Thriving On Low Carbon

A visit with David Velan of Ecodrain: drainwater heat recovery comes of age

Today David Velan of Ecodrain paid us a visit here at South Mountain so my colleague Brice Delhougne and I could see his product firsthand. A few years ago David got interested in capturing the waste heat in shower water and other hot waste water. Others have done that (Powerpipe) but they have been limited to applications where the vertical heat exchanger, usually 4-6 feet long, can fit, and still be higher than the wastewater outlet from the building. Ecodrain's innovation is a wastewater heat exchanger that can be mounted horizontally.

In the photo below, you can see the inlet and outlet, which is 2 inch PVC, and the side taps for the inlet and outlet for the potable water. The unit is about 56 inches long. The flow is countercurrent - the wastewater on top flows one way, and the potable water below flows in the opposite direction and picks up heat across the copper interface. Depending on the relative flow rates, and the pitch of the unit, the Ecodrain recovers 30-45% of the energy in the wastewater.

David went through many evolutions before arriving at his product. It has a double wall heat exchanger configuration that should satisfy codes (the product has a UPC acceptance) and square section tubes on the potable water side to maximize heat exchange contact area. The upper portion of the unit is PVC and is smooth to minimize any buildup of solids. Here's a cross section:

One of the Ecodrain innovations is seen in each square tube - it's a plastic insert designed to keep the flow highly turbulent to increase heat transfer.

The bottom of the unit has a drain hole so that any leaks become obvious quickly.

The unit is designed to work with very slight pitch, like a typical plumbing waste line, but it can be pitched at more severe slopes, and the increased velocity of the wastewater increases heat recovery.

The unit can be plumbed so that the preheated water flows to the cold side of the shower, or to both the shower and the cold inlet of the water heater. The latter equal flow configuration will harvest the most BTUs.

Let's do some Marc's Sloppy Math. Say a family uses 30 gallons/day of shower water and 1/3 of that energy is recovered. That's about 625 kWh/year. At $0.20/kWh (MA residents are about to see a 30-35% rise in the supply portion of their electric bills due to last winter's natural gas price squeeze!) that's $125/year. The unit costs $440, and let's say it ends up at an installed cost of $1,000. There's a simple 8 year payback. With natural gas this will be worse. To get 625 kWh/year from PV would take about 500W of panel, which costs more than the Ecodrain installed.

One application David related to me is using the Ecodrain in cases where people are running out of hot water. Recovering drainwater heat effectively stretches the storage capacity of a water heater.

David kindly left us with one and we're looking for a good application, so we can monitor its performance in an actual situation. We were impressed with the iterative thoughtful engineering embodied in the product, it's rugged construction, and the testing Ecodrain has done to show energy recovered and its clog-free performance.

Energy Positive in First Year

We moved in on November 1st, 2013, and the PV system had been activated on October 20th. The NSTAR net meter read zero when installed, read 99980 (it goes down from zero if the energy is net exported) on Nov 8th 2013, and read 98852 on Nov 10th 2014, for a net energy export of 1,128 kWh in that year. The PV system made 6,198 kWh November through October, so we used 5,058 kWH for the year (these numbers are slightly off because of the non-simultaneity of the utility meter "year" and the fact that I read the PV meter on the 1st of each month). The monthly energy flow to and from the grid are shown in the graph below, assuming for simplicity that the NSTAR meter read zero on November 1st 2013.

Usage was a tad bit higher than I projected (somewhere around 4,700 kWh/year). One reason is the minisplit heat pump - it took me until early March to change the settings at the deeper levels of the controls that were enabling the unit to operate by cycling on at full speed then shutting down, repeatedly, working at its least efficient operating point. The subject of another post to be sure...

The eMonitor wasn't up and running until late January, so I don't have a full year's breakdown of where the energy is going, but I'd say the heat pump used almost 35% of the energy, the heat pump water heater just over 10%, and the fridge and chest freezer about 15%. The remaining 40% is cooking, water pumping (we planted several dozen trees and shrubs which we irrigated), lights, the HRV, and plug loads.

