How is infiltration factor related to ACH50?


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Old 12-05-08, 03:06 PM
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How is infiltration factor related to ACH50?

Hi everyone,

I have spent most of my day today reading the threads in this group. What a great site!

I am in the market for a new boiler to save money on my gas bill and as I'm not sure how long the latest kludge on my current boiler will hold up (currently have a Crane from maybe the 50s or 60s?).

Anyway, I downloaded the SlantFin software today and played around with it a bit. My current boiler does appear oversized, but the calculation is very sensitive to the infiltration factor.

I have results from a blower door test on my house. Does anyone know how SlantFin infiltration factor is related to ACH50 or ACHnat?
 
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Old 12-05-08, 05:55 PM
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I'm not entirely sure what they want. I googled the term and did some reading and the best direction I can guess they are going is, if "you prevent 98% of the infiltration" which was a phrase used in discussing cellulose insulation, then you have remaining, 2% infiltration. Which is .02 a number which seems to be in the range of what they are looking for. BUT how you convert CFM50, natural, or ACH to that number, I couldn't find. I guess I'll be waiting along with you to find out.

P.S. there is an 06 thread on this forum that came up where N J Trooper comments on the variable nature of applying that number, so perhaps he will weigh in again.

HH
Bud
 
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Old 12-05-08, 10:14 PM
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What's your number at 50 kpals? Do you know where that rates? If so apply where it rates in relation to picking an inflitration factor. I think 4.5 is the limit for not using an HRV.
 
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Old 12-06-08, 08:35 AM
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Oh, your responses have both helped very much. Maybe infiltration factor is the % heat loss due to infiltration? In that case, you couldn't just convert a blower door ACH number into an infiltration factor.

The highest infiltration factor in the Slantfin drop down menu is for rough construction with 4 outside walls. The number is 0.068, or approximately 7%. I don't know much about rough construction, but it sounds kind of drafty. On the other hand, this room would probably have a lot of hear loss through the walls with no insulation too. Maybe 7% is reasonable in this case? I really don't know.

@ Who, our ACH50 (at 50 kPa) is about 7.5. I don't know where that rates, but from what I understand, you can convert to ACHnat by dividing by 20. So, our ACHnat is about 0.4. This seems really low for a 1952 house, but we do have inch thick concrete-like plaster. Maybe that helps? Anyway, from what I've found the minimum ACHnat recommended for adequate IAQ is 0.35. So, our house seems pretty tight from an infiltration standpoint.

Slantfin provides these recommendation for infiltration factor.

* .009 for new construction with extra heavy insulation
* .011 for tight homes with heavy insulation
* .018 for average homes with moderate insulation
* .027 for older homes in good condition with minimal insulation
* .036 for loose construction and older homes with no insulation and weather stripping

Since our house appears to be pretty tight*, I guess I could go with a lower number. For some reason though, they provide infiltration factor recommendations based on tight vs. loose houses and heavy vs no insulation. I've been using 0.03 since my house is older with no wall insulation. Using 0.03 vs. 0.015 increases my heat loss by ~ 20,000 Btu/hr. It would be nice to know which side of that 20,000 I'm on. My husband is concerned I'm not being conservative enough. I did see this heat loss reference (Energy Code & Heat Loss Calculation Information) in one of the previous threads. I'll try that one next to see how it compares.

* I say appears since I did the blower door test myself with a loaner blower door. It seemed pretty easy, but the number seems too good to be true.
 
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Old 12-09-08, 01:36 PM
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For what it's worth, I did a heat loss calc with Slantfin and with the Pikes Peak Bldg Dept spreadsheet. I used the same U-factors and exposed areas where possible.

Slantfin agreed with the Pikes Peak spreadsheet when I used an infiltration factor of 0.02. The Pikes Peak sheet assumes an ACH of 0.5.

An infiltration factor of 0.02 doesn't quite make sense from the Slantfin guidelines below for my house, but it is what it is, I guess. I think my house is fairly tight with an ACH of 0.4, but the insulation could be better. The attic insulation is pretty good at about R-30 but I estimate main floor wall r-values at about R-3 and basement wall and basement floor values between about R-2 and R-3.

Anyway, the good news is that I am fairly satisfied with my heat loss calc and can now learn about boiler types.

It's off topic, but if anyone knows of ways to insulate plaster walls in a brick veneered house (beyond drilling several holes in every stud cavity and potentially destroying the plaster), I'd be very interested.

Thanks again.
 
 

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