Hi everyone, 

   A couple of my colleagues were discussing the benefits of air sealing, when I was asked if I could quantify the actual value of air sealing based on how many BTU's per CFM(50) of air leak.  The software does the energy savings calculations of air sealing for my clients, but I was never asked the actual value per BTU. I guess if I could answer that I would have a better understanding of the math behind the savings. 

Can anyone help me with this? Is there a formula?

Thanks

Luis

Tags: CFM, air, energy, saving, sealing

Views: 1524

Reply to This

Replies to This Discussion

Bud,

"In an article from one of the developers of the "n" number he stated the resulting estimate could be off by minus 50% to plus 100% so calling it a ball park estimate may be generous."

Do you recall the article you found this in?

Thanks,

Graham

I have looked but found no reference to that article.  But my background in research saw those numbers and concluded it wasn't much better than a wild guess, or the old empirical divide by twenty,  But since the results bracket the "20" pretty much, in general they are ok.  Part of what we do is to recognize what fits the many assumptions and what doesn't.  Unfortunately, that might improve our accuracy. but reduce our consistency and it is consistency that our system really demands, even if our numbers are wrong.

Consistently relying on software to generate our answers can be a risky way to do business.

Bud

If you were using TECTITE from The Energy Conservatory, you have this value right away.

Asking for an 'actual value of air sealing' is a research project.

Blower Door guided air-sealing works well for those of us in the real world.

Give the Advanced Infiltration tool a try from Residential Energy Dynamics (RED); I think it will do all you need. It is based on the Alberta Infiltration Model (AIM-2) and it includes TMY3 weather data for North America. It will give you the fuel use (therms, kwh, etc) for infiltration. Start with the pre-weatherization CFM50 and then the post-weatherization CFM50 to determine the savings from air sealing. The tools also does many other useful things.

Here is a link to this free tool: http://www.residentialenergydynamics.com/REDCalcFree/Tools/Advanced....

As a matter of disclosure, I am the author of this procedure, along with my business partner, Charlie Holly.

Bob,

Where does the value 1.08 come from?  I am more familiar with the  "0.018 BTUs per cubic foot to raise its temperature 1° F" that Bud cited.  Using a hypothetical 125 CFM nat. with a delta T of 50 F and your equation I get 6750 BTU/hr.  If I use 0.018 and the same equation I get 113 BTU/hr. Markedly different results.   

There is a factor of 60 difference between the answers that Graham is pointing out.

Reply by Graham Irwin 2 hours ago

Remember to convert CFM (cubic feet per MINUTE) to cubic feet per HOUR (i.e. ft3/hr = CFM/60) if you want an answer in BTU/hr."

Bud

1.08 represents the sensible heat constant. Any time that I've done these calcs it was left out because the complexity was beyond the scope of what I was trying to accomplish, but if someone with the time and expertise wants to expound on this I'd like to know more as well. 

I'll just turn Bob's equation around:

Q (BTU loss/gain) = 1.08 x CFM (natural) x ▲T (F°)

The short explanation is, we need to convert CFM to hours as Graham posted. Multiply by 60 min/hr

Then we have to specify the thermal capacity of air: 0.018 btus/ft³/F°

When that all gets factored in we get (60 x 0.018) which equals the 1.08 and the units of the equation come out in btus/hr.

I scratched that all out, but typing it in is a pain, so you got the short version.

Bud

Bud,

You explained it just fine.  Makes sense. 

Yes, and when I cannot sleep I do my own version counting the length of time my FAN runs to reset my internal temp then take into account the current Delta T of the night and then, finally when does the furnace come back on!

I obviously need help

Don't put any faith in standard formulas that convert cfm50 reductions into air sealing savings.  A house doesn't naturally experience air exchange at a 50 pascal pressure difference.  A blower door will cause air to enter the house from every penetration indiscriminately.  Under natural conditions, air exchange is caused from mechanical ventilation, wind and the stack effect.  An installer who cuts CFM50 in half by sealing windward and ceiling leaks will create MUCH more savings than another installer who just spray foams the sill box.

If anyone is aware of a formula that requires sealing location data to estimate savings, please let me know.  Until then, this is huge opportunity for building scientists to advance our industry.

Dmitri Martin

www.greenstarwi.com
Facebook Twitter Google+ LinkedIn

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

Rob Madden, Solar Home Broker commented on Alicia Hobbs's blog post 5 Solar Facts that You Should Know
"From personal experience, that $20,000 savings over 20 years will be a low number based upon the…"
6 hours ago
Rob Madden, Solar Home Broker posted a blog post

Phoenix Solar Home Sales Jump in September

The number of solar home sales in Phoenix jumped 13.6 percent from the previous month, according to…See More
6 hours ago
Profile IconGina M., Brian J. Mulhern, James Tucker and 2 more joined Home Energy Pros
8 hours ago
Casey Gesell replied to Trip Smith's discussion Where to spend my marketing dollars?
"Trip, Check out our sister company Contractor Nation: https://www.contractornation.com/ Pay Only…"
8 hours ago
Casey Gesell posted a discussion

Looking for home improvement companies to join our dealer network!

My name is Casey Gesell and I'm the Business Development Manager at Dr. Energy Saver, Inc.Dr.…See More
8 hours ago
Casey Gesell replied to Trip Smith's discussion Starting a Home Weatherization Business. Considering it...
"Hi Trip, Check out our website: www.drenergysaverdealerships.com "
8 hours ago
Alicia Hobbs posted a blog post
8 hours ago
Mark Wozniak posted a status
"I am looking for energy efficiency software training in Massachusetts."
10 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service