The net exported energy would power an electric vehicle for over 3,000 miles/year.

 

What we started with

This house has an interesting history. The owner of the place had been living on the lot in a structure that began its life as the body of a box truck. It was 8'x16' and had a small attached shed that housed the water pressure tank and the water heater. A small gambrel loft had been built on top - I could just barely sit up inside. There was a small gas heater, a 100A electric panel, a sink, and some built-ins. No shower. There was an outhouse on the property. You might say this was a tiny house before the Tiny House movement began.

The owner learned that there was a house in Edgartown that the property owner wanted to remove, and that he could have it for free if he moved it. The house was a stubby L shape, with a 16'x32' section that contained a kitchen, bedroom, and bathroom, and a 22'x25' section with living area. What happened is conjecture on my part - I think in haste to get the project underway that it is possible that the new foundation was put into place before the conversation with the house mover occurred. I hypothesize that the house mover looked at the 22'x25' piece and said it would be difficult/costly/impractical to move. So in the end the 16'x32' portion was severed from the rest and moved, and the 22'x25' section was duplicated new on site. All this happened in 2002-2003. I suspect the portion that was moved wasn't much older, as the glass in the Andersen windows was dated 1996, and the walls were framed with 2x6s.

The interior finish of the house was no great shakes - green carpet and white vinyl flooring, and inexpensive kitchen cabinets. There were some unpleasant odors, traceable to some rodent activity, and a 140 ft2 patch of basement floor where the water came in from the well and the pressure tank and water heater sat that didn't have a concrete slab, just crushed stone over filter fabric. Smelly soil gases. Plus, the insulation was in the ceiling of the basement, so the basement ran cold, which on MV means mold.

There was a very creative outdoor shower, complete with clawfoot tub (bathtubs are another tale in the future):

The floor plan had only one bedroom, and the south side was very modestly glazed, so we knew we were in for an interior gutting project and major re-framing of the exterior openings. Note to self - this is expensive, it's better to buy a house with a floor plan and orientation you like! Once the drywall and fiberglass were removed, it was clear there was some creative structural design incorporated. The 16'x32' portion had no structural ridge or ties across, so it was held up by paint. The rear had three substantial wood ties across at roughly 6-7 ft centers, but we needed to remove the drywall to see that each was attached to the wall with a single 1/2 inch diameter lag screw - about 1/10th of the fastening capacity the design load would merit. Good thing it hadn't seen a significant snow load.

A sensible person may ask, why did we buy this house anyway? We looked at a couple of parcels in West Tisbury (we were clear we wanted to stay in this town) and I felt that I didn't want to go through the disturbance process we do with raw land. I've been preaching that we need to fix what we have already built. I can assure you now that I have put my money, far too much of it in fact, where my mouth is. When all was said and done, we paid about $100,000 more for this house than a parcel would have cost, and we got an excellent well, a Title V compliant four bedroom septic system (this is a good thing - we could expand), two underground electrical services, one 100A serving the outbuildings, and one 200A service for the house, two funky but useful outbuildings, an excellent concrete foundation housing 1,000 ft2 of basement, and a developed site. You might say everything above the mudsills was a modest bonus - a decent floor frame in the portion built on site, a ten year old roof in excellent condition, a 12'x24' deck. Oh, and a buried 250 gallon propane tank, still mostly full of gas.

The house had a gas water heater and a gas furnace. The furnace was in a tiny attic-like spot in the 16'x32' part, and I could see outdoors through the eave vents from the furnace location. The thermal boundary was, like the framing, creative. The blower door number was a tad over 3,100 CFM50, about 0.63 CFM50 per ft2 of shell area. We aim for 0.05 CFM50 per ft2 of shell area at South Mountain and frequently do better. In this project we reduced the leakage ratio 25:1. More on that to come.