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Why do we need a draft regulator?

            Draft regulators or barometric dampers are devices used to regulate the draft on oil-fired furnaces, boilers, and water heaters. Yes, even water heaters! I can’t tell you how many times I have seen water heaters installed without a draft regulator installed, and a stack temperature over 900° or 1,000° F, and a draft over the fire that (as we used to say) would suck the chrome off a bumper.  Well, that’s when car bumpers had chrome.


            To be sure you understand me, the draft regulator I am referring to here is used only on oil-fired heating equipment, not on gas-fired equipment. That’s a whole different animal.


            On oil fired equipment, the draft regulator is typically a round device with a swinging door, which will only swing one way, and with an adjustable weight on it, which will allow you to adjust it to control the draft. You adjust the weight to control the amount of the opening of this damper which in turn controls the amount of excess air that can enter the flue and chimney when the oil burner is operating. In this way you adjust the draft over the fire in the appliance.


            To be certain of the terminology, the straight run of the tee is called the ‘run” and the 90 degree connector, in the middle, is called the ‘bull.”  The draft regulator is to be inserted into the ‘bull” of a smoke tee between the heating appliance and the chimney, and usually a minimum of 18″ from the appliance.  How many times have you seen it installed in the ‘run” of the tee?  We used to install the tee so that the flue pipe would make a right angle from the appliance to the chimney, remember?  It was so convenient to check and clean out the chimney base, but it was wrong!  Well, we were all wrong.  There are so many still like this in the field, don’t leave it like this! 


Why do you need a draft regulator at all? Air that is used for combustion is called combustible air. That air may be in the boiler room or delivered by mechanical means, but it is still combustible air. This air is combined with the fuel and is part of the combustion process. The gases that are produced must be discharged back to the outside environment. The gases moving through this process is called the draft. 


Too much draft will decrease the efficiency of the heating appliance, increasing cost, but it will also increase chimney temperatures (stack temperature) to an unsafe level.  Too little draft can result in incomplete combustion, soot, puff backs, and in an extreme case, carbon monoxide.  Since the chimney draft varies greatly, we need a way to stabilize it.  This is the job of the draft regulator.


            Let’s say that you measure the draft July and get a reading of -.04 in the smoke pipe.  Do you think that reading would be the same in February without the help of the draft regulator?  The winds and the outdoor temperature all affect the draft.  Also a second unit using the same chimney as the heater, say a water heater, will affect the draft. Also, a major problem to affect draft is outside boiler rooms.  In many homes the boiler room is in the back of the house and accessed through a separate door from the house. I have never seen an outside boiler room with any more space in it than what the heating appliance needs. Most, if not all, of these rooms had outside air vents when they were first designed, but over the years they have been closed off, had a new roof installed over them or just simply removed because no one knew what they were there for. When you do your testing in the summer, you will leave the boiler room door open. When I did my testing however, as uncomfortable as it may be, you need to close the door. If you don’t, things will change. Generally, the draft regulator is set to the lowest draft that gives good combustion and proper oil burner operation. Higher draft will waste money for your customer.


Typically, we try to attain draft readings of 0.02 0r 0.03′ WC over the fire and 0.04 to 0.05″ WC in the breech. You can’t verify these readings without test equipment.  Don’t forget that changing the draft will affect the CO2 readings you will get, so test, my friend, test.  While these draft readings are typical, there are units that operate in a positive draft mode.  Therefore, it is very important to read the instruction manual that is supplied with every new unit.  The manufacturers will send you any instructions you need for their appliance just ask them for them.


A lot is to be said about combustible air and how much is needed and where it comes from, but that’s for another time.

2 comments

  1. In the summer when we ran the central air, the blower fan is set for high,(( in winter it is set for low, ))
    Last year for the first time we noticed when the air conditioner ran and the oil fired water heater ran at the same time, it seems we get a smell, a mild exhaust smell,,,,,,, it was like the oil fired furnace blower blowing cold air in the home was also drawing exhaust fumes from the pipes while the oil fired water heater was running at the same time,

    I checked the chimney, cleaned it, damper works, its just such a draw from the furnace blower that it seems to suck the exhaust from the pipe, last year was the first time it ever happened in 10 years.

    If I tried to make the furnace air tight so the draw from the fan does not draw from the cellar would that help

  2. Two things to keep in mind>

    1.When the furnace is not operating: if there is just a little wind outside it will create low pressure in the chimney which will draw air into it. The little weight on the “damper” will become irrelevant , the damper will remain fully open and the air from the basement will be drawn into the chimney. Now that air has to be replaced in your basement and that will be replaced from the air in hour living quarters. This means that outside air (cold outside air, very cold outside air will be sucked into you living quarters through every little opening, open door, leaking window, electrical outlet, kitchen stove vent, dryer vent, you name it.If you are not convinced just stand next to your basement flue damper when it is windy outside and see what happens. What goes out has to be replaced it’s as simple as that. A furnace that is shut down doesn’t create any carbon dioxide or monoxide problems!

    2. If the furnace is on it gets all the air it needs from a vent on the furnace. And yes, that air is also drawn from the basement. But most of the time your furnace is not running. The problem with the “balancing dampers” is that they will stay open with the slightest breeze which is most of the time. Now someone might argue you need to be able to control the air getting into the combustion chamber especially if it’s really windy outside but since most furnaces operate on only one flame setting there shouldn’t really be a need to control that unless a windy day can “overwhelm” the “air needs” of the combustion chamber.If that is an issue it seems to me the best way to control that would be with an “electrically operated gate” on the furnace vent opening.You could of course have an electrically controlled flue damper which only opened up when the furnace was on but that would still leave the furnace vent open with the tendency to cool the boiler.

    I once tried to explore the above theories with a furnace maintenance technician who thought the need for the ‘balancing” flue damper was that you had to maintain some draft in the chimney so that the furnace would run better when it started up but since the furnace blower is blowing massive amounts of air into the furnace (which it gets from the basement through what I would call the “furnace vent” I wonder if that really is an issue.

    I am not an engineer so I could be wrong on any of he above but I would like to understand it and I know that when I am working in my basement in the winter and that damper is straight out horizontal with the wind howling through the flue, and the furnace is off something is not efficient in the heating world!

    I also assume “state of the art” furnace don’t seem to need any (visible?) flue dampers!

    2.

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