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Community and Q&A

What is the correct way to manually calculate air infiltration heat loss?

RON LAY | Posted in Mechanicals on

I am designing my own house and I would like to figure out the Heat Loss & Heat Gain numbers to verify if the subcontrator’s calculations are right. I think I’ve got the formulas for those, but I don’t under stand 2 numbers in the equation, if this is the correct equation?

Air Infiltration Heat Loss = Room Volume X Design Temperature Difference X AIR CHANGES PER HOUR X .O18.

My questions are: How are the number of Air Changes per Hour arrived at? and What is the .018?

Ron Lay

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    0.18 is the factor required to multiply the other items on that side of the equation (room volume in cubic feet, design temperature difference in F degrees, and air changes per hour) to come up with a heat loss number in Btu/h.

    The most accurate way to determine air changes per hour is to use a blower door. If the house isn't built yet, you have to guess. If you want to build a leaky house, you make this number high. If you want to build a very tight house, you make this number very low.

    1. User avatar
      Dana Dorsett | | #18

      "0.18 is the factor..."

      Order of magnitude erroe- try 0.018 BTU per cubic foot per degree-F.

  2. RON LAY | | #2


    Let me see if I understand what you are saying. .018 is a factor in the equation, but is it a Constant that is always used in the equation for Air Infiltration Heat Loss, or is it variable? If it is a constant, what is it actually, as in what does the number represent?

    Since this will be a new construction I will have to guess at paragraph two. But what is the average Range of Air Changes per Hour between a leaky or tight house?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Thanks -- your answer is better than mine!

  4. RON LAY | | #4


    I will have to read your recommendations more thoroughly to understand the ACH50 to ACH(nat) method.

    But I wonder about your equation for ACH(actual). Is it just adding all of the possible ventilation in the house? As, HRV is the abbreviation for Heat Recovery Ventilator? What does (HRV eff.) mean, and is ACH(HRV) just the air changes that I would need to calculate for the ventilator? Would the ACH for Direct Vent Fans need to be added in since they are only used occasionally?

  5. Bill Dietze | | #5

    The heat capacity of air at sea level is, on average, 0.018 Btu per cu. ft. per degree F. So the infiltration heat loss calculation is:

    [delta-T] x [ACH(actual)] x [volume] x 0.018,

    where ACH(actual) is the actual air exchange rate, in cu. ft. per hour, for your house. How to get this number? Calculate/Estimate it:

    ACH(actual) = ACH(nat) + (1 - HRV eff.) x ACH(HRV) + ACH(direct vent fans) + ...

    Here, ACH(nat) is the "natural" leakage of air into the house. Getting from the blower door test ACH50 to ACH(nat) is an interesting issue. Some people refer to this web site:

    For ACH50 > 1, then ACH(nat) dominates the result.
    Search the web using the terms “ach50” and “achnat” to gather more opinions on the subject!

  6. Bill Dietze | | #6


    ACH(actual) is the air leaking into the house per hour from all sources, unintentional leakage as well as due to all intentional "leakage".
    In the above post:
    HRV = Heat Recover Ventilator
    HRV eff. = the efficiency of the heat recovery ventilator

    For a rough approximation (that’s what it is anyway, even with more “precise” divisors) use ACH(nat) = ACH50 / 20. House with more stories or a climate with bigger delta T? Then you get a larger stack effect and more natural leakage, so you might divide by 12 rather than 20. I’d take all this with a grain of salt, but the general principles hold. Makes me wonder how (exactly) the PHPP software handles this...

    Note that in the original post I had (HRV eff.) x ACH(HRV) but I should have written (1 - HRV eff.) x ACH(HRV). If the HRV is 75% efficient then HRV eff. = 0.75, if the HRV is 100 % efficient, then there is no heat loss! Sorry for the confusion.

    If there are direct-vent fans on only part of the time, then multiply by the fan duty cycle (the fraction of the time it's on) to get an average heat loss for the entire day for that fan: a 100 cfm direct-vent bath-fan on one hour a day in an 18,000 cu. ft. house? Then ACH(bath fan) = (100/24) x 60 / 18,000 = 0.0139ACH. This seems small, but in a house with ACH50 = 1 it’s not an insignificant part of the total infiltration, but in terms a Btu/h of heat loss it’s not much unless it’s quite cold outside.

    I hope all this helps.

  7. Doug McEvers | | #7

    House volume x ACH (nat) x 24 x hdd x .018 for the annual infiltration heat loss in Btu's. I believe Denver uses .015 but I can't say as to the altitude conversion factor.

  8. RON LAY | | #8


    After reviewing your posts last night, I want to Thank You for the excellent information and very thorough explanations. You have made me understand so much that I just could not find otherwise.

    I do have a couple of questions though:

    I'm inferring that a Blower Door test actually needs to be done to arrive at a meaningfull ACH50 number? Although, after typing that, maybe that should have been obvious to me? What information is used to reach the ACH50, I'm thinking it might be the CFM50, Duct CFM50 ? & House Volumn. What would the formula be?

    And about your example of the ACH(bath fan), I don't understand why you would divide the 100cfm by 24?
    It seems like I would just take cfm X 60 (minutes) / house volume?

