Largest and Most Forgotten Duct…The Envelope

Duct, Duct, Duct – Goose. The “goose” in this case it‘s the envelope, and it causes contractors to chase around trying to catch “comfort“. It‘s no secret that all of our conditioned air is contained in a single box with a bunch of holes. Besides affecting health and comfort, the envelope determines the size of the heating and cooling system(s). So then why are most HVAC contractors ignoring the infiltration of the envelope?

Without looking for the answer, what do you think the percentage of envelope infiltration is on the heating system? Many guess 3% – 5%, you would be correct if this was very detailed well-sealed Passive House (0.6 ACH50). The uncomfortable reality is most homes are around twenty to plus forty percent (I used words to avoid those looking for an answer). That is a significant impact on the heating load for a component that can be adjusted – sealed.

The infiltration on many existing homes averages 30% – plus 40% of the heating load. This can calculate to an entire size in a furnace or heat pump. Existing homes are what feeds the industry (& technicians) and many replacement units are “size out = size in“, it was working before – do it again without evaluating the condition of the home.

When you perform the ACCA Manual J, what infiltration method are you selecting? Refresher course, options in the simplified method are “tight, semi-tight, average, semi-loose, loose” OR actual blower door test results [CFM at 50Pa]. Based on feedback I get, contractors are instructed to use “loose“ all the time, “it’s the safest selection.“

For older existing homes “loose” may be the best default selection, but how leaky is the envelope? A simple blower door test will determine the actual infiltration. This can be twice the simplified method estimation of “loose.” This can take estimated infiltration from 30% of the heating load to an actual blower door tested 45%-50%. Now you have comfort complaints and callbacks. If half of your heating load is a leaky house, you find yourself chasing comfort.

The challenge with new construction and the infiltration simplified method estimation of “loose – is the safest selection” can drastically over estimate the calculated load. The IECC 2012 requires a blower door test and an Air Change Rate (ACH50) of 3 in most climate zones (CZ 3-8). The estimated infiltration of “loose” can be 3x the code requirement.

Over over-sizing!

[For new construction, the blower door infiltration method can be reverse calculated for code compliance to estimate the “Test air flow CFM at 50Pa” using this formula CFM50 = Volume x ACH50 ÷ 60 IECC CZ 1-2 requires ACH50 = 5 CZ 3-8 requires ACH50 = 3. NOTE: this is the blower door cfm result required to pass, however, the envelope could be tighter.]

If you have some of the common arguments that I receive in the classroom – I will try to deflect some now.

  • Southern climate and the heating load is not your focus, then how about the impact of infiltration on the latent load. The percentages are the same; it can be a half-ton or more on an AC unit.
  • Sealing up the home will cause mold and other issues. There are multiple standard about how tight a home can be before mechanical ventilation is required. What is “V” in H_AC? If it is missing, it needs to be installed to avoid the potential issues. Otherwise, you’re considered a “HAC”
  • IECC 2012 requires mechanical ventilation on new home with less than 5 ACH50.

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