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Thread: Heat Recovery Unit for shower

  1. #16
    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by hj View Post
    Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels?
    Please look at the actual design. The drain water is flowing down the walls of the drain pipe not in the quad coil, single coil or dual coil. This produces a relatively thin film. Being vertical and open it is somewhat self cleaning. Soap scum and the like will likely reduce heat transfer only slightly over time unless the build up is substantial. However, it is a valid concern and one reason that I would want the ability to remove the assembly for cleaning.

    An area of more concern to me, but related to any build up is maldistribution of liquid in the falling film. Maldistribution is a factor that many old school engineers never adequately considered, so it is one of the first things I examine for problems in designs. However a mitigating factor in this design is the nature of the flow and the relatively high conduction rate of the copper should compensate for a fair amount of maldistribution. If some manufacturer wants to test this I've got a good location in mind as long as I get to keep the unit when I'm done. Wouldn't be hard to test off-level installation, some simulated hanging hair balls, or soap build up that create maldistribution.

    High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows.
    Contact time, residence time or whatever one calls it is not a relevant criteria for heat transfer. By itself it is meaningless. In fact, this greater contact time the average person think improves heat transfer in fact IMPEDES it. Film heat transfer coefficients increase as velocity increases. So for a given surface area higher velocity = higher heat transfer coefficient = lower contact time/residence time.

    One uses more surface area/smaller flow channels where possible to increase velocity. Surface area is typically limited by material/fabrication costs or geometry as well as pressure drop. Decreasing flow channel size or increasing length is limited by pressure drop if not by geometry or plugging concerns.

    The square quad coil design is attractive because it keeps pressure drop in control in greater overall drain pipe lengths and with higher flowrates (as in several showers or other users drawing water at the same time.) The squarish coils are needed to provide good contact with the pipe, something round coils don't achieve--this I can attest to after seeing too many jerry-rigged tubing coils on pipes in plant settings.

  2. #17
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    Quote Originally Posted by C Williams View Post
    More accurately, I am planning a new home in a rural location on Vancouver Island so my water will be drawn from a chilly well.
    The water from the hole itself should be relatively unchanged throughout the year, but by the time it runs through the well tank and piping in the ground it, of course, is much cooler in winter than summer. But the longer the winter and shorter the summer, the greater the impact.

    Being on a well (or any system where supply press. to the home might not be all that great), pay careful attention to pressure drop in the device at anticipated maximum numbers of hot water consuming fixtures running simultaneously. No, not everything turned on at once, but say two showers running at the same time and possibly a sink faucet, maybe the clothes or dishwasher as well. In that case the well supply pressure might be a bit low too and then there is the pressure drop to the house. I'm on city water and have to use a PRV to keep the pressure down, but I would still size the system to not drop the hot water tap pressure too much with all three of our showers running simultaneously (which we do once in awhile.)

    The main shower in the house presents and ideal installation as its drain immediately drops one story to the main drain below grade. Because the concept works best when drain flow matches inlet flow I wouldn't bother re-designing the main stack to accommodate one of these as the shower is the only load in my house that matches the basic criteria.
    Same basic issue for me. 2 of 3 showers in our home would be served by this, serving 3 of the 4 occupants and over 75% of the showering.

    The only question in my mind is can I use the drain waste heat recovery unit to warm the shower's cold water supply or with this present a problem for the mixing valve?
    Yes, you can. That is the preferred way efficiency wise because it will increase the overall heat extraction because the delta T at the outlet will be greater (even with greater heat recovery) than it would be at a lower cold water flow rate. Q = U*A*LMTD Where Q = dury, U = overall heat transfer coefficient, A = area, LMTD = log mean temperature difference.

    For simplicity sake just think of LMTD as the average delta T at both ends of the heat exchanger. (The formula is slightly more complex but this is close enough for understanding.) When you increase the cold water flow at a given duty the hot water outlet temperature wouldn't change, but the cold water temperature rise would be less than before. In reality the duty is not fixed, but area is, as is the drainwater supply temp. (U will increase slightly because of the flow increase, but since the tube side coefficient is probably not the limiting side, it won't have as much impact.) The LMTD increases, so the duty also increases.

