Practically off the grid!

drain water heat recovery unit

Drain water heat recovery pipes are designed to recover the heat from your shower water before it goes down the drain.  The units consist of a large 3 inch copper pipe with smaller copper pipes wrapped around the outside.  They are essentially heat exchangers and are designed to conduct as much energy as possible from the outgoing hot water to the incoming cold water.  As your hot shower water drains through the 3 inch pipe it warms the pipe exterior.  This heat is then transfered to the cold incoming water which is flowing through the smaller tubes on the pip exterior.  It is a very simple design and quite effective because there is lots of surface area and copper is highly conductive.

The installation of the DWHR pipes is simple.  They are basically designed to be inserted into your main stack in a vertical position.  In most houses, the best place for this will likely be right before the drain exits the house.   However, there may be some cases in multiple story dwellings that it would be more efficient to place the DWHR pipe closer to the point of hot water use, such as a second or third story shower.  If you have a huge home and always shower on the top level your drain water may have cooled substantially by the time it winds its way to your DWHR pipe down in your basement.  Therefore, to recover as much heat as possible from your drain water make sure to install the DWHR pipe a close as you can to the hot water source.

I bought one of these units based on claims that they could save up to %40 of your hot water heating load and I have to say I have been very pleased.  For the first few weeks I would find pleasure going down to the pipe and feeling the temperature difference between the bottom of the pipe where the cold water entered and the top of the pipe where the warmed water exited on its way to the water heater.  The difference was substantial to the touch but of course I wanted more precise measurements so using my infrared thermometer I measured an incoming temperature of 9 degrees celsius and an outgoing temperature of 18 degrees celsius.  Those are the stats with my shower running.  I have a low flow shower head that uses only 2.6 l/min (0.6 gal/min).  I thought the small amount of water might effect the temperature readings on the DWHR pipe so I did another temperature reading using the kitchen sink as a hot water source.  The water ran at a higher rate and the sink drain is also about 8 feet closer to the DWHR than the shower drain.  These readings were 7 degrees celsius at the bottom and 22 degrees celsius at the top.  The lower incoming temperature can probably be attributed to the faster incoming water (ie. the cold incoming water would have less time inthe house to warm up before entering the pipe) and the higher outgoing temperature can be attributed to either the increase in water volume or the closer proximity of the drain or some of both.  I’ll leave that for you to figure out since it is the shower data that really matters.  Back to that..

So let’s assume an average comfortable shower temperature of 40 degrees celsius.  The drain water will cool significantly by the time it actually gets to the DWHR pipe but that amount is difficult to measure.  What we know is that when the incoming water temperature is 9 degrees celsius the DWHR pipe can transfer enough energy to raise the temperature of this water to 18 degrees celsius.  This temperature increase gives my water heater a free head start.  Instead of having to heat the water from 9 to 40 degrees it only has to increase the water temperature from 18 to 40.  That means 30% of my water heating costs are immediately eliminated!  

I still believe the claims of 40% savings because my test with the kitchen sink showed better results and there are also longer DWHR pipes available that would increase the heat recovery ability.  These tests were done on my 42 inch pipe.  The pipe retails for $577 and sells under the brand name Power Pipe.  These DWHR pipes are definitely the most economical way to save energy in the long run.  There are no parts to break so it will last forever and the 30% water heating cost reduction will allow me to quickly recover the money that I invested in the unit.  I am even more appreciative of this savings in energy since I use a solar hot water heater.  To me, the free energy I get from the sun is precious and it pains me to see any of that wasted.  If your striving for off the grid living or just cutting back your energy demand I guarantee you will definitely appreciate the savings you get from your own DWHR pipe!