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The correlation between ACH50 reduction and energy bill savings?

auadcmKqvw | Posted in Energy Efficiency and Durability on

Hi Folks,

It is well known that if we reduce air infiltration in a building will reduce the cost of heating/cooling that building. In fact, energy cost can be calculated by plugging in variables such as, Degree Days, cost of fuel, BTU per unit of fuel, etc. We also plug in pre- and post-measure CFM values, subtract one from the other and the difference will be the anticipated savings resulting from the preformed measures. But this is based on CFM’s and does not take into account the volume of the home.

On my manometer, one of the parameters I can read to measure air infiltration is ACH50. I enter the volume of the home and the manometer will calculate the air changes per hour at 50Pa.

What I would to know is if I reduce ACH50 in half on a 24,000cuft home and this results in a dollar savingsof X%, could I expect X% savings on a home with a volume of ANY size if I cut the ACH50 in half? That is, if I cut infiltration rate the same percentage on each home then the percentage savings of each will be the same?

Also, as we reduce ACH50, does the savings increase in direct proportion or at an increased rate? For example, if we reduce ACH50 by 25% and this results in a $300/yr savings, will a 50% ACH50 reduction result in a savings of $600. Or would it be more (or less)? Assume variables to be the same.

The bottom line is, with objective data in hand, I would like to give my clients an likely idea of what savings can be expected based upon ACH50 reduction. Thanks in advance to all. Bob

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  1. Riversong | | #1

    It sounds like you need to go back to energy modeling class.

    The only way to determine the anticipated energy costs (or reduction in energy costs) of any particular home is to do a complete heat loss/heat gain analysis. Once uncontrolled natural air exchange is reduced to a level below ASHRAE 62.2 minimum standards, mechanical ventilation is required and that exchange rate is determined by code, so additional tightening is not going to result in a linear cost equation.

  2. 50_Pascals | | #2

    You are partially correct. But what you are not doing is converting that cfm to ACH.

    These are "rules of thumb" based on my HERS training. If all your cfm reduction is in the basement rimjoist, it's almost always totally worthless for natural air changes or energy efficiency. New York State has paid many home performance contractors to spray foam rimjoists with pretty much zero energy benefit.

    So, assuming you fixed the top of the house....

    That conversion factor is the LBNL N-factor. It is based on the number of stories of the house. You divide the ACH50 by the N-factor and this is the natural air change rate, and thus the savings can be calculated.

    So, to use your example, reducing the cfm50 by 1200 will result in the same energy savings in any house - of the same height. Because 1200cfm50 reduction will convert to the same natural air change rate in any house of the same height.

    As far as being "too tight." We always went as tight as possible, and installed a ventilation system (Panasonic bath fan and timer). The building works better and will last longer due to reduced air leakage driven condensation. Plus, in my experience, "too tight" (and thus low ventilation rates) doesn't exist when you factor in normal bath fan usage, clothes dryers, range hood, and door opening.

    Moisture problems and stale air have not been a problem in very tight houses we've done with cellulose. These are generally younger active families. Their daily functioning is providing air changes that are difficult to quantify. But start stepping closer to the cliff and use closed cell spray foam, shut-in senior citizens who smoke, laundry hanging in the basement - that's when you'll have a problem.

    Our rule of thumb for pretty much any weatherization project was at least 20% energy savings. We had some that were up to 60% savings, but those were rare. Yes - we verified through follow-up energy bills and analysis.


  3. HDendy | | #3

    Not all volumes are created equal.

    You have to look at the big picture. Infiltration is just one component of energy efficiency. Two houses with equal volumes could have very different envelope areas and performance. For example, if you have poor insulation and high U windows that trump the infiltration wastes then you would get less overall percentage return for improving the ACH50 than you would on a well insulated house. Using ELR standards instead of ACH is a much better comparison since air leaks through the envelope, not the volume.... you'll still need to look at the big picture though.

    In my opinion, setting ACH or ELR goals should be a standard practice, not an upgrade. This is quality assurance and "doing the right thing." The upgrades I discuss with my clients are better insulation materials, better performing windows, more efficient equipment etc.

  4. Riversong | | #4

    Robert Susz,

    If the total building CFM/50 is reduced, the energy savings will be the same regardless of where the reduction takes place - rim joists or ceiling.

    The LBL normalized leakage is determined not just by ACH/50 and building height but also by climate and wind shielding - since delta-T and wind exposure also effects natural leakage rates.

    Reducing the CFM/50 by a given volume will have a decidedly different impact on energy costs from one house to another, depending primarily on climate.

    It sounds as if you also need to go back to energy modelling class.

  5. Riversong | | #5

    To put it another way:

    There is no correlation between ACH/50 reduction and energy savings, since ACH/50 is a measure of the forced air exchange of a building impacted by a 20 mph wind on all surfaces - a completely arbitrary and artificial standard which has no necessary correlation to actual air exchange rates.

    To determine heating and/or cooling cost savings from any envelope improvement requires a whole house energy analysis which includes either measured or assumed natural and/or mechanical ventilation rates. The cost savings from any improvement is dependent upon the climate, the whole-house U-value, the air exchange rate, the occupancy load, the solar gains, the cost of fuels and the efficiency of appliances.

  6. 50_Pascals | | #6

    I was trying to think of nice things to say.....but I give up.


  7. Michael Blasnik | | #7

    This thread is mostly filled with misinformation. I'll try to cut to the chase. First, you want to work in CFM50 and not ACH50 and you want to work in absolute numbers and not in percent terms. You can then get a rough estimate of energy use as about 7 therms of natural gas per 100 CFM50 of measured leakage in a moderately cold climate (about 6000 HDD).

    The number should be a little higher than 7 in 2 story homes and very wind exposed homes and a little smaller in 1 story homes and well shielded homes, but you this should put you in the right ballpark. If you have a ventilation system and tight house, then things are good deal more complicated..

    None of the models are very precise and no one knows the actual distribution of leaks or wind speeds any given home will see, but there are still very strong correlations between actual energy use and CFM50 values once you analyze large groups of homes.

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