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Drain Water Heat Recovery

Norman Bunn | Posted in Mechanicals on

I plan to install a drain water heat recovery unit in my new home. Does anyone know of any guidelines on sizing? For example,there will normally be 2 people in the house, but on occasion that could easily double or more as we have extended stay guests (a week or even more). And we will have a 65 gallon heat pump water heater.

As the DWHR units tend to get pricey due to the copper involved, I want to make sure I get the most bang for the buck.

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Replies

  1. Norman Bunn | | #1

    CZ 3A

    Norman

  2. Expert Member
    Dana Dorsett | | #2

    A heat pump water heater takes away the economics of drainwater heat recovery unless it's additional performance allows the water heater to be down-sized to a smaller/cheaper unit.

    A 65 gallon HPWH is enough for most 2- person families, but at 4 people the slow recovery time would require some scheduling of showers (2 showers in a row or one tub fill would pretty much deplete it) unless a decent sized drainwater recovery unit were in place. The 65 gallon unit is good for 2, maybe three sequential showers, an 80 gallon unit would give you enough for 3-4, but so would adding drainwater heat recovery, and possibly more. If you don't have any big tubs to fill a decent drainwater heat recovery would allow you to safely downsize to 50 gallons.

    The "right size" for drainwater recovery units is always "the fattest and tallest one that fits". The installation labor is about the same no matter what size gets installed. The increased efficiency performance of fatter + taller "pays back" quicker in terms of reduced energy use, and provides more "apparent capacity" in showering mode. Anything smaller than a 4" x 48" (~50% recovery efficiency at 2.5gpm) isn't going to make a huge difference, but a 4" x 72" or 4" x 80" or taller (if it fits) can do quite a lot, returning >60% of the heat back into the incoming water stream at 2.5gpm, 70% or higher if your showerhead flows are under 2gpm.

    So you're looking at about a grand (give or take a couple hundred USD) for the hardware cost of a drainwater heat recovery unit worth owning. Compare that to the cost of upsizing to an 80gallon unit, bearing in mind that the bigger water heater doesn't save ANY energy, and thus never "pays back" in energy whereas the heat recovery unit does deliver an energy use dividend, and it will last 3-5x as long as the water heater.

  3. Jamie B | | #3

    Hey Dana,

    I've always wondered this and you might have some insight.

    Would a drainwater heat recovery unit effect water pressure? I look at them, and see how the larger the unit, the more length of tubing the water has to travel through a coil before going back to the water heater and up to the fixture. I assume this would create a pressure loss.

    Thoughts?

  4. Expert Member
    Dana Dorsett | | #4

    Any heat exchanger will have a pressure drop that varies with flow. To mitigate against that have multiple paths off a manifold at each end (Renewability Powerpipe has that on all of their lineup) or is split into two equal paths, one wrap at the top half of the drain, and another at the bottom half (multiple vendors.) This is more important for taller units and/or higher flow applications.

    http://renewability.com/wp-content/uploads/2017/05/Power-Pipe-1024x1024.jpg

    https://buildingscience.com/sites/default/files/migrate/jpg/4.4_Drainwater_Heat_Recovery_web.jpg

    http://corvallisdowntownliving.com/Drain-Water%20Heat%20Recovery_files/8%20Power%20pipe.png

  5. Norman Bunn | | #5

    As a follow up, I notice that many (all?) of the manufacturers do not insulate their units. Is there a reason, other than cost for not doing this? Is there any advantage to doing so?

    Also, it would seem advantageous to remove the toilets from the DWHR unit if possible, since that flow would tend to wash away any residual heat. Of course, in a small household that may not be an issue. May be worth a discussion with my plumber as we scope out the plumbing plan.

  6. Lance Peters | | #6

    From what I've read, it is beneficial to route the main water line through the DWHR unit so that all incoming water will go through it. It makes sense as the higher the cold water flow rate the more heat will be recovered, though this can create an issue if the flow restriction is high. It might be worth looking at two shorter units stacked on top of each other and plumbed in parallel rather than one long unit.

    Insulating the DWHR unit would probably help, just like insulating your water pipes would.

  7. GBA Editor
    Martin Holladay | | #7

    Insulating a drainwater heat recovery device has very little effect on the unit's efficiency or performance, which is why insulation of these units is rare.

    You don't want all of the home's water flow to be piped to one of these units. Whatever tap a family uses to get a glass of drinking water should be as cold as possible, and therefore not routed through a drainwater heat recovery device.

  8. Expert Member
    Dana Dorsett | | #8

    Don't worry about toilets sharing the same drain. Heat exchange efficiency goes way DOWN at high flow rates in the drain, since the heat exchange is at the film of water clinging to the interior side of the vertical copper drain. A high flow has a much thicker film, and a lot more "waterfall" volume that never touches the walls of the heat exchanger. I have a 4" x 48" downstream of an upstairs bathroom- there is no detectible change in the temperature in the shower when somebody flushes.

    The surface temperature of even a fairly tall heat exchanger is rarely much over 80F (and never over ~75-ish with my 4" x 48") even at the output end, and below room temperature at the input end. Insulating the heat exchanger would be counterproductive, since over half of it would normally be room temperature or lower when operating. While 75-80F water is WAY above the temp of 35F incoming water, it's still pretty tepid, and losses from the pluming to the room (even a cool basement) are small, unlike, say, 120F water in the hot water distribution plumbing which SHOULD be insulated. R3 is the current code-minimum.

    The whole issue of tepid-water distribution out of the heat exchanger into the cold water distribution is also a non-problem. Opening up another tap increases the flow on the potable side, reducing the heat exchange per-gallon, delivering a lower output temperature. Even if it's 80F at 2gpm, it will be room temperature or a bit lower at 4gpm. Most other cold water uses can tolerate room temperature water.

