Published Monday September 30, 2013: Updated January 20, 2014
Drain-water Heat Recovery Unit
After heating and air conditioning, hot water heating is the highest energy consumer in a household. It takes lots of energy to make cold water hot. To raise the temperature of 40 gallons of tap water by 50°F takes 16,600 BTU or 4.88 kWh of energy. That doesn't even include all the inefficiencies that occur in the water heater.
Natural gas water heaters are only about 55% efficient. The most efficient on-demand gas water heaters are only 82% efficient. Electric tank water heaters are 90% efficient.
Regardless of water heater efficiency, after you take your shower, wash a load of laundry or dishes, nearly all the energy that went into making the water hot and all the money you spent on that energy, is thrown away, down the sewer pipe.
If only there were a simple and affordable way to re-capture that wasted energy and money.
Drain Water Heat Recovery
Enter the Drain-water Heat-recovery or DWHR unit.
When water runs down a vertical sewer pipe, rather than falling down the center very quickly, it glides down the inside wall of the pipe. In a heat recovering drain, a section of the vertical sewer pipe is replaced with a thermally conductive copper one where a smaller, copper, culinary water pipe is wrapped around the copper sewer pipe. In this manner, heat from warm/hot waste water (and nothing else, ew!) is transferred to the cold water on its way to the water heater.
A typical system costs $600-$1000 depending on the sewer pipe diameter and length of the energy recovering unit. The ROI ranges from 2-9 years depending on energy costs, temperature of the incoming cold water supply and how much hot water you use. Much of the heat from shower water is reclaimed because there is a continual flow of warm waste water preheating the continual flow of cold water entering the water heater.
After thinking about it over the past few years, I finally made the decision to buy and install one.
DWHR and I backstage while competing on Dancing with the Stars ;)
To maximize heat recovery, I wanted to buy the longest DWHR unit that would still fit in the sewer stack.
Unfortunately the plumber who originally installed our home’s sewer stack put in a 45° section about ½ way up that tied to the horizontal sewer pipe.
To gain more vertical drop in the stack, I removed the long 45° section, replaced it with a longer vertical, longer horizontal and a sweeping 90° section.
I was hoping to end up with 68” of vertical fall but soon realized a tee and a clean-out tee lower down the stack could not be consolidated into one fitting. With only 56” of vertical fall available, I settled for a 48” DWHR (which requires 54” of clearance for installation).
Tip #1: Whenever you’re cutting into a sewer stack, make darn sure that nobody is using any fixtures above you, especially toilets. Ew!
The removal and installation took me almost all day but it was fairly uneventful.
It looks so good, I can't bare to cover it all up inside the wall. So I won't.
The DWHR unit all displayed like a trophy.
Most of the water in Kaysville City comes from alpine sources in the Weber water district. In the winter time, these streams are just a few degrees above freezing. As the frigid water travels along the underground pipelines, it is warmed up by the ambient ground surrounding it.
If you live close to the source of a mountain water supply, the tap water entering your home will be much colder than it would be for someone who lives farther down the water pipeline.
A drain-water heat-recovery system would make a ton of sense for anyone living close to the Alpine water source.
Due to record high July and near record high August summer temperatures, the city’s culinary water supply was a luke warm, (70-72°F).
Look how much the water temperature varies throughout the year. Wow!
Thanks to the Weber Water Disctric for providing this data on seasonal water temperatures in Kaysville, UT (from the pipeline off Highway 89 near Crestwood Dr)
BTU's vs. kilowatt-hours
The British Thermal unit is based off of weight and temperature units that the British abandoned a long time ago but the BTU it is still an industry standard.
BTU is the amount of energy required to raise 1 pound of water by 1 degree Fahrenheit. That's about as arbitrary as using furlongs per fortnight to measure velocity.
I would much rather use the kilo-watt-hour. That way, every energy measurement, (whether gas or electric), can be compared on the same energy measuring stick.
1 kWh = BTU/3412.
The water temperature for a typical shower is about 102°F. Much of the water's heat in a shower goes toward:
- Warming the occupant
- Warming up the air in the bathroom
- Warming up the walls and floor of the shower itself.
The DWHR unit will only recover remaining heat in the drain water as it goes down the sewer pipe.
After several months of use, the results are in:
For a 1.5 gpm shower, the efficiency of our 48" DWHR unit is about 64%.
|Time||Drain In °C||Water In °C||Water Out °C||Efficiency|
|9 min||21.6||7.8||16.8|| 65.2%|
Efficiency is lower at the beginning of a shower (as the drain pipes warm up) and higher at the end of the shower. Efficiency will be lower for a higher flow-rate shower, (one more reason to switch to lower flow-rate shower heads). When using ultra-slow-mode on the shower head (0.75 gpm) efficiency of the DWHR shoots up to nearly 81%.
While local energy recovery efficiency is very high, the whole house water heater energy savings end up coming in around 35%, which is still not bad at all.
By following the water saving tips (in this previous article), you will reduce your household water usage. When the water savings is from hot water, huge energy savings will result.
Our house is large and spread out. The kitchen is on one end of the house, the master bathroom is on the other and the water heater is in the middle of the basement. Most of the bathrooms drain into one sewer stack and the kitchen and laundry drain into another sewer stack.
It would be ideal if everything flowed into a single sewer stack, that way the DWHR could recover all that wasted drain heat.
Although roomy, comfortable and luxurious, on a thermodynamic, “conservation of energy” standpoint, our house does not have an energy efficient plumbing layout. Oh well, it is what it is.
It takes a while running the faucet before the water gets hot and there is a lot of sewer line that separates the bathroom showers from the vertical stack where I installed the DWHR unit.
I already hyper insulated the hot water lines going to each fixture. I’m starting to think that I need to insulate the horizontal sewer lines as well. That way the drain water will stay hotter for longer and more heat can be recaptured in the DWHR.
"Insulate sewer lines? Nobody does that! You are insane!
That's crazy talk!"
Sewer pipe before and after being insulated.
Insulating the horizontal sewer lines have not made any measurable difference in efficiency gains to the drain-water heat-recovery unit.
It still takes a couple minutes to pre-heat the drain pipes anyway. The water travels down the drain pipe so quickly, there isn't much time for heat loss, through the sewer pipe, into the ambient air to occur. Insulating the sewer pipes will probably help efficiency during another shower 10-15 minutes later since it will keep the sewer pipe warmer for longer. I have not collected enough data to confirm this though. Gains are probably so small that it isn't worth worrying about.
Adding insulation to the sewer pipe does make it significantly quieter though. So maybe it isn't such a waste of time/money to do this after-all. I paid about $12 for a single roll of R-13 insulation, enough to insulate a 30' length of sewer pipe. It took about 40 minutes to install it.