Holy Cow! Now that Prices have changed, Which Way Should I Proceed?

Discussion in 'Water Heater Forum, Tanks' started by DEDon, Mar 18, 2013.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    Fluid dynamics is a complex science, but the fluid does tend to stick to the sides of the pipe.
  2. Terry

    Terry Administrator Staff Member

    Bothell, Washington
    I've keeping track of the heat reclaim pipe you show. I have a location in my home, where I could replace a 3" stack and place my tankless water heater by it. The tankless unit vents with PVC, so condensation would be taken care of. And the washer would be nearby for draining the condensation. It means a little remodel, but I'm adding a bathroom on the first floor in that location anyway.

    Years ago, maybe decades ago, I was installing heat exchangers for commerical use. I love the idea of preconditioning and not wasting that energy that would normally just find it's way outside without kicking in a second use of the first earned energy.
    In the mid 80's I was building passive solar homes with a Southern exposure. Those worked pretty well too.

    And yes, water does cling to the outside of the pipe. That's one reason when we dump sink waste into a floor sink, we cut the end of the pipe like a baloney, at an angle. That way the water doesn't spin around and miss the drain.

    Last edited: Nov 6, 2013
  3. Caduceus

    Caduceus Master Plumber

    Pittsburgh, PA
    Do you cut the baloney on the horizontal or the vertical part of the pipe? And if on the vertical, then where do you point the baloney side? Towards the middle of the floor drain or towards the inside wall of the pipe? This sticking and clinging, is it to the outside or inside of the pipe?
  4. Terry

    Terry Administrator Staff Member

    Bothell, Washington
    I'm either at a 45 or at vertical.
    With a 45, the long side is down. On the vertical, I tend to keep the open side over the center of the drain, and the long side near the outer edge.
    That doesn't mean I'm doing it right though. I only know that if I don't cut in the angle, the water spins off the end. If I cut in an angle, it tends to follow the pipe to the the end, and falls off the pipe there. Much cleaner. That's the sort of thing that you hear from someone, and hope that you remember for the time when you may need to do it.
    It enables me to get the waste water to the floor sink, and maintain an air gap.
    Last edited: Nov 5, 2013
  5. Caduceus

    Caduceus Master Plumber

    Pittsburgh, PA
    Okay, got it. I was just trying to determine which one of the fundamental forces was at work inside the pipes. The 'sticking' and 'clinging' threw me off for a minute but I guess it's kinda like gravity affecting the water travelling vertically and when in free fall the rotational influence of the earth in concert with gravity results in an expanding circular pattern until it reaches the wall where the dilation cannot continue. Of course this is only if the incoming volume of water doesn't exceed the rated flow capacity of the pipe. And then by cutting an angle at the end of the pipe, there is a subtle abridgement of the gyre reducing the splash. Sorry for any confusion. Fluid dynamics get me all goofy. I guess that got cleared up with out getting messy.

    See, Terry...I DO have a sense of humor.;)
  6. Terry

    Terry Administrator Staff Member

    Bothell, Washington
    That explanation had me going for a while :)
  7. you are cutting the balony real thin here....

    you know , I have noticed when I take a piss, the stream sort of spins out the end of my pecker and spins in a counter clock wise
    direction down into the toilet bowl.....

    is that the theory that Dana is talking about that the water seems to cling
    to the side of the pipes as it passes through...does it spin down the pipe ??

    Depending on your age the psi that you are pushing urine out with could differ dramatically... I sitll got good pressure

    Now with a normal sewer pipe I dont think there is much pressure and I wonder how much heat transferrence actually happens....and how hot is the water going through the pipe???

    now, if I lived south of the equator would it be spinning it out of my pecker
    in a counter clock wise direction and should that be factored into the equation??

    Dana I think you should facator these things in to your heat transfer equation.....

    also if you put your hand on the flue pipe,
    it will most likely come close to burning you..... that is where I would rather wrap the copper coil..... that is probably close to 195 degrees...

    next time you take a piss , try passing your hand through your stream and you should get a feel for the heat...
    but it will only be around 98.6...luke warm at best......and it would drop dramatically once it gets into the toilet...

    that is some serious heat loss.....

