Fresh Air is Almost Free

At 3/15/2024, look for naivete that might be criticized in overlooking cost of conditioning moisture in the air. Google: air conditioning: air conditioning: the relative cost of heat gain and humidity regulation. Choose this result: https://www.achrnews.com/articles/146975-nrel-discovers-humidity-is-why-a-c-units-consume-so-much-energy Find that for a portion, perhaps all,  of Cooling Degree Days, air conditioning cost should be doubled. The math gets messy, but this is a moot point. Whatever the cost, we must have that fresh air exchange. It doesn't matter whether air to be expelled, then somehow replaced, is driven by a clothes dryer, or a bath fan, or holes in walls of an otherwise airtight home. Where few of us will ever have opportunity to invest in expensive HRV or ERV, the neglect is not a dominant concern. The cost of comfort will still be mostly from defects of insulation perfection and window placement. The cost will be directly regulated by generation efficiency as in upgrade to heat pump technology.

At this post of February 23, 2023, Costs of Running A Miele Heat Pump Clothes Dryer

I share a surprising finding that conditioned air dumped outdoors in USA  ordinary clothes drying is replaced at very modest cost of about $6 to $16 per year, dependent on HVAC efficiency.  The very simple math for average-USA climate, is indisputable.

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. The best insulation envelope is fully filled, precluding air convection, air-tight all-around, not determinant by a blower door.

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.

And then natural gas cost ramped up. Don't expect cost to come back down, ever. Pain boosts interest in all-electric heat pump HVAC.

Costs of Running A Miele Heat Pump Clothes Dryer

Matt Risinger shares this comprehensive review of Miele Laundry Machines - W1/T1 Washer & Dryer

The Case For Ventless Dryers - My Experience  , January 10, 2023

The heat pump (ductless) clothes dryer, HPCD,  is the greater Miele novelty, and the subject of some math on operating costs, in comparison to ordinary clothes dryers.  

Search:  miele t1 dryer energy consumption

Find this Energy Star table with Estimated Annual Energy Use of 133 kWh/yr, a number also  shared by Matt Risinger. A button at this number in the table declares:

The estimated annual energy use is based on the Combined Energy Factor and an annual usage of 283 cycles per year, as referenced by the U S. Department of Energy test procedure. Code of Federal Regulations, Title 10, Section 430, Subpart B, Appendix D2. Actual energy consumption will vary on your usage patterns, including how often your run the dryer, what cycle you select, and load size.

https://www.energystar.gov/productfinder/product/certified-clothes-dryers/details/2308247

Matt's math is for his city, Austin Texas, where the incremental cost of electricity is 11.4 cents per KWh, 133 KWh then costing $15 per year. Matt finds that an ordinary electric clothes dryer for the  same demand would draw 608 KWh, costing $69 per year, higher by times-4.6.

Matt then is aware of further cost in the heating or cooling of outside air to compensate for air expelled outside by the drying operations, further disadvantaging the ordinary clothes dryers. But, he despairs of doing the math for this. I will now do that math, with assumptions that Matt has offered;

In each dryer cycle the discharge rate is 200 cfm, and cycle time is 45 minutes. 9000 cubic feet of conditioned air pushed outside, is .replaced by the same volume of ambient air. Let this be for Austin TX. Calculate the year-average  temperature difference in conditioning this air as: 

(Sum of Austin TX annual Heating Degree Days and Cooling Degree Days), divided by 365.

Find HDD and CDD, at web site degreedays.net, for Year 2022, choosing the daily weather station data for Camp Mabry. For HDD choose reference temperature 65°F. For CDD, choose reference temperature 75°F. Results are 2047 HDD65 and 1817 CDD75, total 3864 degree days. The year-average  HVAC temperature moderation is 3864/365 = 10.6°F.  A number near 11°F will apply for most of us in USA, with our different mixes of HDD and CDD.

The heat capacity of air is taken as 0.018 BTU per cubic ft, per °F.

0.018 BTU per cubic ft per °F * 283 cycles * 9000 cubic ft per cycle * 10.6 °F= 486,000 BTU

The cost of this make-up heat with a modest natural gas furnace is about $2 per therm, $.00002 per BTU, about $10 per year. The cost is less at actual present incremental fuel cost and with better HVAC efficiency as with a heat pump. The highest cost is with electric resistance heating. At Austin TX rates: 486,000 BTU * KWH/3412 BTU * $0.114 per KWH = $16 per year. To speak in round numbers here, think that Matt Risinger's missing number is: $10 per year. More precisely it is a variable dependent on HVAC efficiency.

The conditioning of makeup air with an ordinary clothes dryer adds little to the savings in comparison to a heat pump clothes dryer. It is not a larger number.  Fresh air is nearly free, due to its extremely small heat capacity, while energy remains cheap in USA, and endlessly in a post-carbon better future. 

Insignificance of Fresh Air Dumped With An Ordinary Clothes Dryer

Compare math of fresh-air conditioning attendant to an ordinary clothes dryer, to that of needed air exchange in a healthy home. Judge needed exchange with this graph:

Want of 0.5 air changes per hour is reasonable.   Times-twenty, this is 10 ACH50. Covid has pushed-up what we tolerate as a fresh-air goal. We must all know what this costs, and that it costs very little vs. other collective investments for family good health.

Consider  a 1000 sq ft home  with eight foot ceilings, house volume 8000 cubic feet, in Austin TX.

0.5 air exchanges per hour * 24 hours per day * 365 days = 4380 air exchanges per year. The exhaust flow rate is a not-huge 4000/60 = 67 cfm.