    The one negative of having the shower cold water leg go through the heat recovery system is that as the system warms up, your "cold" water supply to the shower will be warming as well, so you will likely want to adjust the mix of hot/cold manually during the shower. Other hot water users cycling on/off during the shower will impact this some too. So some sort of thermostatic control would be desirable for this (particularly for children...or a cranky spouse.)

  3. #18
    Retired Defense Industry Engineer jadnashua's Avatar
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    A thermostatic shower valve adjusts how much cold is used to dilute the hot. It tries to keep the preset temperature constant. If either the cold temperature or the hot temperature changes, your outlet temp on a conventional valve will change. If you take a long shower, the cold will become colder and the hot will cool off. The only valve that will keep the temperature relatively constant for you is a themostatically controlled one. To me, worth the increased cost. It can never get hotter than the hotter input, and if the cold side got hot, it would rise to the cold temp as it turned off all of the hot.
    Jim DeBruycker
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  4. #19
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    Quote Originally Posted by johnfrwhipple View Post
    I have three little girls and they all have bath and shower time after dinner. This is my biggest demand time for hot water.
    I have two suggestions for you that I used with my children. The first is to get one of those stick on 5 min hourglass "shower coach" thingies from the orange box store or another place. This works even if you give them 10 mins/each. The kids like playing with them anyway...so it wasn't hard to get mine to use them.

    The second is to consider a lower flow showerhead for their shower. The two I like are in the 1.5-1.6 gpm range. I use the Evolve Roadrunner in my shower and one of the kids' showers and we have the High Sierra in another. I researched this for awhile before selecting these to try. They are both non-aerated designs that gave good full spray patterns (rather than hollow cone patterns.) An aerated design can give the impression of fuller flow, but also results in more mist generation requiring somewhat greater hot water mix for the same temp. Some of the ultra low flow aerated designs also have more of a needle like impingement sensation to them from what I've read.

  5. #20
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Quote Originally Posted by C Williams View Post
    Your recovery time may be shorter but I can see no cost savings here. It strikes me as an inefficent use of space and money. Given that your "holding tank" will be absorbing BTUs from a conditioned space, a space that you paid to condition, there is no net gain in BTUs. You're not capturing lost energy, you're mearly transfering energy....

    I do not pay to heat the space it is my mechanical room.

    If I use the return loops to further heat the secondary tank I'm actually get more for my money anyway since I'm told my Loch N Var boiler will fire at the same rate to heat water at 10 degrees as it does 18 degrees.

    If my kids take two baths and one shower a day this can easily be 40-60 gallons of hot water. If while they are filling the tub or showering and the hot water tank is getting feed by warm water I would think the recovery time and the amount of heating energy used would be far less.

    I could pick up a spare tank for under $400.00. Maybe $40.00 in fittings. 2 beer on the weekend. Done.

    I would see a return on this investment in less than two years I would think. Maybe sooner.

    JW


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

  6. #21
    Moderator & Master Plumber hj's Avatar
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    quote; Contact time, residence time or whatever one calls it is not a relevant criteria for heat transfer

    That is a ridiculous statement. The concept for "multipass" heat exchangers is to maintain the two fluids in contact as long as possible. As for the square tube being the most efficient, a trianglar cross section would be even better, because it only has two unheated sides which are radiating the captured heat into the surrounding area.
    Licensed residential and commercial plumber

  7. #22
    DIY Senior Member jch's Avatar
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    I have had a Drainwater Heat Recovery Unit installed in my house for the past year (since Dec 2010). I had what is probably a relative of yours (Dave W.) install it here in Victoria -- he had never seen one before.

    Full results and actual temperature measurements are in my thread here:
    http://www.terrylove.com/forums/show...y-installation

    In a nutshell, this $650 PowerPipe unit saves me 50% on all my shower hot water bills. Payback period for our family of 4 is less than 3 years.

