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Dear Energy Wiz,
Insulation is being added in a standard attic bringing it to R-38. A natural gas flue vent is routed through the attic to the roof. This flue exhausts gases from both the natural gas furnace and water heater. Should the natural gas flue be insulated as well? One source indicates it should be insulated, and the other source counsels against it. What does the Wiz recommend?
The Energy Wiz!
It's really not a bad idea to insulate the flue pipe. The reason is to try to get water vapor in the combustion gases all the way through the flue before the gases have a chance to condense.
For a water heater, if the water vapor is allowed to condense in the flue, it will most likely end up dripping back down into the hot water heater, and possibly shortening it's life due to corrosion. A better way to look at this might be to say that, insulating a water heater flue may help to increase the life of your water heater. Insulating the flue of a hot water heater will not save energy.
Not all insulation would be appropriate for a flue pipe. Some types of insulation have a very low ignition or melt temperature, and should not be used in an application such as this. There are ceramic types of insulation available for just this type of application. There are also water heaters available that condense the water out of the flue gas, just as a condensing furnace does. These condensing water heaters are expensive, but will save energy, anywhere from about one-fifth to one-third of the energy used in a conventional water heater, depending on the water heater you compare them to.
The installation of insulation on a flue should probably be addressed to a water heater contractor, and not an insulation contractor. Flue gas stacks for water heaters or furnaces should come with a certain amount of insulation.
The Energy Wiz
The staff at the Energy Office respond to many inquiries on a variety of topics from Nebraskans. From time to time, the Quarterly will share some questions and the answers with readers.
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