  9. Lance Peters | | #9

    Martin,

    I've been playing around with DWHR plumbing ideas the last while, doing a bunch of reading and trying to figure out what would make the most sense. I completely agree with you that the main source of drinking water should bypass the DWHR unit for the reason you state. My current plan would be to run the main water line to the DWHR unit with one line branching off before that for our water filtration system and on to the kitchen sink, fridge etc.

    Routing all other water through the DWHR unit makes sure that the water going to both the water heater and the water going to the cold side of the shower are both being pre-warmed. The higher flow rate through the DWHR unit also means higher overall heat recovery.

    Regarding insulation, with cold ground water circulating through exposed copper pipe I believe the same concerns regarding condensation exist. Water demand along with drain water flow through the DWHR would keep the temperatures elevated, but code does allow multiple shower drains where not all are routed through the DWHR unit. In that scenario, cold water will be flowing through the DWHR coil with no drain water to warm it, in which case insulating the DWHR coil would be as beneficial as insulating any other cold water line.

  10. Jon R | | #10

    Think about just how low you can go on shower head flow (eg, 1 GPM) and whether there are any concerns about hard water deposits in the heat exchanger causing a reduction in efficiency (and more pressure drop).

  11. Norman Bunn | | #11

    Where I live hard water is not an issue, thankfully. I would think a whole house water filter would be good idea, regardless.

    Does anyone have any recommendations? The last ones I used were GE ones that leaked from the pressure relief valve all the time. Totally worthless.

    Also, what about pressure regulators? Given the potential pressure drop should these be set higher to compensate? If so, won't that impact the seals on faucets and other valves?

  12. Davor Radman | | #12

    @Dana

    Wow, those are some scary numbers you gave for shower water needs. 65 gallons for 2 showers?! I can't fathom how could anyone ever use that much water for a shower? Do you just leave it running the whole time, and take extended showers? Isn't this water also mixed with some cooler water? Or is it low temp?

    The way we usually take showers is, use it for a few seconds to a minute max, to get wet. Turn off. Soap. Rinse up to a minute. With mostly low flow shower heads.

    The general consensus here, when I was sizing, is that 50 gallon (200l) is more than adequate for 4 people. Though most go for 30 gallons (120l), and some go for 80 gallons.

  13. Norman Bunn | | #13

    Davor,

    HPWH do not recover as fast as standard ones; it's a trade off for the extra efficiency. So, you need a bit more capacity. In fact some utility rebates for HPWHs require a larger tank than would normally be considered. Adding the DWHR unit will allow the water heater to rise to the occasion when we might have multiple showers going on at the same time with extended stay guests.

  14. Expert Member
    Michael Maines | | #14

    The average shower length in the US is 8.1 minutes, though teenagers are notorious for taking much longer. Turning the water off when you're lathering up makes a lot of sense but it is definitely not the norm in the US. Since 1992, new showerheads have to be 2.5 GPM or less, though older showerheads or multi-head units may be higher. There are some decent low-flow showerheads that only use 1.5 GPM. Water heaters should be set at 120°F to 140°F. Typical shower temperature is 105°. So when you shower you're using 75% to 88% hot water, or 15 to 18 gallons of hot water per shower.

    On the more efficient end, a 1.5 GPM showerhead, 140° hot water and a 5-minute shower would be just under 6 gallons of hot water per shower. But if you set your tank at 120° (for less heat loss at the tank) and take a 15-minute shower with a standard 2.5 GPM showerhead, you'll use 33 gallons of hot water per shower. (As an aside, that's roughly 15¢ to 90¢ of hot water per shower at typical electric rates.) So two showers can use from 24% to 130% of the hot water available from a 50-gal tank. Or 15% to 83% of an 80-gal tank.

  15. Expert Member
    Dana Dorsett | | #15

    @Davor: Yes, US-Americans are real hot water pigs compared to most Europeans! Most have the water running the whole time. Michael Maines' quoted 8 minutes is just the average, with some people regularly going beyond 12 minutes. Luxury ensuite showers often have multiple side sprays at 1gpm+ each too, and need huge water heaters.

  16. Jan Juran | | #16

    Hi Norman: the newest generation of heat pump hot water heaters feature significantly better efficiency. I recently installed a Rheem model XE50T10HD50U0 (50 gallon tank) in my house; it features a 3.5 Energy Factor. Price was $1199 at the local big box retailer (before 50% local utility rebate). Rheem 65 and 80 gallon models are available also. Perhaps the most cost effective
    "capacity and energy saving" idea would be using high efficiency showerheads such as the 1.25 gpm Niagara Earth model cost=$7 or so each; I have had many guests in my 4 shower house but zero complaints re these so far. Drainwater Heat Recovery devices are almost always cost effective if they a)fit your vertical space b)can be fitted during new construction at reasonable installation cost (I would like to install one but I would have to disassemble too much of my house to retrofit one) and c)if they allow you to downsize the hot water heater tank size. Two bonuses: it will be difficult to run out of hot water; also a good 60-75% efficient DHR device will remove lots of load off the heat pump which should extend its longevity.

  17. Norman Bunn | | #17

    Jan,

    Great minds think alike. I am getting the XE65T10HD50U1 (65 gallon tank) on sale for $1399 ($250 off the regular price of $1649), plus it has smart capabilities (Econet) built in, another $150 or so savings vs having to add it on. Unfortunately, Duke does not offer a rebate in my area unless I use one of their "approved" contractors. Those typically run higher than my preferred plumber, enough so that the rebate is meaningless.

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