What is a Draft Hood?
A draft hood is a device that is usually installed on water heaters, gas boilers, and non-current gas or oil furnaces. A few applications can even be found externally attached, meaning that they were not installed as regular equipment, but the majority of them were installed as a piece of the appliance. The draft hood is a device that sits in between the appliance and the chimney, allowing a break to form in the system. This device allows better draft control of the system, as well as helping gather toxic fumes from within the appliance and expelling them out of the building through the chimney pipe or vent.
Toxic gases and fumes are extremely hazardous to any of the occupants within a building, and if they are not expelled properly, they can cause various serious medical conditions, including death. The draft hood acts as a collection reservoir that funnels the toxins out of the appliance and into the chimney, which effectively prevents any contamination from occurring inside of the building. This is a safety feature that is imperative when it comes to healthy living. Even though these devices are more commonly found on older models of appliances, some newer versions have a replica of this device for this particular safety feature.
The second reason that draft hoods are installed is to help control drafts. Appliances such as these need to have a combination of combustion air and diluted air in order to operate correctly and safely. The hood allows air to be sucked into the system from the external areas around it, while stopping severe back drafts from occurring, such as from a brisk wind or heavy rain storm. An appliance, and the system attached to it, must have the proper amount of air going in and coming out in order to work effectively. Without the draft hood on some of these models, this requirement would not be able to be met.
The two most common types of draft hoods that are used are inverted cone shaped models and boxed styles. The cone shaped draft hood is more commonly found in smaller houses and buildings, while the box type hood is installed in larger buildings and homes. The box shaped draft hood can handle more air flow, as well as more toxic fumes, so they are generally installed when the appliance is larger than an average unit. Both types work to effectively solve any draft problems while helping the system expel dangerous gases and fumes.
Why make the air cooler as it enters the chimney?
To slow down the rate of exhaust movement. Otherwise the resulting high suction would lift the flame off the burner and blow it out.
Prevent chimney degradation and/or fire. The hot exhaust gases can reach 500 degrees F. The mixing room air reduces the temperature to a safer level, but not low enough for condensation to occur.
I've read many articles on gas conversions of boilers and hwh's, and the complaints about condensation in chimneys, all types, stainless steel and clay lined, inside and outside chimneys, etc. I recently read about dilution air through draft hoods.
As a novice, I ask the question: why make the air cooler as it enters the chimney? Is it just a catch 22 that in order to have the down draft protection this design is necessary? I understand how it controls draft through the appliance, etc. Please, if you could enlighten me, it would be helpful.
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