Final answer:
Negative pressure in a building can cause backdraft, which results in decreased venting efficiency of appliances, as exhaust gases are drawn back into the building.
Step-by-step explanation:
The effect of negative pressure in a building on the venting of an appliance is that it may cause backdraft. Negative pressure occurs when the air pressure inside the building is lower than outside, potentially causing air, including gases from appliances, to be drawn back into the building rather than venting out as intended. This condition can lead to dangerous situations, such as carbon monoxide entering the living spaces instead of being expelled outside. Venting efficiency, in this case, decreases as the intended path of the exhaust is reversed due to the pressure differential.