Why not Energy Star rating for dryers?

That's a tough question. Cost efficient or CO2 efficient? The answers could differ depending on the source of electricity.

Propane is a tougher call than natural gas. Propane is better suited as a chemical feedstock for a cracking plant rather than as a fuel and is more expensive. Natural gas on the other hand is the best of the fossil fuels where available.

When I was doing my recent dryer purchase I stuck with electric although I'm still questioning the decision. Operational cost wise natural gas would have been far cheaper over the long haul. I had several reasons for staying with electric though: 1. I would have had to modify the piping and vent runs (about $500 worth of work.) 2. Plus I didn't want it to interfere in any way with my plans for new supply and vent runs for a condensing furnace and perhaps some form of condensing water heater. It was #3 on the priority list. 3. I'm displeased with the poor performance of the newer FVIR gas systems (in particular my current water heater) and didn't want to add another. 4. I was concerned about gas running a bit hot--it's been a few decades since I had a gas dryer but I remember it drying fast and hard. I'll freely admit that 3 & 4 were my own fears, and probably were overly cautious.

Electricity's main downfall is that even if one lives in a place that is primarily non-coal or 100% renewable, it still prevents the transfer of some fraction of capacity to other regions that might otherwise reduce coal plant production (as in shutdown/not build new). The exception would be off grid renewable. I'm next to a coal plant, so for me the CO2 output of choosing electric is as bad as it can be. Yet, I went that route for other reasons.

 

I guess now is a good time to interject that a well integrated dryer could be quite an efficiency improvement. For example if all of the drying air was interchanged with outside air and/or inserted into the vent distribution system for the home it could be positive depending on region and season.

However, there are various reasons that this is not presently feasible/desirable. The most obvious is lint/dust clogging. I clean my dryer vent about once a year and scoop out a lot of accumulated matted lint that I would not want distributed throughout the home. Without a very effective lint trap I would not want that material passing over the AC coil, furnace heat exchanger, air handler blower blades, or into the ductwork/registers.

The second major concern is what "off gases" are in the dryer discharge? Laundry detergent residue, dryer sheet compounds, and various other chemical treatments will be in that air. This could pose some air quality or window/wall residue type of concerns.

The third factor is humidity control. For such a system to really make sense it should be integrated into the HVAC control. For about 5 months or so of the year, the heat and humidity would be a positive where I live. In the other months it would be best dumped outside after preheating the dryer feed air.

 

That's great if you only need to dry a single load per day when the weather is nice and predictable.

Show the background as gray cloudy skies, about 35-45 F with occasional rain for the past seven days and you will be wondering about the "efficiency" of a system that can't dry a single load in a week...AKA, the 2nd week of October here.

I've used clothes lines and such when I can, but they are extremely unreliable (little thing called weather) and can result in relaundering of the same load...sometimes twice when various junk gets blown onto them (thanks southwest Georgia subtropical weather.) Plus the extended stints in high UV don't exactly extend the life of the clothing... Drying racks in the utility room work better, but have limited capacity, and primarily are used for delicates. For 800 kwh/year or so I'll use the dryer instead. If I lived in an arid climate I would air dry more.

As I've said before, my aim (and that of most consumers) is not to waste a lot of extra time and effort each day or week for modest energy savings.