When you think about what goes on energy wise when you take a shower, its mind boggling wasteful. Your hot water tank heats up several gallons of water from about 60F to about 120F typically using about 6000 BTU worth of energy. The water flows over you once, and then right down the drain taking nearly all of that 6000 BTU of energy you just put into it right down the drain.
Tyler has worked out a pretty simple heat exchanger to extract some of the heat from the shower water as it goes down the drain and use this heat provide some of the heat to the incoming shower water.
in Tyler's design, the cold water on its way to the shower goes through a heat exchanger where it picks up heat from the draining shower water.
Thanks very much to Tyler for sending this in!
From Tyler:
Design and Construction:
I use a heat exchanger in my shower drain that runs the cold water line of my shower through and takes the grey water from both my showers and my tub. The exchanger is just 4" ABS with a bunch of loops of 1/2" pex inside.
This shows the new 4 inch ABS pipe section and the coil of PEX that will be used to make the drain water heat exchanger..
Putting together the bunndle of PEX that will be placed in the new section of shower drain. The PEX connected with brass fittings with crimp rings.
The bundle of PEX being placed in the new shower drain. Once the bundle is in place, a cap with holes for the two PEX lines is installed and sealed with silcone caulk.
The new section of drain stack with the PEX in it is on the left. The newly routed 2 inch shower/tub drain comes in near the top left of the new drain stack. The new drain stack is joined back into the main stack as shown on the picture on the right. All of these pipes will be enclosed in a new planned wall with access for any required servicing. All of this plumbing is located in the basement.
The 4 inch pipe on the right is the orginal stack that served all the showers toilets and sinks. I just separated the the shower/tub drain and run them through the new stack on the left with the PEX heat exchanger, and then rejoin the two stacks as shown in the right picture.
The shower drain water just gravity feeds through the PEX heat exchanger area.
The top of new shower drain stack is sealed with a pipe cap with holes drilled for the PEX and then sealed with silicone caulk.
There are valves in the ceiling to isolate the two PEX lines going to the heat exchanger that allow it to be bypassed if there is a problem with it.
Performance and Cost
After the first trial it warms the incoming cold water from 18c(66F) to 25c(77F) with 35c(95F) drain water. I thought it would do more but since my cold water is fairly warm already (stored in the basement) the lower temp differential probably hurts efficiency... It is quite noticeably hotter in the shower and I'm able to turn it down quite a bit!
If you assume a 10C (18F) warmup of the water going through the heat excanger (based on somewhat cooler cold water than Tyler has).
The, the energy saving for a 10 gallon shower is about 1500 BTU per shower -- an about 30% saving in energy to heat shower water.
If you assume three showers a day, and an 90% efficient water heater, the saving per year is about 2.0 million BTU per year, or nearly 600 KWH per year.
Further optimization of the heat exchanger and drain pipe might gain further energy savings.
The total cost of materials was about $180.
The saving for 600 KWH of electricity here in Saskatchewan is about $70.
Potential Improvements
Efficiency improvements- there was room for more pex loops which would slow down the flow and increase surface area but I left some space as I was afraid of water backing up. I think there could be a few more added though without any worries.
I had thought of different ways to use the 4" abs as a tank kept full at all times but I couldn't come up with a good design to avoid buildup of dirt/sludge and it would have a greater potential for leaks or spills. Also it would lose heat to the basement and have a slower response whereas this system gives a fairly immediate benefit. I would think a horizontal type exchanger would be more efficient but would be a little more difficult to fit and support.
Tyler
March 17, 2014
Other Gray Water Heat Recovery References
A couple links to other grey water heat recovery ideas and projects:
