The draft hood of a chimney vented appliance beit be a boiler furnace or water heater will always play the same role. Its role is to supply excess secondary air for clean safe combustion. The distance between the bell to the appliance and the diameter of the vent pipe along with many hours of testing to comply with ASME Section 4 manufacturing codes and standards.
Let's start with a draft hood that was supplied by the manufacturer and was not altered in any way.
The draft hood is designed to fit a specific product line or partial line. It must control the amount of secondary air flow across the burners to control CO. The control of secondary air will also help control the temperature at which we see flue gas condensation in the flue passes. The primary air is controlled by the orifice gas injection and gas pressure. The second thing it does is control the amount of air flow through the boiler which can change the heat removed by the boiler. So, to say it another way it affects safety and efficiency.
Font color="000000">When the draft hood is altered it can create an unsafe condition. If the draft hood is shortened it will raise the CO and can spill into the home causing sickness or even death. The distance from the appliance to the draft diverter or bell on the draft hood is set by design and testing by the manufacturer. This distance will vary between product lines from the manufacturer and even different models in the same product line to meet the code specs. The Code specs will control the maximum CO that can be emitted and boiler drafting properly.
The below drawing affect the airflow as the draft hood is reduce in height. You can see less room for air into the draft hood and less secondary air in the bottom of the appliance.
On the other hand, if the draft hood was extended up the secondary airflow will be increased which will pull more secondary air into the appliance which will usually keep the CO within limits, but more heat will be removed from the boiler which reduces efficiency by raising vent temperatures.
If you are replacing the draft hood, it must meet the specs from the manufacturer for that appliance. If your draft hood has been altered, it should be replaced for safety reasons. If you are not going to replace an altered draft hood, at least put a CO detector in the mechanical room and pray there are no health issues because you will be responsible. How can you tell if the draft hood was raised or lowered?
If the draft hood was raised which is very uncommon, there will normally be a pipe joint between the diverter and the appliance. If the draft hood was shortened the easiest way to tell when the pipe is cool lift it up and look at the end on the appliance. It should be a straight edge. If it were cut the edge will waver up and down due to being cut by hand and not sheared by machine as they are when manufactured.
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