DIY heat exchanger for your shower

Update December 2011: You may now buy the new affordable DWHR unit for stand alone shower cabinets at the HeatSnagger Shop  (Norwegian link: Varmegjenvinner for dusj )
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Update April 2011: Arthur and I have joined forces and are now working on a commercial, low cost DWHR unit based on this blog post. You may follow our progress at Meander Heat Recovery
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Nov 2009:
I got an E-mail from Arthur Kimmels. He has come up with a DIY concept for a small shower heat exchanger, a drain water heat recovery (DWHR) unit. I really like the simplicity of this design. His mail speaks well for itself, so here it is:

Hei Svein,

With interest I read your blog about miljødusj http://urge4lessenergy.blogspot.com/2008_11_01_archive.html. I had been walking around with a similar idea of building a heat exchanger for the shower (before I read about miljodusj, but inspired by some products on the Dutch market), and after some months of thinking and building I installed it today. Attached you will find some pictures of the unit. It is built of wood, coated with fiber glass and polyester. It measures 85x32x12cm. The water channels have an inclination of approximately 1cm/m. The heat exchanger is built of ca. 4m copper, 12mm diameter and soldered together. It works as 'counterflow'. A disadvantage of this setup is the pressure drop/flow reduction due to the long PEX/copper/PEX length (ca. 8m), but I measured this to be within acceptable limits for my setup. The measured temperature increase at a flowrate of ca. 10liter/min was 23.3(out) -8.8 (in) or 14.5degC. Total material cost was ca. NOK 1500, including PEX tubes to and from the unit. (The thermometer in the picture measures at the in and outlet, I removed the 'indoor' sensor from the housing and soldered it to some wire.)

Feel free to add to your 'blog'.

Kind regards, Arthur Kimmels

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Summary in Norwegian: Arthur har laget en enkel varmeveksler for dusj. Avløpsvannet fra dusjen brukes til å forvarme kaldtvannet som kommer inn i dusjen. Temperatur økningen på kaldtvannet er målt til 14.5 C. Det medfører at man bruker mindre varmtvann og dermed sparer energi. Spillvarme gjenvinning i praksis.

The most efficient way to boil water

This is a follow up of a great article by Pablo at the Treehugger http://www.treehugger.com/files/2009/11/electric-kettle-stove-or-microwave.php where he tested the efficiency of different ways of boiling water. By bringing 350ml of 17°C water to a boil and measuring the energi used, he found the efficiency of using a pot on the stove to be 30.5%, a microwave 47%, and an electric kettle 81%.

I have been thinking of how to make coffee in the morning. I can use the drip brewer or the Frensh press pot. So I first repetead the Pablo test with our 2000W electric kettle. It took 77 seconds to bring the water the 83 C° up to a boil. Actually slightly shorter, but I timed it to the point where the kettle turned it self off. The specific heat of water is 4.180 kJ/kgK, which means that it takes 4,180 kWs to heat one liter one C°. Needed energy transfered to the water is then 0.350 l * 83C°*4.180 kWs which is 121.4kWs. The kettle averaged 2000 W and used 77 seconds, which is 154 kWs. From this I get the efficiency of my electric kettle to be 78%, which is close to what Pable got. Then I tried the drip brewer. It averaged 950 W and after 153 seconds it was done, but the next 15 minutes it turned it self on again four times for 6 seonds each time. So at the time of automatic shut off it had been running for 177 seconds. 168.2 kWs turns out to be an efficiency of 72%. I did this test with 350 ml so it could compare to Pablos test.
For us, a more realistic water use is typical 700ml, so after waiting for an hour, I redid the tests with twice as much water. The result from this was that the drip brewer was 95% efficient and the electric kettle 88% efficient. So for our family, the drip brewer is the most efficient way boiling water for coffee.

Summary in Norwegian: Det kreves store mengder energi for å varme opp vann. Derfor er det lønnsomt å være bevisst på hvordan man varmer opp vann. Konklusjonen fra denne testen er at en kaffe trakter er den minst energikrevende måten å lage kaffe på.