doug Horgan on Mon, Nov 22, 2010 @ 03:54 PM
Whether you are upgrading your current gas furnace, or adding a new one as part of a remodeling project or addition, there are some new options worth considering.
Furnace technology continues to evolve. Today's better options are safer, quieter, and save money on energy costs. Plus, they can even make your house more comfortable compared to the old standard types.
Here are three categories of gas furnace for your renovation:
Old fashioned "atmospherically vented" furnace
If your house dates back before the year 2000, it was almost certainly built with one of these old-style furnaces.
The easiest way to identify
an atmospherically vented furnace is the metal flue pipe. These furnaces need high-temperature metal flues, because they leave a lot of heat energy in the exhaust.
The highest efficiency you can get in this style is around 80%, meaning the other 20% goes right up the chimney! That's one fifth or more of your fuel dollar. Older atmospheric-vented furnaces were even less efficient, as low as 60%; they cost even more to run.
In these furnaces, it's the buoyancy of the hot gasses that removes the exhaust from your home--and that's all. The hot flue gasses rise up the flue, but not very powerfully. Any chimney obstruction can reverse this gentle buoyancy, and bring flue gasses into your living space. Even a competing "backdraft" from exhaust fans, fireplaces, or wind effects can suck flue gasses into your house.
In the best case, the flue gasses have a lot of moisture and carbon dioxide, with traces of nitrogen and sulfur. None of these belong in our living spaces. In the worst case, backdrafting furnaces can even produce a lot of carbon monoxide, which can cause serious health problems or even death.
To me, the possibility of combustion gasses entering the house is even more a detractor than the poor energy performance. Better to choose one of the upgrade options below.
90% Efficient "Condensing" Furnace
To leave both inefficiency 
and potential backdrafting behind, opt for a condensing furnace with an efficiency level of 90%.
These are easily identified by the two white plastic vent pipes, which replace the metal flue. A fan brings outdoor air to the furnace in one of the pipes, and sends flue gasses out through the other. Because the entire air pathway is sealed off from the house, flue gasses can't get into indoor air.
These efficient furnaces remove so much heat from the burning gas, that the exhaust is cool enough to be piped out in plastic. The water vapor in the flue gasses condenses in the furnace, leaving even more heat behind--that's where this category of furnace gets its name.
Though the furnace itself usually costs a bit more up front, it's quickly paid back with energy savings. In some situations, the installed cost is the same or less, because the plastic flue pipes are much easier to install and can run out the side of the house. That saves boxing out a chase through upper floors, a necessity with metal flues.
I had one of these installed at my house a few years ago, when our old 80% furnace died. It's nice, but if I had the chance again, I'd upgrade to the next step up:
95%+, Electronic Motor Furnaces
The most efficient furnaces take the basic sealed combustion path from a 90% furnace, and upgrade it with staged burners and a highly efficient motor.
A staged burner has a low setting that is even more efficient than "all the way on". Most of the winter, houses don't need 100% of the heating capability, so these furnaces operate in super efficient low stage most of the time.
Another great feature is an "electronically commutated motor" (ECM) which is quieter, works well at low speeds, and uses much less electricity. People also like the quiet operation at startup and when the fan runs on low speed.
An ECM also helps air conditioners work better in our Mid-Atlantic climate: a special, lower air flow mode helps dehumidify better in part-load conditions. When paired with a high-efficiency air conditioning compressor, a 95% efficient furnace can deliver more comfort for less money compared to any other gas furnace.
These highest-efficiency, best-comfort units have been out for a while now and have been reliable and comfortable. Many of our clients have decided to install one in their renovation or addition. Everyone who has chosen one has been very happy with it.
