Drain Water Heat Recovery

The other week I finished installing a drain water heat recovery (DWHR) unit (shown in photo to right)

If you don't know what these are, they are a VERY simple device that recovers heat from the sewer stack (primarily from shower water) to pre-heat cold water coming into the house. They are sometimes called gravity film xchange (GFX). The optimum configuration is to pre-heat both the cold water going to the shower and supplying water to the hot water tank as illustrated below:







I ordered mine from ECOInnovation Technologies Inc. in Quebec. I've had a couple of dealings with these folks, and they are very helpful, responsive and good to deal with. The first time I ordered the wrong size - a 4" diameter unit, when in fact my sewer stack is the more standard 3" diameter. Doh. They were very good about doing the return.

In the end, I ended up getting a S3-60 which is a 60" long unit and has a heat recovery efficiency of 68%. The unit has a 3/4" water supply, which splits into two 1/2" lines around the sewer stack. The purpose of this is to minimize pressure drop and increase heat recovery efficiency. My unit cost about $980 once shipping and tax was included, so not that cheap. Given the amount of copper in the thing, I'm not surprised. Then there is another $50+ dollars with of misc. plumbing parts (couplings, pipe, fittings, etc.). You can get smaller units (40" long) which are only slightly less efficient for $500 to $600. Those are probably a better return on investment because your get more bang for your buck.

Note that Energy Solutions Centre is now including DWHR devices in the Good Energy program, and you can get a $100 rebate from them.

Energy Savings
These units have been validated by NRCAN through a number of studies you can find online. In fact, they've commissioned an on-line DWHR calculator at http://www.ceati.com/calculator/. I ran my setup through this calculator to see what my savings would be. I logged our family showering patterns for a couple of weeks (ya, what a nerd) - we take on average 1.35 showers a day, with an average shower time of 9 minutes (I shower WAY longer than Georgi). So, based on the calculator and our unit, we should be saving about 550 kWh / year, or at a marginal electrical cost of $0.14 / kWh, that is about $77/year. When Finn starts showering, obviously these savings will increase. Furthermore, if marginal electrical rates rise to $0.22/kWh as proposed by Yukon Energy, the savings during winter time (when we use a lot of electricity for space heating), would be $121 / yr (although less in summer when we use the cheaper first block electricity).

So, at current prices, the simple payback is about 12-years, again a bit mediocre as far as energy efficiency upgrades. From a return on investment perspective, the unit has a IRR of 8% over a 25 year period. Quite a bit better than any of my RRSP's are returning these days!


Installation
Theoretically, the installation of these should be pretty easy. In new construction, I think this is a no-brainer. However, things never go smoothly as planned....

1) because my sewer line is HIGHER than my basement, I don't have space on the sewer stack in the basement to install the DWHR, so I had to cut the wall open on the main floor and install the unit in the wall cavity. That was okay, because I wanted to rip the drywall off anyway to install wood paneling in my front entry, however....

2) the stack was in the wrong place (not where I designed it to sit in a nice big wall cavity), rather it is in a skinny section of wall, too narrow to accommodate the diameter of the DWHR. So, I had to fur out the wall to accommodate the unit, and....

3) my lovely plumber also put a "jog" in the sewer stack there, so not only did I have to cut out the stack to for the DWHR, but also had to move the jog in the stack- making the overall length of the installation too long for the hole I cut in the wall. So, I had to cut the hole bigger, meaning I have to patch the drywall (yuck). Then....

4) I decided to install the unit on a Monday night, after 8pm when we put Finn to bed. So, that means the store is closed if anything goes wrong. There was no shut off valve between the supply to the house and the hot water tank, so I had to shut off all the water to the house.

I hooked up the unit with relatively no problems, but once I tested it, it didn't seem to be working right (the cold water supply to the shower wasn't warming up). Then I realized I had installed the water supply lines BACKWARDS!!!. I had to completely disassemble the nice plumbing job I did, and in the process, got totally soaked, spraying water all over the place and creating a minor flood in the basement. A very Chevy Chase like scenario. AND, I was running out of PEX fittings, so was totally stressed about not being able to complete the job. If it wasn't hooked up, then I couldn't turn the water on, and we'd be without water over night. I was not getting impressed looks from Georgi.

Finally, I did get it installed, and it works great now. When showering, you set the shower a bit cooler than previously (because the cold is being pre-heated, reducing the amount of hot water used). Also, you really can feel the heat on the outlet to the DWHR, so it does work.

Overall, it is a very simple device, and does work. As with most things, install it as part of new construction for far less "reno-pain”.