    If I were to move to another house, I would immediately install another one. No brainer.
    ----------
    - John

  8. #23
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    Quote Originally Posted by hj View Post
    quote; Contact time, residence time or whatever one calls it is not a relevant criteria for heat transfer

    That is a ridiculous statement. The concept for "multipass" heat exchangers is to maintain the two fluids in contact as long as possible.
    Completely wrong. As I said before this is a common misconception. It is not a reflection on one's intelligence or educational level. Other engineers often make the same mistake because few of them have actually done their own heat exchanger design. In the design groups I've worked with we can tell when someone has never designed an exchanger when they start talking about residence or contact time with respect to the heat transfer. I've designed a lot of exchangers and redesigned or reconfigured a lot of others that were either not meeting requirements or needed to be able to handle greater heat loads and/or throughput. Multipass exchangers are used for a variety of reasons, but not for the reason you claim.

    The primary reason for using a multipass is to increase the tubeside velocity, and therefore the heat transfer coefficient. Look it up. (Increasing velocity can also prevent settling/fouling, and even corrosion in some systems...while it can cause erosion in others.) Multiple passes can be used to keep the overall length of an exchanger at a reasonable value or to balance resistance to flow from a common header (most exchangers are not throttled on the cooling water side so if one exchanger is designed with only 1 psi of drop and another with 5 psi of drop, the lower resistance exchanger will get far more than its design flow, robbing others in the network.

    Here's the kicker: For a given shell diameter and length increasing the number of tube passes REDUCES the total residence time in the heat exchanger when the flows are fixed (metered.) The reason is simple. Everytime you add another pass you have to add a partition in the exchanger head (or employ U-tubes that have an empty zone within the minimum bend radius of the tubes.) The partition obstructs part of the tube sheet and requires the elimination of some tubes. Fewer tubes = less tube side volume and lower residence time. Again, this comes from experience. Doing tube counts and examining tubesheet layouts is part of the design process.

    Another reason I've seen multipass exchangers used was because a refinery had specified that all of the exchangers in a unit have a given tube length when it was built decades ago. They varied the pass count and shell diameter to achieve the desired duties. While I was doing some cooling water distribution analysis and redesign for them I recognized that I could debottleneck their capacity limiting alkyl unit by putting a new head on a small exchanger to reduce the pass count and thereby increase the cooling water flow and duty--a super cheap, easy project. To put it in terms more familiar to you: it had way too much "residence time" for the cooling water and this was restricting the amount of cooling it could accomplish. The tubside film coefficient was high...but so was the exit water temp.

    There are various negatives to using multipass tube exchangers: reduction in the LMTD, potential for temperature crosses, increased bypass stream flow on the shell side (empty zones in the tube sheet for partitions or U-tubes), higher pressure drop on the tubeside, complexity/reliability problems of the gasketing of the head partitions, limitations on head side nozzle sizes and orientations, etc.

    As for the square tube being the most efficient, a trianglar cross section would be even better, because it only has two unheated sides which are radiating the captured heat into the surrounding area.
    An interesting proposal, but it would most likely have the opposite impact because of the geometry. Square tubes against one another will have less external surface area than triangular tubes with one face to the pipe. Essentially the two sides of the square are not exposed to the environment. The flat outside wall is. But for the same triangular tube width, neither of the two outside walls will be touching one another. They will both be losing heat to the environment. So the effective losses for the same width could be as much as twice as high for a triangular tube. However, various effects would tend to suppress the result for the triangular tube and increase that of the square tube somewhat.

    Another problem with a triangular shape will be the lower flow. The tube will have half as much open cross section as a square tube, actually even less than that because of wall thickness. So at a given pressure drop the flow rate would be half that of the square tube. This would defeat the purpose of having parallel flow paths. Without running through calcs to verify I suspect the change in the hydraulic radius would improve heat transfer coefficient marginally for a given velocity though.

