Economical Energy Savers

Get the most out of a tankless water heater with
drain water and passive heat recovery systems

If you’re concerned over high energy costs these days, you’re probably looking for ways to save money on home essentials such as heating hot water. Whether you have a traditional tank hot water heater or a newer energy efficient tankless hot water system, adding a drain water heat recovery system should dramatically increase your savings.

1. Using drain water heat recovery

The most well-known drain water heat recovery system is probably the GFX (gravity film heat exchanger) unfortunately the site isn’t very helpful, but how it works is almost too simple to comprehend.

The basic premise is that the GFX takes the hot water currently being wasted down the drain, such as during a shower, and uses it to heat the cold water flowing into your water heater, thereby significantly reducing the energy required to heat fresh hot water.

Do a web search for waste water heat recovery for the latest technology. One that looks very promising is EcoDrain, a Canadian company that will be operational very soon. What is nice about this product is that it will operate horizontally in a 3 foot space.The heat it recovers can be used to preheat water at the tankless intake.

In a typical home, plumbing water is transferred through an input pipe (frequently metal) to the hot water heater where it is heated to the correct temperature. After it reaches the right temperature, the water goes to the faucet and then down the drain to the sewer line.

FYI: the Dept of Energy estimates that 350 billion kWh of hot water goes down the drains annually. If there is a drain water heat recovery system installed, much of the hot water is recovered and considerably less energy-consumed.

How it works

The drain water heat recovery system is a section of copper pipe that replaces a vertical section of your existing drainpipe. This section is tightly wrapped with copper tubing. Copper pipe and tubing provides the longest service life and most productive heat transfer.

The pipe handles hot water; the coils around the pipe are connected to the incoming cold water. The cold water flows upward through the coiled tubing while the hot drain water falls downward.

During this process, the falling hot water runs down the walls of the copper tube to heat the incoming cold water in the surrounding coils as it flows either to the water storage tank or the cold water faucets. So, you are using the temperature of the draining hot water to raise the temperature of the incoming cold water but not mixing the two together.

The Drain Water Recovery System is particularly beneficial in homes with tankless or on-demand hot water heaters or solar water heaters. No matter what type of hot water heater you currently have, for the DWRS to work properly, both hot and cold water must be used simultaneously. Interestingly, the greater difference in temperature between the incoming cold water and the hot used water, the greater the efficiency of the unit.

The illustration to the right shows preheated water mixing with hot water from the water tank. For a tankless water heater it would be much more efficient to pipe the preheated water to the heater intake. Note: some models caution against preheating water above 75 F degrees.

With a system like this, some estimates claim, that it is possible to recycle 50 percent of waste water heat. Units come in various lenghts, there may be units available for $300, but I’ve seen only $500-$1200. To qualify for tax credit units must be at least 48 inches tall. Bob Vila says this is a good DIY project.

An electric tankless heater as an example:

1. Heating ground water from 50 F degrees to 115 F degrees could typically cost - for a 20 kw heater (2 GPM max) - $2.00 per hour.
2. Using drain water heat to raise water from 75 F degrees (room temperature) to 115 F degrees would decrease cost to $1.33 /hour or increase flow rate to 3.4 GPM.

Note: This system of preheating water only works when there is hot water running through the drainpipe. There should be at least 48 inches of vertical drain pipe that can be replaced.

For more information, go to www1.eere.energy.gov
http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13040 for their paper on wastewater heat recovery, or search “drain water heat recovery systems.”

2. Using Passive storage for heat recovery

Many people discard their old water tank when switching over to a tankless system but there may still be life left, if it's still in good condition.

Leaving the old tank in place allows it to be used as a pre-heater. For this to work the insulation should be removed and it should be in a basement or utility room that is warmer than ground water temperature. There is no need to have the power on; it’s doing its job by soaking up heat from the room.

One person wrote that he uses his tank with the timer set to turn on for an hour a day, this gives him the extra flow rate his tankless heater needs in the winter.

An electric tankless heater as an example:

1. Heating ground water from 50 F degrees to 115 F degrees could typically cost - for a 20kw heater (2 GPM max) - $2.00 per hour.
2. Using passive heat to raise water from 65 F degrees (room temperature) to 115 F degrees would decrease cost to $1.54 or increase flow rate to 3 GPM.

A search will bring up several water storage tank distributors Like Plastic-Mart
www.plastic-mart.com/class.php?cat=33
That offer plastic models for water storage. They may also be found under RV supplies.

If you do decide to use your old tank, be sure to put a drip pan underneath and check that you have necessary shut off valves.