Condensing Dryers

December 22, 2016
Spring 2017
This online-only article is a supplement to the Spring 2017 print edition of Home Energy Magazine.
Click here to read more articles about Appliances / Plug Load

Consumers have never had more options when it comes to choosing a clothes dryer. Dryers are now styled like high-end cars and include nearly as many features. They have multiple Temperature and Spin settings as well as moisture sensors, alarms, and delay starts. You can select models with different colors, sizes, stacking configurations, and energy performance. With so many options, selecting the right model can be a bit overwhelming.

Obviously, the greenest option is to air dry on a clothesline, but this isn’t always practical. In our house, we dry as much of our clothing as possible on the shower curtain rod, only using the dryer for smaller items or when we are in a rush. When you’re living in 840 square feet, you don’t always want to pull out the drying rack.

Our primary concern when selecting our equipment was size, since the laundry closet was narrow and required stackable units. We also wanted the dryer to use as little electricity as possible. We opted for Bosch’s 24-inch-wide Ascenta (WTB86200UC), since it met our size criteria and is a condensation dryer, which uses much less energy than standard gas or electric units. This Energy Star-qualified appliance uses at least 20% less electricity than a standard dryer. Here I’ll explain how condensing dryers work and review our experience with the Ascenta unit.

Installed stacking appliances.

Technology Overview

Conventional clothes dryers draw in house air that’s heated using gas or electricity and circulated through the rotating drum. The humid air is then exhausted through a dryer vent ducted to the outdoors. This approach has worked for decades, but new technologies show that it is not the most energy- efficient option.

Unlike conventional dryers, condensing or condensation dryers are not vented to the outdoors. They circulate the warm humid air across a cooled heat exchanger. As the air passes over the heat exchanger, the moisture in it condenses. The heat exchanger acts like a cold can of beer on a hot humid summer day. The condensation is then piped out of the home through the clothes washer drain. The Ascenta also exhausts some air into the surrounding area, so it’s not a completely closed system (more on this later).

But Isn’t Venting a Good Thing?

The conventional wisdom that dryers must be vented simply does not apply to condensing dryers. Vented dryers take house air that’s heated or cooled and exhausts it to the exterior. This puts the home under slightly negative pressure because every unit of air removed from the home must be replaced with an equal amount of air (got to love physics). The new air enters the home through holes in the building envelope. All this means that the heating-and-cooling system will have to work a little harder to compensate for the dryer running.

When a vented dryer isn’t running, there is typically some air leakage around the exhaust pipe and through the exhaust pipe itself, especially if the damper isn’t functioning well. For this reason, condensing dryers are a great option for any home, especially those striving for extremely tight envelopes. It can also be challenging to run dryer vents in apartments, condominiums, and town houses. And finally, any time you put a hole in a home, you create a new opportunity for water to get in.

The one drawback is that most condensing dryers exhaust some air into the surrounding space. We actually appreciated this the wintertime, because it provided a little extra heat in the laundry room. The opposite is true, however, in the summertime, when we’re trying to keep the home cool. Some dryers have additional heat recovery, which channels the warm air into the heater as preheated air.


We learned that stacked washer and dryer installation is different enough from standard practice to warrant hiring a qualified professional. After a less-than-successful attempt at having our contractor do the installation, we hired Howard Payne Company to reinstall the refrigerator, clothes washer, and clothes dryer. During the initial installation, we learned the hard way that refrigerator feet are not meant to level the appliance, but rather to adjust the height and alignment of the French door. Similarly, we discovered that the washing machine should be hooked up to cold water only, and that it comes with large bolts installed to keep the drum from shifting during shipping. The bolts are visible only from the rear and are easy to miss. We discovered the bolts when we used the washing machine for the first time and it shook so violently it felt like it could have leveled the house.

Another quirky detail is that the clothes washer plugs into the dryer. Our contractor missed this detail, so we ended up with two specialty electrical outlets (one 240V three prong and one 240V four prong).


Have you ever looked at the outside vent of a standard dryer? They tend to get dirty and clogged up. With a condensing dryer there is no outside vent. Instead, the internal heat exchanger acts as a filter and needs to be routinely cleaned. Under regular use, Bosch recommends monthly cleaning of both the heat exchanger and its cap. The moisture sensors should also be cleaned monthly. Interesting fact—in reviewing the Bosch manual, I discovered that the material lint filters collect is known as “fluff,” not actually as “lint.”

Immediately behind the maintenance flap is the heat exchanger cap. The cap is easily removed by turning both locking levers towards each other and sliding it straight out.

Behind the heat exchanger cap is the heat exchanger, which is removed by pulling straight towards you.

Here are the heat exchanger and cap side by side after about two months of moderate use. I generally wash off any accumulated hair and fluff with the bathtub faucet.

Final Verdict

I had read a number of reviews online, and I found that in general condensing dryers have a reputation for drying very slowly. This isn’t necessarily true from our experience. The moisture sensor does seem to deem the clothes dry while they are still damp to the touch. I think this is due to a combination of factors: not selecting the right Dry setting, not making sure that the moisture sensor is kept clean, and perhaps that the moisture sensor may not be that accurate. For this reason, we tend to use the moisture-sensing setting first and then run the dryer again for an additional 20 minutes. This is not a perfect system by any means, but it works.

Condensing dryers definitely require additional routine maintenance, but this is a small inconvenience for the energy efficiency gains. That said, I’m curious to see how the next generation of dryers develop, and how they integrate heat pump and heat recovery technology.

Abe Kruger is coprincipal of SK Collaborative, which helps homeowners, contractors, architects, manufacturers, and nonprofit agencies create better buildings and products. Kruger is an authority on single- and multifamily green- building consulting and certification and on industry education and training.

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