Calculating BTU lost per cfm leaks

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: 4715

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.

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

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