Which reading is more reliable on a Minneapolis Duct Blaster?

I have recently switched to using a Minneapolis Duct Blaster and have noticed that switching the rings has been resulting in varying results. My question is which reading is more accurate?

 

I know that each ring is used depending on the amount of leakage in the duct system, however it doesn’t seem clear which ring I should be using. Yesterday I test a 1.5 ton system in a condo and I used the #2 ring. I got a result of 240 CFM then I put in the #3 ring (smallest one) and got 180 CFM. Which one is accurate? 

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Hi Stephen, if we don't get a more knowledgeable answer, my blaster experience is very limited, we can summon someone from TEC.  But from this link: http://www.energyconservatory.com/sites/default/files/documents/dbq... it looks like ring #2 (90 to 300 cfm) would be the best choice.  However, there are areas of crossover between the different rings and I'm unsure if two different rings would yield the same number.  Example would be ring #2 giving a reading of 100 CFM, should one switch to ring #3 and if so would they see a big difference?

Bud

Thanks for the reply Bud. I usually start with ring #2 and if its on the low side I will try ring #3. If its on the high side I will go with #1. It seems like the answer is a gut feeling of which one is right, the instructions only say to use which ring you "think best matches" but thats far from scientific.

I had been using the retro tech duct blaster which seemed to have closer numbers when you changed the rings or wouldnt give a reading if the leakage was out of the rings range. However, the Minnieapolis one seems to give a different number for each ring and doesnt really show out of range.

Number 3 ring should be more accurate. This is according to my HERS training which I was taught if you can achieve the pressure the smallest ring is the most accurate or the fan speed as high as possible. This is true also of the Blower Door.

I use Retrotec and always move to the smallest ring I can to achieve readings. Is see small difference but never 60 cfm maybe 1-10 cfm 

Are you certain you had the correct ring set on the manometer to match the blaster?

Thanks for the info, I have been going with the number that I get with the smaller ring so that makes me feel more confidant.

Yes I changed the setting on the manometer.

Stephen, curious if you changed back and forth between #2 and #3 and got the same reading with each ring repeatedly, or just made the switch once?

No I only switched once.

I got to ask - When was the last time you did a calibration check because based on Minneapolis Flow Tables a 180 is impossible to get on the smallest flow ring. You may wish to check the manuals page http://www.energyconservatory.com/support/manuals for how to field check your fan & gauge.

I can't recall does EC have a can't reach 25 feature where it will extrapolate the number for you? If so & you are not reaching 25, that tells you immediately that you need to switch rings to a larger size 

With that as Glen points out - the higher the fan speed & smaller the ring the more accurate reading you will have. The biggest catch is making sure everything is set up right (no one standing on a hose) and you double check your settings - if any of these is off so is the result.

Choosing a size - I generally base mine on the size of the house, what I see, and prior results by the contractor. Fortunately even if I need to switch rings, it isn't a big deal - just don't forget to change the manometer setting

Thats why I was confused on my readings because I'm still getting a reading outside of what the CFM should be for that ring. If it is out of range does that mean its wrong?

I did a field calibration and I'm slightly higher than the CFM of 106 for fully calibrated, I read CFM 109.

Just checking all the bases.  Did you remember to switch configurations on the dg 700 to account for the different rings?  That would certainly make a difference.

 

I built a test box- 2 ft square where the duct blaster attaches to one side and I can vary the openings on the other side.    When I switch rings, I always get readings that are  the same (within the margin of error), even when I am outside the ideal range of the duct blaster.  Makes me think it is a setup, or calibration error.

Cheers,

Eric

You were testing leakage to the outside and total leakage, and also doing a Blower Door Test on the condo?  

If so,  what surfaces were your sealed vents on.  Drywall? Laminate/tile Flooring? Carpet?   How did you seal those?  How windy outside? Are your ducts inside conditioned space?  Outside in traditional areas, attic or crawl?  Outside in service areas unique to multi level structures? How well did your temporary sealing hold up?  I did one last night and pulled a less then perfect seal into the duct. As you would expect that 4 x 12 return increased leakage by 200 CFM @25.

In general the higher speed, smaller ring is more accurate, both BD and DB.  How does your test result compare with the standards.  Code or Energy Star is  CFM / 100 sf of conditioned space; ACCA is 10% of system air flow.  Before you got there did you expect to hit one of these targets?  If so, how close did you come?  

To reiterate what Glen said, always use the smallest ring that allows you to achieve the desired level of (de)pressurization. It will always be the most accurate reading available. 

It may not be the Blaster but the water vapor presser inside, air speed outside.  I have also found duct leaks change with attic temp.  I have found return air leaks if in a stud of wall change with Relativity Humidity in wood and sheet rock.  super dry out the wood to 2% then when spring hits with 100%RH day after day its less.  In my own home I did a duct blaster each morn for 90+ days  and got different  reading each day.   Hood test are better at repeating same CFM.  but still +- 5% each day with temp and RH.  The tighter the building the closer the reading.   I opened a main duct snap in plenum  with high static presser .4"wc and dropped an extra 450 CFM into the attic- the heat and presser from hanging 30' kinda put it self together and lowered the CFM 450, with lower presser.  The  local who made the duct needs to replace the tooling on his duct maker as it does not snap together well.  

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