I have been in framing mode, after having closed up the old basement stairs. I thought it was a good idea to tackle the 1st floor bathroom floor next.
But… there is always a but. This one has to do with plumbing. I have to install the water closet drain, floor drain, and the bathtub/shower drain for the 1st floor bathroom. Along the way, I should think about energy conservation.
All right then, let’s put the framing square aside and get the pipe wrench out.
Plumbing and energy conservation
To reduce our energy load, we included a drain water heat recovery (DWHR) unit in our plans. The DWHR is a heat exchanger with a copper drain pipe that is tightly wrapped with small copper tubing coils.
The unit is installed in a vertical drain stack. The drain water coming down the stack tends to cling to the pipe walls because of the water’s surface tension.
While hot drain water flows down the copper pipe in the DWHR, fresh cold water is flowing up the heat exchanger in the coils and, in the process, picks up the heat from the hot drain water.
In other words, hot drain water enters from the top and leaves cold at the bottom, while fresh cold water enters at the bottom and leaves warm at the top. Makes sense, doesn’t it? Let the drain water go, but keep the heat for which I already paid.
Not recovering that heat can get expensive. The Department of Energy estimates that 80% to 90% of the energy we put into hot water is lost down the drain.
Because the DWHR process relies on the drain water surface tension, it is only effective in vertical applications. The longer the heat exchanger unit, the greater the heat recovery. Also, a larger pipe diameter in the heat exchanger typically yields a better recovery rate.
Side note:
A DWHR can lead to pressure loss in the cold water supply, while it is flowing through the small coils around the heat exchanger.
By running multiple coils in parallel around the central drain pipe, some manufacturers have overcome the pressure loss problem. This is something worthwhile to look out for.
Location challenge
What is the best, or the most effective location for the DWHR unit in our plumbing system?
It should be installed at the lowest point possible, which would be somewhere in the basement. That would allow us to pick up and recover heat from the 1st and 2nd floor drains.
Because the heat exchanger would be located on or above the basement floor, we cannot route any of the basement drains through the unit, thus we will not be able to recover any waste heat from basement fixtures.
Waste heat sources
What generates hot or warm drain water that should be routed through the heat exchanger?
The water closet flushes with cold water. No energy recovery there.
The clothes washer is in the basement, thus below the heat exchanger. That doesn’t matter too much, as most of the laundry is done with cold wash cycles these days. Not much energy recovery there either.
What about the sinks in the kitchen and bathroom? There will be some potential for drain water heat recovery. Considering our plans for WaterSense low flow fixtures and the very intermittent use, I suspect a very small recovery yield.
The dishwasher uses hot water. The problem is that it doesn’t draw any cold water when it discharges the hot water. I need both at the same time for the heat exchanger to work.
There are the showers. They will also be fitted with WaterSense low flow fixtures, but they will run for at least five minutes. And I will draw cold water through the heat exchanger while I have the hot water from the shower going down the drain.
After consulting with some energy experts as well as with manufacturers of DWHR systems, I was reassured that I will get the biggest bang for my buck by recovering waste heat from the shower drains.
Layout
Quick recap: The DWHR unit will be somewhere in the basement where it can pick up the drain water from the 1st and 2nd floor showers.
The 1st and 2nd floor showers should drain through their own two inch drain stack routing the water into the heat exchanger. The bottom of the heat exchanger will be connected to the sanitary sewer.
Careful placement of the DWHR unit into one of the basement storage rooms leaves the door open for a future gray water collection system. It would fit in the space between the heat exchanger and the sanitary sewer.
