Static-Pressure Probes: Measuring Combustion Draft
Looking for a draft in a flue by measuring just the pressure in the flue—with reference to the combustion appliance zone, or CAZ—is a bit like discussing the speed of a moving vehicle without referencing the distance travelled or the time taken to get there. It’s interesting, but it doesn’t tell the whole story. The velocity of the airflow in a flue can be calculated by knowing the velocity pressure and the density of the air. The velocity pressure is the difference between the total pressure and the static pressure (see “Pitot Tube”). The total pressure is the impact pressure of the flow. The static pressure is exerted uniformly throughout the gas or fluid. The static pressure pushes out against the walls of the flue or duct. Despite the fact that the static pressure is exerted uniformly, to get an accurate draft reading, the probe used must be oriented to eliminate the impact of the total pressure on the static pressure.
For combustion safety testing, the analyst is seeking to determine whether there is or there isn’t draft moving up the flue. Are the combustion by-products moving effectively up the chimney and out of the house? If they are moving down the chimney and into the house, there can be nasty, devastating results! If the static pressure in the flue is negative with reference to the CAZ that should indicate that the flue is working, carrying the nasty stuff out of the house. We use static pressure to save us from doing the math required to actually determine the volume of combustion gases moving up the chimney.
Most HVAC texts recommend a draft pressure of -0.01 to -0.02 inches of water column (iwc)—about -2.5 to -5 Pa. BPI requires adherence to the formula
(Toutside/40) - 2.75
An outside temperature of 10°F would result in a required pressure difference of -2.5 Pa or -0.01 iwc. Any outside temperature warmer than that would result in a lower required draft pressure. A 40°F day, for example, results in a mere wisp of a draft pressure of -1.625 Pa (-0.0065 iwc).
Tamasin Sterner, president and chief coach of Pure Energy Coach, LLC, documented the results of using either an L-shaped static-pressure probe or a straight tube (see Table 1). The straight tube resulted in generally higher static-pressure readings. Pure Energy Coach decided to stay with the straight tube. These pressures averaged almost 0.9 Pa better draft with the straight tube.
I used the probes described in this article to test the draft on an oil burner. The results are shown in Table 2. Note that these oil burner numbers are considerably higher than the gas systems numbers shown in Table 1. Because of the relatively high pressures generated with the oil-fired boiler, for these measurements the variations in the position of the probe would not have been great enough to fail the appliance. But the variations are great enough that in the case of appliances close to the required draft pressure, the position of the probe will make a critical difference between passing and failing.
Dwyer A-303/ 1
These probes are L shaped, with a tapered point at the end of the short leg and holes on the sides of that same leg. The long leg of the L is 4 inches and the short leg is 2 inches, ending in the pointed tip. There are two holes 0.04 inches in diameter, located ¾ inch and 11/8 inch from the tip. The pointed tip should be oriented to face into the airstream (see photo below). The point helps to straighten out the airflow around the sides of the tube and to reduce the effect of the total pressure on the static pressure. In this orientation, the holes are perpendicular to the airflow.
There is a bracket surrounding a magnet that spaces the foot of the probe 31/16 inches into the airflow. When a ¼-inch hole is drilled in the flue pipe, the magnet will hold the probe in place. The leg of the probe extends another inch beyond the bracket, allowing the probe to be connected to a hose that can be connected to the Input or plus side of a manometer (when measuring flue pressure). Both Dwyer and UEI Test Instruments have fastened a ¼-inch bulb or bump to this end of the tube, which works well with some of the fatter hoses that are used for combustion analysis but are too big for the 1/8 inch-inner-diameter hoses used by most building analysts. With a bit of effort, the bulb can be removed.
If the flue is 4 inches in diameter, when the probe is mounted in the flue, it will be about 1 inch away from the back wall. If the flue is only 3 inches in diameter, the probe will press up against the back wall and may not engage the magnet; in this case, it will not provide an accurate reading. To obtain consistent readings, a spacer of some sort should be added to move the probe tip out to the middle of the flue—1½ inches for a 3-inch flue and 2 inches for a 4-inch flue.
The straight tube is simply the 1/8 inch metal tube commonly supplied with manometers, but a piece of stainless steel tubing will work just as well. The standard tube is 6 inches long, but a longer piece can be effectively used when it is attached to the manometer hosing to extend under doors or through a hole in a basement wall to get an outside pressure reference reading.
When it is used for combustion draft measurement, the tube should be held perpendicular to the flow in the middle of the flue. If it is allowed to dangle at some random angle, the reading will also be random and certainly not consistent from one house to the next.
The ACX probe is a 1/16 inch-diameter tube that can be used to probe into a variety of places because of its extremely small size. The probe is 12 inches long, so it tends to be bent in use, which makes it difficult to insert directly perpendicular to the flow into a flue. The flow numbers generated, however, are very similar to those produced by the 1/8-inch-diameter straight tube.
(For the purposes of full disclosure, the ACX probe is manufactured by and available from Heyoka Solutions, LLC, the author’s company.)
Many combustion analyzers can also be used to measure draft. Testo’s 327 flue gas analyzer, for example, has a function for measuring draft that includes a bar graph that indicates when the probe is at the optimum point in the flue for highest temperature and maximum draft. It displays the draft pressure, stack temperature, and maximum temperature. The UEI C155 Eagle 2X has a dial setting for “Prs” or the flue pressure. The reading can be made at the same time as one makes the other combustion analysis measurements. Combustion analyzers commonly have cone-shaped fixtures that will hold the probe steady in the flue.
A pitot tube is a probe that has a hole at the end that, when pointed directly into the airflow, admits the total pressure into the tube. The pitot tube has a second concentric tube that has tiny holes on the sides that allow the static pressure to push in. The two tubes are connected to a pressure-measuring device, allowing the separate pressures in each of the two tubes to push against each other, resulting in the velocity pressure.
The point of measuring the static pressure in the flue is to be sure that there is enough draft to carry the combustion gases out of the building. The static-pressure readings from a straight tube inserted perpendicular to the flow will provide higher draft readings than the same probe inserted at an angle, and a straight tube will provide higher draft readings than an L-shaped static-pressure probe. Since it is always better to give preference to health and safety, I would recommend using the static-pressure probe, because it is most likely to indicate a weak-draft problem. However, it must be used properly.
The Dwyer A-303 or the UEI ASP1 static-pressure probes should be pointed into the exhaust flow from the boiler, furnace, or water heater, and they should be located in the middle of the airstream. They should not be pressed up against the back of the flue pipe.
The straight tube should be held perpendicular to the flow at least 1 inch into the pipe. A magnetic clip can be used for this purpose, with a hole through the magnet and the clip to support the tube. Mark the tube at 1-inch increments, and it can be inserted into the flue to a consistent depth and angle.
Contact the author at firstname.lastname@example.org.
Watch an on-demand webcast presented by Paul Raymer titled Ventilation systems and the ASHRAE 62.2 standard at www.ecw.org/university/ecuevent.php?ecuid=487. Free to view, pay for credit.
For a long time, measuring the static pressure has been an acceptable surrogate measurement for draft. It may not be perfect, but it has worked most of the time. As homes get tighter, however, it is increasingly important to make consistent, reproducible measurements. Proper instruments and proper technique are both important.
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