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: 4949

Replies to This Discussion

Temp Difference x Actual CFM Leakage x 1.08 = BTU loss/gain.

Thanks Bob!

I just want to make sure I am understanding this correctly... If the outside temperature average is 33, the inside is a heating design of 70 and the actual leakage is 14,200 CFM, the formula would be:

(70-33) x 14,200 x 1.08 => 37 x 14,200 x 1.08 =  567432 BTU loss/gain (is the a per hour figure)

I really appreciate your help understanding this!

Luis

Luis, where are you getting the 14,200 CFM?  Is this a very large leaky building?

Bud

Yes, that is an actual reading... but I do not want that number change the main thing I am trying to understand how to do.... what about if the building leak was only 3500 CFM50?

You have to forget about CFM50, this isn't the condition the building normally operates at. You must use ACTUAL leakage rate under normal operation conditions, which is difficult to measure. This will vary considerably based on wind speed and outdoor temperature.

OK, I already forgot about CFM50... If the outside temperature average is 33, the inside is a heating design of 70 and the actual leakage is 2,200 CFM, the formula would be:

(70-33) x 2,200 x 1.08 => 37 x 2,200 x 1.08 =  87912 BTU loss/gain (is the a per hour figure?) Is the formula used correctly?

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.

Maybe this will help.

Air requires 0.018 BTUs per cubic foot to raise its temperature 1° F.

CFM50 divided by "n" approximates the natural leakage, or just divide by 20.  Here is a link for determining what "n" should be in your area if you want to be more specific. http://www.waptac.org/data/files/Website_docs/Technical_Tools/Build...

But that's just a snapshot for one delta T.  If you want to know savings per year you need to convert ▲T to HDD.

I would recommend you pick up a copy of Residential Energy by Krigger and Dorsi as it covers all of these equations and a lot more.

Bud

If I'm reading that right the 14,200CFM50 would be about 710CFM in normal conditions?

The divide by 20 was an early attempt to convert from CFM50 to natural where they ran BD tests on a bunch of houses and then used tracer gas to determine the actual air leakage.  Dividing by twenty was a happy medium.  The "n" from LBL came later in an attempt to account for wind and exposure of the building.  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.

As for the 14,200 CFM, I think my BD only goes up into the 6,000 or 7,000 range and even with the "can't reach fifty" that may be beyond a single BD.  But I think Luis was just picking a number as an example.

Residential Energy shows an equation for annual air-leakage heating costs but I haven't figured out what units they are using and they multiply by "n" which confuses me.  Maybe Darrel will stop by.

Bud

Correction, I was looking at a 4th edition so pulled out my 5th edition and they have indeed changed that formula.  For Luis, here is what they give, just as they show it.

Credit Residential Energy by Krigger and Dorsi 5th edition:

ALH = ((26 x HDD x Fuel Price X CFM50)/(n x heating efficiency)) x 0.6

ALH is annual air leakage heating cost.

HDD is annual heating degree days.

n is the LBL correlation factor

The number 26 combines the heat capacity of air (0.018) with the factors 24 and 60 for relating CFM to HDD.

Now, they may have a newer edition, but this looks reasonable.  If you like it we can dig through it to determine what the needed numbers look like.

Bud

WoW, That looks fantastic!  I have the Krigger book... I have to dig it out and start reading this section! I am taking the BPI heating professional class in February so I am trying to get prepared to actually learn something from the class! Thank you Bud, you gave plenty of food for thought... I be practicing this formula this Week End!

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