American Water Heater (A)BCG38T500-8P ユーザーズマニュアル

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Servicing should only be performed by a Qualifi ed Service Agent

A lack of combustion and ventilation air can create a negative ambient air pressure in the 
installed space. The vent system on one or more gas fired appliances can experience down 
drafts due to the outdoor air pressure being greater than the ambient air pressure in the 
installed space. Where multiple gas fired appliances are installed, one or more gas fired 
appliances can “pull air” through the vent system(s) of other appliances installed nearby.

Downdraft Caused by Kitchen Vent Hood

 

Figure 2

One common example is in a restaurant installation where exhaust vent equipment was not 
considered in sizing make-up air requirements. This condition may result in air being back 
drafted by the restaurant exhaust equipment through the water heater, causing the draft 
proving switch to open and/or erratic heater shutdown.  See Figure 2.

Down drafts can cause poor mixing of fuel gas and combustion air in the burners.

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Down drafts can cause flue gases to spill into the installed space.

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Down drafts can cause common service problems such as ignition failure. Down drafts  

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  can sweep pilot gas away from the spark igniter during ignition and prevent ignition of 
 the 

pilot.

Down drafts may cause extremely dangerous conditions such as flame rollout where the  

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  flames from the main burner “roll out” of the combustion chamber.