  9. Bill Dietze | | #9


    Since air leakage is determined by the actual construction, the only way to know it is to measure it after you’re done. No formulas here. You have the same issue when using a R-value for the walls, ceilings, and floors: voids in the blown in insulation? fiberglass bats put in poorly? more lumber in the walls than originally anticipated? slightly different product actually used? After good design, quality control is king. Since you are designing/building the house, can you specify the tightness at the beginning of the process and require blower door test(s) to ensure you get approximately what you wanted?

    Perhaps you are over thinking this. Remember that the formula for going from ACH50 to ACH(nat) is approximate (+/- 20% ??) - don't be lured into the illusion of absolute accuracy by the equals sign. I assume that your subcontractor is trying to calculate the heating system design capacity. If you design for ACH50 = 2 and you end up with a tighter house then no big deal (?). Push the numbers around and see how they change the total heating system design requirement. Seems like your contractor is on top of it.

    Regarding picking a reasonable (or meaningful) leakage rate for the house, here's a good article:
    You know also that very tight houses with ACH50 < 0.6 are possible from reading here at GBA, but all that air sealing will cost more. That's house design!

    As for dividing the bath fan cfm by 24, in the example above, the fan is on 1 hour a day; that is, it's on 1/24th of the time and the average flow is 4.2 cfm.

    1. User avatar
      Jon R | | #12

      Old post, but should be corrected. ACH(nat) depends greatly on if you are talking about annual average or short term (relating to peak load calculation).

      > the formula for going from ACH50 to ACH(nat) is approximate (+/- 20% ??)

      On a short term basis, ACH(nat) could vary from near zero to ACH50 - depending on wind and stack effect.

      1. Bill Dietze | | #13

        Jon, you are correct. ACH(nat) is actually very hard to infer from ACH50. The blower door result doesn't really care about the location of the holes in your envelope or the wind and ACH(nat) does. If all the holes are at the neutral pressure plane, then ACH(nat) is much reduced. If you have a four story house, its the middle of a very cold winter and all your holes are split between the attic and basement, then your stack effect is larger forcing ACH(nat) to larger values. Calculating ACH(nat) is not to be taken terribly seriously, but you can get a crude estimate from the above and add that to your heat load for the house. I recommend assuming ACH50 of two in the design phase and then aiming for an ACH50 of one or less. But you have to tailor that to the type of construction, if it's new, or just measure ACH50 if it's an existing house. Upon reflection I'd put the accuracy at +/- 50% rather than +/- 20% just to emphasize the inherent unknowns, but that's a non scientific seat of the pants judgement on my part. I've only used the above to estimate peak heating load, not ventilation. I'm using an ERV for ventilation.

  10. RON LAY | | #10


  11. Neeraj Yadav | | #11

    ACPH = {60Q/vol. of room or area}.


    ACPH = number of air changes per hour; higher values correspond to better ventilation
    Q = Volumetric flow rate of air in cubic feet per minute (cfm), if using Imperial units, or cubic meters per minute if using SI
    Vol = Space volume L × W × H, in cubic feet if using Imperial units, or cubic meters if using SI
    Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person).
    The conversion between air changes per hour and ventilation rate per person is as follows:
    Rp= {ACPH*D*h/60.

    Rp = ventilation rate per person (CFM per person, L/s per person)
    ACPH = Air changes per hour
    D = Occupant density (square feet per occupant, square meters per occupant)
    h = Ceiling height (ft, meters)

    0.018 is altitude conversion factor at sea level.

  12. Tom May | | #14

    q = - k a (T2 -T1) where k = conductivity of the material/s (BtU/ hr * ft2 * F) or
    (1 / Resistance eg 1 / R-value)
    a = area (ft2)
    T2 -T1 = temp out/inside (F)
    q = heat loss/gain (BTU/hr)

    Air infiltration can be determined with blower door test, otherwise it's a best guess.
    Air exchange is usually considered in an hvac system and is different from heat loss/gain.

  13. Hugh Weisman | | #15

    I did a quick heat loss calculation for btu/hr at a 10 degree design temp for the addition we're planning, Martha's Vineyard, zone 5, using coolcalc (tight house) and also manually. Total square footage is about 1,100, volume 11,360 (cathedral ceilings)

    Coolcalc results in a total load of 18,000Btu/hr. of which about 2K is infiltration. Manually, I come up with about 14,500 in heat losses for walls, ceilings, etc, …That’s fairly close to the 16,000 cookcalc comes up with. But when I try to calculate the additional heat loss due to infiltration assuming only 1ACH, I'm coming up with about 12K. That seams totally out of whack, even given perhaps allowing an additional 25% for the additional volume of cathedral ceilings. It also seems like a disproportionate amount relative to the other heat losses. What am I doing wrong?
    Here's my formula:
    11,360cuft x 60 degrees temp delta x 1ACH x .018 = 12,269Btu/hr

    1. User avatar
      Jon R | | #16

      That's correct if 1 ACH is occurring. But it will rarely be windy enough to cause that much infiltration.

      On the other hand, ignore all talk about the long term average ACH(nat). You want ACH(design day) for design day load calculations. Possibly around 10% of ACH(50).

  14. rrbryce | | #17

    .018 is the specific heat of air. it is a constant that explains how many btu's are required to heat a cubic foot of air by one degree. to raise the temperature of one pound of water by one degree, the specific heat of water is 1, meaning it takes one btu to raise the temperature of 1 lb water by one degree. so you can see it takes a great deal more energy to heat water than to heat air. .018 is a constant, so when doing your infiltration calcs, use it. it doesn't change.

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