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.
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