  9. #24
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    Quote Originally Posted by jch View Post
    I have had a Drainwater Heat Recovery Unit installed in my house for the past year (since Dec 2010). I had what is probably a relative of yours (Dave W.) install it here in Victoria -- he had never seen one before.

    Full results and actual temperature measurements are in my thread here:
    http://www.terrylove.com/forums/show...y-installation

    In a nutshell, this $650 PowerPipe unit saves me 50% on all my shower hot water bills. Payback period for our family of 4 is less than 3 years.

    If I were to move to another house, I would immediately install another one. No brainer.
    Thanks for the info. Always helpful to see actual installs and get some numbers. Your install is very similar to how mine would look, although I hope to squeeze in a slightly longer version. According to their specs a 54" would fit here.

    I've used digital thermometers to read shower temps and water at the drain. The loss you saw there is similar to what I observed.

    One quibble I have is that your numbers seem to indicate less than 50% reduction in hot water from showering: $188.55 reduction from a $493.14 baseline. That's a 38% reduction. The confusion is coming when comparing that to efficiency of the unit itself, which appears to be 50% based on the incoming water temps and coldside temp rise.

  10. #25
    DIY Senior Member jch's Avatar
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    For maximum efficiency, the flow of warm water down the drain pipe should match the flow of cold fresh water up the outside of the pipe. i.e. The DWHR unit should supply *both* the cold (now warm) feed to the shower *and* the hot water tank inlet.

    If you only feed one with your DWHR unit, then your heat recovery will be less.
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  11. #26
    DIY Senior Member jch's Avatar
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    I have a DWHR unit that I use with an old-fashioned two-knob hot and cold handled shower.

    The temperature stabilizes within 30 seconds of turning on the shower. No need for a thermostatic valve when you have a Drainwater Heat Recovery Unit.
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  12. #27
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    Quote Originally Posted by jch View Post
    I have a DWHR unit that I use with an old-fashioned two-knob hot and cold handled shower.

    The temperature stabilizes within 30 seconds of turning on the shower. No need for a thermostatic valve when you have a Drainwater Heat Recovery Unit.
    And if your daughter flushes the toilet while your showering???


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  13. #28
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by jch View Post
    I have a DWHR unit that I use with an old-fashioned two-knob hot and cold handled shower.

    The temperature stabilizes within 30 seconds of turning on the shower. No need for a thermostatic valve when you have a Drainwater Heat Recovery Unit.
    The original question was what if you install it in the cold supply to the shower...then, the cold water is getting warmer and the hot water is cooling off as you empty the tank...the shower outlet temp will change. If it is installed per the instructions, to the WH inlet, things should be much more stable over time.
    Jim DeBruycker
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  14. #29
    DIY Senior Member jch's Avatar
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    Quote Originally Posted by johnfrwhipple View Post
    And if your daughter flushes the toilet while your showering???
    You feel it!

    The most efficient way to install a DWHR unit is to have its output feed both:
    - the cold water side of your shower mixing valve, *and*
    - the cold inlet of your water heater.

    That way, the water flow rate down the drain will match the water flow rate up the outside of the DWHR unit and maximum heat transfer will occur.


    Aside from that, yes, a thermostatic valve is always a welcome step up from a regular shower valve, regardless of whether you have a DWHR unit or not.

    My point was that the temperature of the Cold/Warm output stabilizes very quickly (<30 seconds) and so you don't need to be constantly adjusting the hot/warm mix in your shower.
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  15. #30
    ACO Shower Drain Sales johnfrwhipple's Avatar
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    LOL

    I thought so.

    I'm going to tackle this issue and do some solid number crunching on the money it takes to heat up one 60 gallon tank with no thought to pre-heating the supply water and another with my secondary tank working.

    If I isolate the shower's supply line and feed the showers mixing value with the drain water pre heated cold I should improve things even more.

    I had pushed these projects to the side lines but I think this summer I'll tackle them.

    JW


    jfrwhipple@gmail.com - www-no-curb.com - 604 506 6792

    Always get construction advice double checked by your local city hall. Flood Test Every Shower - Every Time.

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