Should Furnace Chimney be insulated?
#1
08-20-04, 08:00 AM
Should Furnace Chimney be insulated?
The whole house is heated with hot water and a boiler. The boiler/furnace has a chimney that goes up, through the attic and through the roof.
We live where it gets -10 below at times and have very deep snow.
I am trying to prevent ice daming and and water build-up on my roof and icecycles. The attic floor is insulated and there is a ridge-vent and the eaves are vented. This chimney pipe goes up threw the middle of the attic. Should I be wise to insulate it so that heat is not escaping into the attic causing the roof to melt the snow, under the snow?
We live where it gets -10 below at times and have very deep snow.
I am trying to prevent ice daming and and water build-up on my roof and icecycles. The attic floor is insulated and there is a ridge-vent and the eaves are vented. This chimney pipe goes up threw the middle of the attic. Should I be wise to insulate it so that heat is not escaping into the attic causing the roof to melt the snow, under the snow?
#2
08-22-04, 07:07 AM
The easiest way to describe free and adequate attic ventilation is to dip a straw into a fluid and putting your finger to cover the end of the straw and taking the straw out of the fluid. The fluid remains inside the straw until you remove your finger from the end of the straw.
Adequate free venting is illustrated by removing your finger from the end of the straw and Inadequate free venting is illustrated by leaving your finger on the end of the straw. You can compare this to your situation where there no snow on your roof covering the ridge vents (adequate free venting) and snow on the roof covering the ridge vents (inadequate free venting).
As far as insulating your chimney flue, it is not advisable to do so. The reasons for this are sizing and flue stack temperature to safely dispose of the gases produced from combustion. The hotter a gas is, the faster it will rise. As the gases produced from combustion rises from either your boiler and/or furnace, it draws in a certain amount of fresh air into the combustion chamber. The size of the flue and the temperature of the flue stack gases are designed to provide optimum efficiency. Of course there are other considerations when determining flue stack size, but for this explanation they need not included.
For example, the average efficiency of a furnace is 80%. This explicitly implies that 20% of the heat produced by the furnace goes up the chimney. This heat or energy is used to safely remove the combustion gases from the structure. The size of the flue and the flue stack temperature determine this. You decide to insulate the chimney flue in the attic. It will affect the flue stack temperature and more than likely increase the temperature because as the gases rise it loses temperature. Which affects how fast the gases rise.
If the flue gases rise faster as a result of insulating the chimney flue in the attic, then more air will be drawn into the combustion chamber and more heat (energy) will go up your chimney. Though you remove the flue gases more quickly, the impact of this can be significant. Your heating system is designed to heat your home on the coldest day of the year for your area based on 80% furnace efficiency. The increased flue stack temperature created by insulating the chimney flue in the attic extracts more heat/energy than it was originally designed for resulting in a drop of efficiency to 60%. You might not be able to heat your home on very cold days.
Another possible result of this is not having enough hot water or constantly running out of hot water when taking showers. Though admittedly there are other possible causes for inadequate domestic hot water supply.
Adequate free venting is illustrated by removing your finger from the end of the straw and Inadequate free venting is illustrated by leaving your finger on the end of the straw. You can compare this to your situation where there no snow on your roof covering the ridge vents (adequate free venting) and snow on the roof covering the ridge vents (inadequate free venting).
As far as insulating your chimney flue, it is not advisable to do so. The reasons for this are sizing and flue stack temperature to safely dispose of the gases produced from combustion. The hotter a gas is, the faster it will rise. As the gases produced from combustion rises from either your boiler and/or furnace, it draws in a certain amount of fresh air into the combustion chamber. The size of the flue and the temperature of the flue stack gases are designed to provide optimum efficiency. Of course there are other considerations when determining flue stack size, but for this explanation they need not included.
For example, the average efficiency of a furnace is 80%. This explicitly implies that 20% of the heat produced by the furnace goes up the chimney. This heat or energy is used to safely remove the combustion gases from the structure. The size of the flue and the flue stack temperature determine this. You decide to insulate the chimney flue in the attic. It will affect the flue stack temperature and more than likely increase the temperature because as the gases rise it loses temperature. Which affects how fast the gases rise.
If the flue gases rise faster as a result of insulating the chimney flue in the attic, then more air will be drawn into the combustion chamber and more heat (energy) will go up your chimney. Though you remove the flue gases more quickly, the impact of this can be significant. Your heating system is designed to heat your home on the coldest day of the year for your area based on 80% furnace efficiency. The increased flue stack temperature created by insulating the chimney flue in the attic extracts more heat/energy than it was originally designed for resulting in a drop of efficiency to 60%. You might not be able to heat your home on very cold days.
Another possible result of this is not having enough hot water or constantly running out of hot water when taking showers. Though admittedly there are other possible causes for inadequate domestic hot water supply.