Calculate the cost to condition this replacement air:. 

4380 air exchanges per year * 8000 cubic feet per exchange * 0.018 BTU per cubic ft, per °F * (10.6 °F daily-average Delta T) = 6.69 E6 BTU * $.00002 per BTU = $134 per year*, if with an ordinary gas furnace 88% efficiency. These costs go up affordably in proportion to the size of the home. Costs go down to insignificance, with heat pump HVAC.

Compare this to the $10 per year expended in air exchange with an ordinary clothes dryer and the 88%-efficient gas furnace. The $10 figure probably corresponds with a smaller household, say that 1000 sf home, and increases in proportion to household size.

Conclude:

Saving money by reduced fresh air is foolish. Eliminated dumping of stale air with a heat pump clothes dryer, might need to be made up with other means of air exchange. Maybe, don't add $10 per year in the payback math.

Reducing the fresh air cost by regenerative heat exchange is unlikely, especially with concurrent heat pump HVAC.

$134 per year*

Please compare this number, $13.40 per ACH50, to finding of $10 per ACH50 for a house with degree-days more by 4400/3864 = *1.14 and more-complex math of blower doors, in my Insulation Math. At same degree days, disagreement is by *1.34*1.14 = *1.5. The simpler math gives ballpark confirmation of equations and constants of less-certain, less provable origin. Refinements in the blower-door math are:

Multiply by 0.6 (or divide by 1.67), to adjust the theoretically calculated energy savings for the following interactions:

• The theoretical model assumes that homes need heat at an outside air temperature of 65 degrees. In fact, many homes do not need heat until the outside air temperature is below 65 degrees F.

• Some air that leaks into the house may not get heated to the inside temperature before it leaks out again.

• Air leakage through small cracks reduces conduction losses by tempering surrounding surfaces. This process is referred to as the dynamic insulation effect.

These adjustments would not apply where fresh air is constantly delivered for health reasons. Fresh air math need not derive from arcane little-known, generally-forgotten blower door methods. Methods preserved in my blower door Insulation Math retain interesting details. For example, I may state the size of a net "hole" in a house that with natural draft would provide a wished amount of perpetual fresh air. A 1000 sq ft single-story house wanting ten ACH50 fresh air might have path area 178 sq in, for example a couple of 30" windows opened 3", even in coldest winter. In mild weather, we want much more fresh air, than this. Wishes for filtered air are uncoupled from needs of fresh air.

An airtight home may yet have badly missing and wind-washed insulation, not at all energy-efficient. A blower door can not measure construction quality.

Math Error Found

Sadly, the clothes line math error reported in this post is in the direction of lesser justification for savings effort. The math of Build It Solar is quite correct. Further math for understanding is correct at my web site: Convert KWH at 29.3 KWH per therm and 12 pounds CO2 per therm. About 410 pounds of CO2 results from generating 1000 KWH.  Here is one credible outside source for the conversions. The 410 number looks right for the Northwest mix of sources, mainly coal and hydroelectric. 

Numbers above are from the UC Irvine link.


Allow that my two-person household would be using 860 KWH per year with full resort to an electric dryer. Saving half of that is 180 pounds of CO2 and $30 at $2 per therm, per year. Savings are smaller (by half?) where I have a natural gas dryer.


The error found? It is in the often-cited CS Monitor article. Annually, these dryers consume 1,079 kilowatt hours of energy per household, creating 2,224 pounds of carbon-dioxide emissions.
1079 KWH converts to perhaps 440 pounds of CO2.

More Clothes Line Math

Build It Solar - Clothesline Thoughts


Drying clothes in a gas or electric dryer is energy intensive.  An electric dryer uses about 2.3 KWH of electricity to dry one load.  In addition, as the dryer vents its exhaust out of the house, new air is pulled into the house to replace it, and in the winter, this new air must be heated.  If the temperature outside is (say) 30F, it takes about another 2 KWH to heat this incoming air.  In climates where AC is used in the summer, air pulled in by the dryer must be cooled.  So, drying one load can takes from 2.3 KWH to 4.3 KWH for each load -- a lot.  If you do a load a day, then you probably spend about 1000 KWH per year on drying energy. 


I'll figure 200 loads in a year, for a two-adult household. Without AC, figure 860 KWH. Do the Math on that.


One therm is 29.3 KWH and 12 pounds CO2. 860 KWH is 29.4 therms and 350 pounds of CO2. If I save half of that, it is about fifteen therms, $30, 180 pounds CO2.


Where is my error?

More Thought of Carbon Footprint: Shunning My Clothes Dryer

It is time at last to curtail my luxury of mindlessly tossing laundry in my natural-gas clothes dryer. The conversion from electric heat in a hand-me-down, free dryer was ticket for ease that I let go through another Summer. What is the amount of my savings opportunity?


I refer first to one of my clothesline shopping discoveries. I will just accept for now that a two-person household, with half-time resort to clotheslines, may save 500 pounds of CO2 per year. The saving is sizable, judging by math for my total carbon footprint, about 20,000 pounds of CO2 per year. Every measure that might save another 1000 pounds of CO2, will be painful.


The cost of a durable, convenient and presentable clothesline seems to be more than $250. How does this pan out in a payback calculation, the Insulation Math?


A therm corresponds to emission of about 12 pounds of CO2. 500 pounds is 42 therms. At $2 per therm, my saving is $84 per year. A $300 clothesline cost is repaid in less than four years, 28% simple return. Worth doing. Worthy of incentives. Spending $300 will not be extravagant.