Drainwater heat recovery installation

Discussion in 'Water Heater Forum, Tanks' started by jch, Dec 16, 2010.

  1. jch

    jch Member

    Got my Drainwater Heat Recovery Unit (DWHR unit) last week.

    What this does is use warm drain water to pre-heat the water that I'm using (in the shower). Depending on the size of the heat exchanger, you can cut your hot water heating bill in half (mine should drop by 33%).

    The drain water flows down the wall of the central copper pipe, and fresh water circulates through copper coils wrapped around the outside. Drain water flows down; fresh water spirals up (counter-flow for maximum heat transfer). You pipe *all* of your house's water through one of these (i.e. before it branches off to your hot water tank and your cold supply) so that the flow rates (drain vs. fresh) are equal (for maximum efficiency).

    If you don't like warm drinking water, you can branch off the cold water for your sinks (kitchen and bathroom) before you hit the DWHR unit. So then you'd have 3 distribution systems in your house: Hot, Warm, and Cold.

    It is installed by removing a piece of the main plumbing stack. You need to be careful to design the drains so that the water is sticking to the walls of the pipe, rather than in a free-fall down the centre. Originally, we had a 3" vertical shower pipe connected to the top of the 4" stack with a bushing -- not good. Needed to move the bushing upstream (i.e. before the pipe went vertical).

    Scheduled to finish hook-up this week....

    Here are the temperature values (as measured with an infrared heat gun) when my daughter was taking a shower:

    10C Cold Water feed from street
    14C Ambient air temp (in basement)

    Before shower:
    14C Temp of heat exchanger

    After running shower for 2 minutes:
    40C Temp of shower @ shower head
    34C Temp of bathtub drain (cools a lot by falling through the air, and by hitting the cold cast iron tub and brass drain pipe)
    32C Temp of heat exchanger with no water running through outer coils (not hooked up yet!)

    For this size (Power-Pipe R4-42: 42" x 4"), it's rated at an efficiency of 46.1% which means that we should get 46% of the theoretical temp rise for free ( 0.46 * (32C drain water - 10C cold water feed) = +10 C rise in cold water temp ), so it'll be as if our house is being supplied with 20C water and will only have to heat it +20C rather than +30C -- a 33% savings in hot water heating cost for showers. When it's colder outside (5C water from the street) we'll get +12.4C for free (+22.6C heating required rather than +35C heating: a 35% savings).

    Note that this will only help for showers (where it is draining at the same time as it is drawing new water). It won't do anything for baths, washing machine, dishwasher, etc. that use water at a separate time from when they drain.

    Cost $699 from Sears (cheaper than buying it direct). The manufacturer drop-shipped to me in under a week. Will get back just over $200 from the Canadian and BC governments because it's over 42% efficient. Payback period for us will be under 3 years... ($235/year, based on 60 minutes of showering per person per week x 4 people)

    A bigger heat exchanger would be more efficient (66.7% heat transfer for a R4-84), resulting in 51% savings (if you have the physical room). Here's the chart to see the efficiencies.

    This is the 4" diameter, 42" long version that I got:

    You can see that it is a 4" copper waste pipe:

    The cold potable water flows *into* the bottom of the coils and splits into 4 parallel copper pipes (to minimize flow resistance):

    At the top of the DWHR unit, the 4 water pipes are gathered back into a 3/4" copper header. This is the Warm supply for your house (showers, toilets, hot water heater input):
    Last edited: Apr 20, 2011
  2. jch

    jch Member

    Another view of the top. Notice that the coils are flattened to maximize heat conduction:

    Here's the label on the unit:

    Here's the newly-replaced drain system for the house (the old layout was a mess). 2 showers feed into this: one on the main floor (through a 2" wye that's on the backside), and one from the 2nd floor (which dumps into the 4" stack higher up):

    And here it is once the DWHR unit has been installed. A ProFlex at the top, and a Fernco at the bottom (both supplied with unit). In hindsight, I should've ordered the next size up of DWHR (R4-48") and used a ProFlex both top and bottom -- it would've given me 10% better heat recovery (saved an additional $35/year).

    Tomorrow we will finish up by connecting the supplies and wrapping it in insulation.

    I have no affiliation with these guys. Just saw DWHR units mentioned in my Home Energy Audit (which outlines how much grant money is available for various house upgrades) and started doing my research.

    I have yet to meet anyone in town yet (plumbers, inspectors, home energy assessors) who have ever seen one.

    Once it's up and running (tomorrow), I'll re-measure the temperatures and post them here.
    Last edited: Apr 20, 2011
  3. Dana

    Dana In the trades

    To get a more accurate reading with an infrared thermometer, spray a spot of flat-black paint on the pipe where you want to measure it. The IR-emissivity of bare copper is quite low- low enough to give you false-low readings with most IR thermometers. You can measure water itself at the floor of the shower with an IR themometer quite readily, since it's relatively high-E, but put a spot of something high-E on the copper if you want to be sure about both incoming and exit temps on the potable side of the heat exchanger.

    Generally speaking the tallest & fattest unit that fits will be the most cost-effective. The cost adder of the addtional length is usually proportionally less than the boost in performance derived.

    I like PowerPipe's multi-path design on the potable wrap, since it has lower flow restriction than the single-path slinky versions from some other vendors.

