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: 620

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

Featured Forum Discussions

BPI Anaylist

Started by Larry R Waters in BPI 2 hours ago. 0 Replies

What causes a temperature plane in a home

Started by Energy Wise Solutions in HVAC. Last reply by Brad Cook 11 hours ago. 6 Replies

Velocity Pressure Testing

Started by Horace Douglas Hunt, Jr. in General Forum. Last reply by Horace Douglas Hunt, Jr. Apr 15. 2 Replies

Videos

  • Add Videos
  • View All

Latest Activity

Larry R Waters posted a discussion

BPI Anaylist

Industry leader in the north bay area California. Looking for experienced professionals and…See More
2 hours ago
Larry R Waters joined Diane Chojnowski's group
Thumbnail

Job Board

This group is for posting jobs related to all aspects of the home performance industry including…See More
2 hours ago
John Nicholas replied to Jim Fowler's discussion TREAT modeling of mid-rise apartments low heating load
"So why is tenant behavior significantly different in this building?"
3 hours ago
Jim Fowler replied to Jim Fowler's discussion TREAT modeling of mid-rise apartments low heating load
"Hi John - I'm strictly modeling the entire building, not individual units. I'm looking…"
4 hours ago
John Nicholas replied to Jim Fowler's discussion TREAT modeling of mid-rise apartments low heating load
"Where is this going?  Are you (the housing authority) trying to set the Utility Allowance?…"
8 hours ago
Brad Cook replied to Energy Wise Solutions's discussion What causes a temperature plane in a home
"It is interesting that people respond to questions like this with answers that make some…"
11 hours ago
Ed Minch replied to Energy Wise Solutions's discussion What causes a temperature plane in a home
"We see this in the mid-Atlantic with ceiling feeds on a hot air system, and usually with a…"
11 hours ago
North East Hydronic Radiant Inc posted photos
15 hours ago

Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported 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. Please honor our Guidelines

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service