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Old 01-05-13, 09:22 AM   #1
Steve Hull
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Default lessons learned - dryer energy conservation

Fascinating reading all the posts on how to improve clothes dryer efficiency with HRVs, outside air use and all.

Lessons (over the last 30+ years) learned are to put in gas vs electric, use outside air (or put dryer in garage) and the problems of lint.

The lint issue is a killer (literally - more on that later). Years (actually decades ago) ago, I built an HRV and hooked it up to the dryer. Within one week the HRV was clogged with lint. Back blew the HRV out and installed a large pleated filter set between dryer output and HRV with a capture of 90% at 10 microns.

This filter set (1 inch pleats, 24x 24 inch) clogged in two weeks due to the trapping of fine lint in the filter set. Note I was also being VERY careful to remove lint from the dryer "filter".

So I put in filters with not so tight a weave and yet more pleating (went to 6 inch pleated 24x24" filter set (90% capture at 100 microns).

It worked GREAT and the filter was not clogging, but the HRV performance got worse and worse over a few months.

The HRV got clogged again by lots of fine particles that made it through the 100 micron filter and yet had no resembalance to lint as it was packed in so tight. Back blowing the HRV failed.

Anyway, I did an experiment examing the air expeled from dryers and found out that dryers are essentially lint generators. It is the tumbling of cotton that breaks the longer fibers into very fine particles. And these fine particles are very bad for the lungs (causes emphysema).

I looked into putting in a centrifugal type "spinning filter", but the energy costs of these exceeded the goal. These used the now familiar "Dyson" vacuum cleaner type spin technology to trap particles. But they are noisy!!

Another technology uses a very fine spray of water mist to capture lint water recycling after more coarse 100 micron filtration. But weekly maintenance is required or these water filters clog up. These work well (if maintained), but now there is even more water vapor into the HRV . . .

So I have resigned myself to blowing out lint filled, moisture laden air into the outside air. But I do use an outside air attic supply, a large filter over that (prevents mold/dust/allergens) and a "cold trap" to prevent cold air descending into the washer/dryer room in winter. With three kids we did a LOT of daily laundry and the CFM of a dryer is about 300-500 CFM.

Running a dryer for an hour essentially evacuates the house of conditioned air and brings in unconditioned air complete with allergens/dirt, etc. It was pointless to construct a house tight, put in an HRV and then use a clothes dryer that evacuates it every hour . . .

I now recommend to clients to use outside air to a dedicated washer/dryer room or the more conventional placement of the dryer in the garage . . . (just like many of our parents). Doing an extra spin cycle on the washer also helps to get out more water before drying (huge savings). Drying is expensive, an extra spin is not nearly so.

Use natural gas or propane over electric as the payback time for the more expensive gas dryer is about 2 years (36% return using "rule of 72").

There is a great ACEEE paper out there by Paul Bendt that talks about the inefficiencies of dryers and there is a LOT that can be done.

If you are new to this thread, look of the many superb posts here on dryers, but be aware of a major health issue (very fine lint into home) that has not made it into the public literature if you are venting the dryer into the house.

consulting on geothermal heating/cooling & rational energy use since 1990

Last edited by stevehull; 01-05-13 at 05:12 PM.. Reason: typos
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Old 01-05-13, 06:50 PM   #2
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Do you have a link to the Paul Bendt paper?

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dryer, energy efficiency, lessons learned

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