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I thought I had a GC/Framer lined up to build a 16" double stud wall but they have backed off. I found a builder willing to get <3ACH, code in NYS is 3 ACH@50, but not double stud, only 2x6 and he wants to use spray foam. Being PHPP only list kBTU/(ft²yr), how do we convert that to MAX BTU/Hr?

Is this the correct conversion for average BTU/Hr?

([PHPP kBTU/(ft²yr)] * 1000 * [square ft]) / 365 days / 24 Hr = average BTU/Hr

Being we are not going to get close to <= 4.75, is just doubling the number from above enough? If I lease it up-to the HVAC installer I will end up with a 80+kBTU propane unit for $15,000+. I should also mention we are going with a Zehnder ComfoAir350 with ground loop & Zola windows/doors.

Asked by Jeff Ziegler
Posted Jul 19, 2017 12:35 AM ET


3 Answers

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Does your project have a certified Passive House consultant? If not, you need to hire one.

There are two possible ways to meet the Passivhaus standard; either

1. The annual heating demand as calculated by the Passivhaus Planning Package should be no more than 15 kWh/m² per year (4,746 btu/ft² per year) in heating and 15 kWh/m² per year cooling energy; or

2. The building must have a peak heating load that does not exceed 10 W/m² (3.1718 Btu/h per square foot).

The first number tells you how much energy you need on an annual basis.

The second number doesn't tell you anything about energy. It tells you about the maximum power requirement. (Power and energy are different.) This is basically the design heating load, in BTU/h, divided by the number of square feet.

Your certified Passive House consultant should be able to tell you your design heating load and your annual heating energy demand.

If you want to perform your own calculations, you need to know your building's average U-factor and the heating degree days for your location. The formula is:
Q = HDD*A*U*24

Answered by Martin Holladay
Posted Jul 19, 2017 6:26 AM ET


So you are saying because we are not going with a certified Passive House, PHPP can't help and I am stuck with the high school drop out to fill in the Manuel-J 100% correctly. Or am I missing something?

Max BTU/Hr = 7722 (HDD) * ? * U-value * 24 Hrs

* Is A area in sqft or cubic ft? PHPP calculates this based on 1st floor interior space - walls + a % of the basement (per PHPP consultant even though that will not be heated room)
* Why times U-value as 1 number, each surface will be a different U-value & PHPP calculates that?
* Why times 24Hrs when heating systems are in BTU/Hr and not a day? Or is this the safety factor?

PHPP is saying the following for Buffalo, NY
Heating / cooling period (day/yr)
Annual method: 207
Heating: 303
Cooling: 122

Heating / cooling degree hours (°F.day/yr)
Annual method: 6926
Heating: 7722
Cooling: -1319

I just noticed on the "Heating load" tab has "Heating load PH" = 39,993
BTU/hr. But I still have to figure out how to model the Zehnder geothermal system correctly, so a single 28kBTU split system may work. But now maybe spray foam is needed vs cellulose?

Answered by Jeff Ziegler
Posted Jul 19, 2017 7:36 AM ET


For more information on the formula I provided, and more on U-factor calculations (as well as an all-important discussion of UA), see All About U-Factor.

I suspect that you don't have to calculate UA, which is laborious, because PHPP probably calculates it for you. Can any GBA readers who are experienced with PHPP provide Jeff with useful advice, before he begins his UA calculations?

Answered by Martin Holladay
Posted Jul 19, 2017 8:30 AM ET

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