    With 4 people taking showers even in rapid succession, with one o' these suckers in line you never run out of hot water with a gas-fired tank heater (even an 40 gallon version). I'm sure you could deplete a 40 gallon electric with 4 people if you worked at it though.
    Last edited: Dec 16, 2010
  4. jch

    jch Member

    Yup. I used a piece of flat-black hockey tape on all the spots where I was measuring the temperature. A good Canadian approach, eh!

    During the plumbing reconfiguration, we worked to maximize the pipe length and diameter. Even though the plumber wanted 3" drain pipe, I insisted on 4".

    The vertical spacing between fittings in the basement was 51-3/8". Power-pipe says to subtract 5-1/4" to determine the largest length of DWHR unit (they come in 6" increments).

    That gave me 46-1/8" to play with (so ordered a 42" DWHR).

    In hindsight, I didn't realize they were going cheap-o on the bottom coupling (supplying me a 4-1/8" long CI-to-CI Fernco rather than a second 2-1/4" long ProFlex fitting).

    If I'd used my own ProFlex for the bottom (rather than their Fernco), then I would've have to only subtract 3-3/8", giving me just enough room for the 48" version.

  5. Dana

    Dana In the trades

    So THAT'S what hockey tape is for! (And here I'd been using it to restrain my kid from banging on the glass & throwing stuff on the ice! :) )

    Don't sweat the ~10% difference in heat recovered between the 42 and the 48- it's hardly enough to matter. It would have been a crying shame had you put in a 42 where an 84 might have fit though. Nice rebate they have going there in Victoria- it's nowhere on the public radar in my area (but I installed it anyway.) And my incoming water temps are more simlar to that Pendicton or Kelowna than in Victoria.
  6. dlarrivee

    dlarrivee New Member

    Where are you located Dana?

    I'd have a hard time shelling out $700CDN for something like this...
  7. jch

    jch Member

    I agree that it sounds like one of those "to good to be true" scenarios...

    However, I ran the numbers and for a 4-person household, each using the shower for 60 minutes per week, it pays back:
    $218.53/yr with 10C water coming into the house, and
    $268.20/yr with 5C water coming into the house.

    Totally passive system. Installs in less than 1/2 an hour. Nothing to clog, maintain, or wear out. <3 year payback, even with no grants. After that, it's free money.
    Last edited: Dec 16, 2010
  8. Dana

    Dana In the trades

    Currently in US zip code 01609 (central Massachusetts), but I hailed from Seattle orignally. (I spent quite a bit of vacation time in the B.C. Okanagan decades ago, have relatives in Vancouver, and a great fondness for the region.)

    If you're heating hot water with low priced natural gas and the shower runs fewer than 10 minutes per day it's tough to make the financial argument for $700-1000 in a short-term net-present-value analysis fuel cost savings. It's easier to make at 7.5cents/kwh electric hot water heating and 20+ minutes/day. See: http://www.renewability.com/uploads/documents/en/analysis_dwhr_minnesota.pdf At the $500 post-subsidy cost it's even more favorable, and even at $1/therm (=$2.83/m^3) it starts looking pretty good.

    But it's more than fuel savings, it's also about the hot water heating performance-boost. In showering mode you'll rougly double the first-hour capacity (or more importantly, the first HALF-hour capcacity for serial-showerers) of any tank heater. In deluxe master-bath side-spray gusher-showers drainwater heat recovery can sometimes give you the necessary showering capacity for less money than going with some monster tank with a dragon-burner to achieve that end. The fractional performance falls somewhat with higher flow, but at 4x standard flow rates a ~50% unit will still be delivering ~30-35% (of a much bigger number).
  9. jch

    jch Member

    So now that it's all connected, here are the measured numbers (using an infrared temperature gun aimed at black hockey tape, as before)...

    40.0C (104.0F) Temp of shower water (at shower head)
    34.0C (93.2F) Temp of shower water (at drain -- cools a lot falling through air!)
    32.0C (89.6F) Temp of drain water by the time it makes it to the heat exchanger

    6.0C (42.8F) Temp of incoming fresh water to house and DWHR unit
    19.0C (66.2F) Temp of "warm" fresh water exiting from DWHR unit (i.e. 13.0C/23.4F temperature rise for free)

    46.1% Manufacturer's stated efficiency for this DWHR unit
    50.0% Actual efficiency of DWHR unit, based on temperatures above

    $0.0878 Cost / kWh for electricity here (electric hot water heating) -- the Electric Company has given notice that these rates will be increasing 30% over the next 3 years

    2.5gpm Estimated gallons/minute of shower water used

    4 Number of people in the house
    60 minutes showering/week per person

    $493.14 Cost/year in showers (WITHOUT heat exchanger)
    $304.58 Cost/year in showers (WITH heat exchanger)

    $188.55 Savings/year by having this heat exchanger
    $245.03 Savings/year 3 years from now after electricity rates have increased 30%

    $700 Cost of heat exchanger
    $210 Government rebates ($80 Provincial + $130 Federal)

    Payback < 3 years

    Naturally if you have a smaller household (fewer people) or more-disciplined shower users (our neighbour says he keeps his under a minute!) then it may not make sense.

    But for us (with 2 kids, one of whom is almost a teenager), it makes sense.
    Last edited: Dec 17, 2010
  10. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    North Vancouver, BC
    Outstanding Thread John. Thanks for sharing!

    Can you come over to North Van and install one for me!

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