Grey Water Heat Recovery from Exposed Drain Pipes

I've been thinking a bit more about simple ways to recover some of the heat that goes down the drain when you take a shower.

Think about the shower process from an energy point of view:

- Water comes in from the street at about 50F.

- Your hot water heater heats the water up to about 110F shower temperature. This uses about 10,000 BTU (3 KWH) for a typical 10 minute shower with a lowish flow shower head.

- The shower water flows over you in the shower, and goes down the drain at about 100F, taking with it about 85% of the energy you just spent heating it up -- what a waste!

Multiply this by however many showers your family takes in a day. For a family of 4, just the wasted can amount to about 300 KWH a month -- and about 500 lbs of CO2 emissions a month.

So, there must be some good way to make use of this energy we are sending down the drain?

Here are some existing methods that people have used or thought about:

- There are some commercial solutions that transfer some of the heat in the outgoing shower water into the incoming cold shower water to preheat it.

There are some thoughts on DIY schemes to do the same...

- Of course, if you take tub baths, a simple way to recover the heat in the water is just to let it cool off in the tub before you pull the drain plug. This will recover most of the heat in the bath water down to room temp.

- I guess you might think about a way to run the shower drain into the bathtub to accomplish the same thing?

While it seems like these methods should get a lot more attention than they do, none of them seems to be taking off in popularity.

Another Approach to Recovering Shower Water Heat

This page considers a method that is not as effective as the ones just above, but that is simpler to put accomplish, and appears to have some potential (at least in heating climates) for useful energy saving.

The idea is to recover the heat in the draining shower water for space heating.

This is not as good as preheating the incoming shower water (discussed above) because 1) during the summer you don't need the heat, so its a seasonal recovery, and 2) you can only recover the heat from the drain temperature of about 100F down to about 70F room temperature.

Anyway, the method I was thinking about is to arrange the shower drain plumbing in such as way that the water stays in the drain plumbing until it cools off. The heat it loses while cooling off becomes space heating to the house, and reduces furnace fuel.

One thought on doing this would be to add enough large diameter drain pipe below the existing shower trap to hold about 1 showers worth of water (say 15 gallons). The extra drain pipe would be arranged so that it fills up completely, and then as more water comes in, the water in the added drain piping "overflows" into the regular drain piping and out to the sewer or septic tank as usual.

If 4 inch PVC drain pipe were used to store one showers worth of water, it would take about 23 ft of added pipe to store 15 gallons.

I would want to arrange the plumbing so that the extra 4 inch PVC can be easily bypassed in the summer when you don't want to recover the heat.

The heat recovery potential is about:

Water used per 10 min shower = (1.8 gpm)(10 min) = 18 gallons = 150 lbs

Heat recovered = (150 lbs) (100F - 70F) (1 BTU/lb-F) = 4500 BTU per shower -- about 1.3 KWH per shower

This might be increased somewhat by insulating the shower better, so there is less cool off of the shower water from the shower head to the shower drain... This might get the savings up to about 1.7 KWH per shower -- which is a bit more than half the energy that went into heating the shower water -- not so bad?

I am guessing that this scheme may not meet plumbing codes(?). But, the added "storage pipe" would be beyond the regular shower trap, and would be vented by the regular vent system. So, it seems to me that the potential for health or odor problems is minimal?

I'd like to hear anyone's thoughts on this -- gary... Is it worth giving this a try?

A nice side issue on this is that it can be used in addition to solar water heating -- basically you can get (with a good system) nearly 100% of your water heated by solar, and then recover 50% or so of that heat again for space heating -- what a deal!

A Rough Test of the Drain Water Heat Recovery Potential

This is a quick test to assess the potential of drain pipe recovery.

In my house, there is a conditioned crawl space under the main bedroom/shower area. The drain pipe from this bathroom goes all the way across the crawl space and exits the other side. I thought I might get an idea of the potential for recovering heat from drain water by just measuring how much heat this existing drain pipe transfers to the crawl space now.

The plot below shows the result.

From a heat recovery point of view, it appears that at any given time during the shower, the water temperature drops about 9 to 10F from the start of the 22 ft pipe to the end.

So, the heat recovered to the crawl space is about:

Water used per 10 min shower = (1.8 gpm)(10 min) = 18 gallons = 150 lbs

Heat recovered = (150 lbs) (10F) (1 BTU/lb-F) = 1500 BTU per shower -- about 0.5 KWH

The logic here is that the heat that is coming out of the water (as calculated from its temperature drop) is going into the the crawl space. Since ours is a conditioned crawl space (unvented and insulated), this counts as space heating energy.

These numbers seem worthwhile, and this is just from a pipe that happens to run through the crawl space with no attempt to retain the water long enough to lose all its heat. It should be possible to do much better if the plumbing is rearranged as discussed above to retain the grey water until the heat is all transferred.

Plot shows the drop in temperature within the 22 ft long drain pipe.

Purple -- Temperature of water going down shower drain

Red dashed -- Temperature of outside of drain pipe at start of 22 ft drain pipe run

Blue dashed -- Temperature of outside of drain pipe at end of 22 ft drain pipe run

Green dotted -- Ambient crawl space temperature

All in degrees F.

During the shower period, the water in the drain pipe warms up about 9 to 10F over the 22 ft of exposed pipe.

The purple line is the temp of the water going down the shower drain. The brief increase in this temperature at 50 minutes is caused by moving the sensor briefly from the drain area up to the shower head -- so, you can see that the temperature of the water drops about 10F from the shower head to the drain.

The reason that the pipe temps don't follow the purple shower drain temps more closely is that the drain pipe sensors are mounted on the outside of the pipe (with insulation over them), and there is some lag due to the sensor reaction time and the mass of the pipe.

I did not thing it was a good idea to drill holes in the sewer pipe to put the sensors right in the flow :)

Click on pictures for full size

Temperature sensor at start of 22 ft pipe The sensor is under the taped on pipe insulation.	View down the long section of 3 inch PVC drain pipe that crosses the crawl space.
The Onset Computer 4 channel logger.	Sensor in the shower drain.

Mercifully, no pictures were taken of during the actual shower event.

Gary March 26, 2010