Saturday, March 4, 2023

Fresh Air is Free

The previous post shares a surprising finding that conditioned air circulated in USA  ordinary clothes drying is replaced at very modest cost, say $16 per year.  Healthy provision of fresh air in our homes at required 0.35 ACHnat, 7 ACH50, is at very small cost. That cost, about $100 per year for a well-built 1000 sf home, is unavoidable. If a home is more-expensively built to Passive House standards, less than 0.6 ACH50 , fresh air must be drawn in by other means, to a total of at least 7 ACH50. Those means can be absurd, perhaps with net increase of operating costs. Powered means may be unreliable with expense in operation and repair/ replacements. Occasional reporting of false economy in air tightness, in this blog , is not kindly regarded by readers.

The physics fact in this is that air has extremely small heat capacity, rising or falling in temperature without much gain or loss of thermal energy. In-home air-to-air regenerative heat exchangers save little energy vs. operating costs including maintenance. The value in regenerative heat exchange is related to the heat capacity of the moved fluid, mattering a lot for high delta-T liquid systems, and little for small delta-T gases.

I was not heard, when I spoke out to the full assembly of March 2016 hearings of proposed 2018 revision of the International Energy Conservation Code, Louisville, KY. I was dismayed by the many proposals sought to surrender the enforcement of construction quality, to  numbers in blower door tests. I call that Blower Door Madness.

In a few words I stated an Insulation Math finding that for  my Portland, Oregon 1000 sq ft single-story home with 8 foot ceilings, 4400 heating degree days, natural gas heat at $2 per therm, each ACH50 of fresh air brings a conditioning cost of only $10 per year. Tightening a home is almost irrelevant to the total of typical HVAC costs. The attention should instead be upon perfection of a complete home insulation envelope with thoughtfully-placed windows.

Where it is accepted that a healthy home should have minimum 0.35 ACHnat,  times-twenty, 7 ACH50, air exchange in my home is at natural gas cost of a modest $70 per year. Home HVAC cost is very little related to provision of healthy fresh air. Don't we all know that pulling makeup air through gaps of ordinary good construction 24/7, is better than pulling outside air at a pipe to the HVAC return, only when the air handler is running? Those odd pipes in so many new homes, are not part of any regenerative heat exchange or of thoughtful filtration of the the outside air.

Where annual usage of natural gas might be at cost of $400 per year, the healthy fresh air cost $50 at estimated 5 ACH50 tightness, is about 12% of that total, and is unavoidable.  There is a gas clothes dryer perhaps accounting for 15% and reducible by about $50 per year by the investment in HPCD.  It will make sense for sure, when all natural gas service is terminated, with heat pump HVAC. Heat pump HVAC is interesting now, from need for summertime cooling not now provided.

I have no further possible savings in adding of insulation. Recent gas usage of about $470 per year are probably with comfortable temperature setting near 70°F. Where I lived alone in the house under its reconstruction, thermostat at 55°F, I demonstrated the potential of about 50% savings in a bearable life of dressing very warmly to be comfortable.

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