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: 436

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.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

tedkidd replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Bob, here is where our perspectives diverge. I think people should switch to electric because they…"
5 hours ago
tedkidd replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Here our thinking falls out of alignment. Going to electric is default for me. People can't…"
5 hours ago
Bob Blanchette replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"If you are going to stay with gas service, might as well keep the gas water heater. Switching gas…"
5 hours ago
tedkidd replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"8 people, sometimes under 10 therms. Just not enough energy used heating water to get obsessive…"
6 hours ago
Eric Kjelshus replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Biggest is to keep the CO  or other gas down in building  "
6 hours ago
Bob Blanchette replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Look at your SUMMER gas bill where water heating accounts for most of your gas use. 10-15 therms…"
6 hours ago
Bob Blanchette replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"The marathon fails miserably in operating costs when compared to an ordinary gas water heater.. NG…"
7 hours ago
Richard C. MacCrea replied to Damien Greenfield's discussion Is there is cost saving for you in having a tankless water heater?
"Most of the posts here are far too narrow in scope. To make the best decision you need to…"
22 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service