The draft hood of a chimney vented appliance, be it 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 and the appliance, as well as the diameter of the vent pipe, must comply with ASME Section IV manufacturing codes and standards after extensive testing.
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 regulate the temperature at which flue gas condensation occurs in the flue passes. The orifice gas injection and gas pressure control the primary air. The second thing it does is control the amount of air flow through the boiler, which can change the heat removed by the boiler. To put it another way, it affects both 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 can raise the CO levels and potentially 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 specified by the manufacturer's design and testing. This distance will vary between product lines from the manufacturer and even between different models within the same product line to meet the code specifications. The Code specs will control the maximum CO that can be emitted and ensure proper boiler drafting.
The drawing below affects the airflow as the draft hood is reduced in height. You can see less room for air to enter the draft hood and less secondary air at 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 replacing the altered draft hood, at least install a CO detector in the mechanical room. You'll be responsible, so be prepared for any potential health issues that may arise. 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 is shortened, the easiest way to check if the pipe was cut is to lift it and look at the end of 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.
Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website.