The L-shaped probe grew legs, so this was improvised with a straight metal tube. There seems to be a persistent notion that angling the probe toward the appliance (into the exhaust stream) is the correct technique, but wouldn't this result in interference from ram air pressure of the rising flue gases?

Tags: CAS, CAZ, analysis, combustion, draft, pitot, safety, spillage, tube

Views: 430

Reply to This

Replies to This Discussion

I'm guessing that is the short metal tube that comes with the DG.  If that is the case, go with perpendicular to the pipe.  Might want to trim some of the pipe insulation off as well before the job is completed.  The insulation looks a little close to the vent, based just on the pictures above (clearance to combustables).

Thanks Dan. Yes, it's the straight tube from the DG bag. Interestingly, Minneapolis sells what they call a "Static Pressure Probe" and distinguish it from a "Pitot Tube", which they don't sell. Both are right-angled metal probes, the difference being that pitot tubes measure ram air (or velocity) pressure through an opening in the tip that static pressure probes don't have.

The principle is familiar to the aviation geek in me (aircraft use a pitot tube to measure airspeed and a static pressure port with an aneroid barometer to measure altitude). After further discussion in our office and examination of one the aforementioned static pressure probes, angling the tip toward the appliance would, in fact, align the holes in the probe perpendicular to the exhaust stream for a true static pressure reading. In that context, the technique I hear people cite about pointing the probe into the stream makes sense, as does your recommendation to hold the straight metal tube perpendicular to the flue.

I think the tight clearance is a result of the photo angle here.

A pitot tube on an airplane is pointed forward because you want to measure air speed and if it points into the wind it measures, well, the speed of the air.

Draft in a heater is different - we aren't looking for MPH but Delta P.  These are 2 different things.  Both the pitot tube and the bent static probe have holes perpendicular to the air stream so are measuring the same thing.  The straight pitot tube in a heater flue should point straight in to measure the same thing that the bent static probe measures when either pointed toward or away from the air stream.  

Interesting that, in your pictures, the probe pointed into the airstream measures zero.

Yep - and the static pressure port is usually on the side of the fuselage, perpendicular to the airstream.

The reading pointed toward the airstream fluctuated between -0.2 and 0.2 or so. If we interpolate the "true" static pressure as about -2.0 (halfway between zero and -4.1), then the interference of the velocity air pressure directly entering the tube, which the manometer would interpret as positive, explains the zero reading.

Isn't there a measurement missing here?...

Yeah, it should have been taken perpendicular to the flue, as Dan pointed out.

Too cryptic, sorry.  

Shouldn't there be another tube and another pressure being measured?...

Hmm. I've understood the intent to be measuring the delta P of flue gases and the ambient air, but it's been quite a few moons since I've done combustion testing on a regular basis. What am I missing here?

Ted

Still too cryptic

Where is the 2nd hose measuring caz depressurization?

I see what you mean - but the title of the thread is:

Which is the valid draft test, noir which is the valid CAZ test

Is it a valid draft test without both numbers?

RSS

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Al Heath replied to Bryan Pringle's discussion Dense packed cellulose in basement?
"My favorite was 2x4 directly on cement basement wall, with fiberglass, then plastic inside of…"
40 minutes ago
John Nicholas replied to William Fisher's discussion Can tankless water heaters provide hot water even when the groundwater is fairly cold?
"Simple math will tell you.   Figure the GPM requirements for each fixture you think will run…"
2 hours ago
R Higgins replied to William Fisher's discussion Can tankless water heaters provide hot water even when the groundwater is fairly cold?
"I have that problem, 45 to 50 degree ground water, in summer, not that we have much of a…"
11 hours ago
R Higgins replied to Larry Schaffert's discussion How to correctly insulate exterior wall in a 1900 house?
"I'd suggest trying to replicate the existing conditions as much as possible in your rebuild,…"
11 hours ago
Bryan Pringle replied to Bryan Pringle's discussion Dense packed cellulose in basement?
"I am getting a lot of "bad idea" push back on this.  I fully understand that…"
23 hours ago
Greg Mitchell commented on Larry Ralph Jr's blog post Top 7 Worst Crawl Space Repairs
"Spray Foam should be used in crawlspaces.   To learn about closed cell foam in crawlspaces…"
yesterday
Jerry Lawrence posted events
yesterday
Morgan M Audetat replied to Bryan Pringle's discussion Dense packed cellulose in basement?
"Right you are. You have to ask: what will the cellulose contribute.  Here in Minneapolis we…"
Friday

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service