Draft Hood

A draft hood is a device that is either integral (built into the appliance, typically found on gas boilers and non-current generation gas furnaces), or externally installed, common to typical chimney vented water heaters and virtually all oil fired appliances.

Its purpose is to create a break between the chimney and the appliance itself. It serves one or two purposes. Heating professionals: one or two depending on what school you came out of and what your technical aptitude is (that the polite way to phrase it).

For oil, it is used to adjust the draft. Too much draft and there will be too much air in the flue gas (meaning less efficient), and possibly the inability for the cad cell to properly acknowledge the flame during the trial for ignition.

For gas, it serves one or two purposes, and there is some debate on this whole subject within the heating industry. Unlike oil appliances, gas appliances where never meant to be tuned in the field; although making sure they are operating safely is of course a whole different matter. By tuned we mean this: The gas to air ratio is pre-determined already by the manufacturer. Once the gas pressure is set, the appliance should be operating at factory specifications.

The problem, and the controversy, is this: we have a greater possibility for carbon monoxide threats, there is a potential for pilot light outages, and we cant tune the combustion.

The Draft Hood Controversy

If you bring up this subject to the average plumbing or heating guy, do not be surprised if you get a blank look. We hate to call this subject advanced, but unfortunately many of todays typical technicians do not understand this theory. Hey, we even expect some unfriendly comments regarding our approach at times. As long as we have a safe and efficient home, we can tolerate name-calling.

Gas appliances, once upon a time, did not utilize the modern draft hood, whether integral (built into the jacket of the appliance), or external (like what we see on the common water heater, you may hear the term mushroom for this part). The draft hood literally separates the chimney from the appliance. Now take a moment and conceptualize this idea if you will: If we break the chimney from the appliance, dont we create a potential draft concern for the appliance? Would it not be better if the appliance was directly connected to the chimney? Yes. We can achieve this with a barometric damper. Actually, when an oil fired appliance (there is no such thing as an integral draft hood on an oil appliance) is converted to natural gas, a barometric damper is installed.

Another reason why the draft hood became so main stream is because of the occasional pilot outage. Because wind could travel down the flue and blow out the pilot, this of course became a nuisance for the gas companies in the old days. So, this is the second theory that must be addressed. We at Wilson believe its far better to have a pilot outage once in a while than a potential venting concern Whats worse, a 5 minute pilot relight or the (possibly greater) potential for carbon monoxide poisoning?

The barometric damper on a gas fired chimney vented appliance is not mainstream, nor conventional. If is however safer than what the factory gives us with the integral hood or the field installed mushroom. As an added benefit, it will even save energy, because now we are able to tune the gas/air ratio, just like the oil guys have been doing for decades. The one stipulation is you actually have to know what youre doing with combustion theory. Do not have your heating or plumbing contractor attempt this without a digital combustion analyzer and the proper understanding of what hes doing.

It goes without saying the average oil technician understands combustion theory more than the average plumber or HVAC technician.