    Dana , just try it and see;):p

    here is a study that some psysicists did on
    this subject

    Last edited: Nov 6, 2013
  8. Dana

    Dana In the trades

    I'll accept the measured performance of the engineers who design & test stuff above anybody's armchair theory. I accept Terry's (and other's) less rigorous field test measurements too.

    Most of the water in vertical drains is NOT in free-fall, but rather on the surface of the pipe, which is why these things work at all. The measured heat exchange efficiency of gravity film heat exchangers is well established, no equations necessary, but if you want some of the details of how droplets are clingy whereas bubbles are bouncy, that is well known too. :)

    For the non-numerical perspective, watch what happens to rain-drops as they hit a vertical window at an oblique angle, and how little of the water bounces off compared to what sticks (and that's at a much higher verital velocity than you would find in a drain). It doesn't take a high lateral velocity to go from one edge of a 4" drain pipe to hit the other side and stick to it, even even a 2' drop. The surface tension of the liquid causes it to spread out over the surface of the drain pipe as it runs down, increasing the heat transfer area.

    High temperatures on low thermal mass surfaces like single-wall flue pipes is not a measure of heat transfer efficiency, only a measure of how poorly that heat is re-transfered to the proximate air on the exterior of the flue pipe. Give it a shot if you like- when you take the flue heat exchanger to it's logical limit you will have re-invented the condensing water heater! :)

    I fully accept that not everyone is comfortable with having a potable wrap in contact with the drain, but I also accept that as-built these do not constitute a code violation in most places. The double walled character and the tightness of the wrap (=no mechanical vibe & wear) are probably the differentiating factor that gives the assembly a pass, where potable piping field-assembled in contact with drains would be a violation. Without the minutes of the presiding/deciding body to refer to we can only speculate. But from one vendors' website:

    "Code Compliance

    DWHR double-walled heat exchangers meet the Uniform Plumbing Code, Section 603.3.4; 1995 CABO (Council of American Building Officials) One and Two Family Dwelling Code, Section 3402.3.1; 1998 ICC (International Code Council) One and Two Family Dwelling Code, Section 3402.4.2.1; and the 2000 and 2003 ICC International Residential Code, Section P2902.4.2 - Heat Exchangers.

    If the lack of an ASTM B88 stripe on the brazed manifold on Renewability's heat exchanger design is an issue in your jurisdiction, there are multiple vendors of units that don't have that problem, and there are other vendors who also offer direct from manufacturer website sales shippable to US locations, if there isn't a US retailer or distributor to work with eg:


    Natural Resources Canada maintains a third-party tested performance list here.

    There is another US manufacturer that isn't third-party tested & listed on NRCAN program (probably due to their ongoing allegations of patent infringment by Canadian vendors), but their stuff works too:


    The Canadians tried and failed to measure the heat recovery of room-temp and tepid water drains using these things, but failed. The delta-Ts are too small, and even using the theoretical model the result would be smaller than the measurement error for unbalance high drain-gpm not exactly simultaneous slow-potable flows like toilet flushes. (Heat exchange efficiency falls with higher flow rates & unbalanced flow.)
  9. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona
    There is no way I would even attempt to slog through those voluminous replies, but you can forget about finding a new heater which will match the connections to the old one after 17 years. The dimensions have changed so often that even a 8 year old heater would be different from a new one.

  10. Dana , tha tis a lot of information...
    I think that the canadians would rather sit back and drink a Labats beer than
    waste their time trying to capture the Delta t of one of these devices because
    it is nominal... they probably finally just gave up because it was so small a figure
    to try to capture.....

    I honestly think that its a good idea if you make your own out of 1/2 soft copper which would work ok anything smaller would probably restrict the pipe flow..... but these things are in
    the same league with Tankless water heaters.... a lot of mis-leading information , smoke and mirrors...

    That is why the canadians threw in the towell...

    they are politely saying that its a joke....

    now speaking of flow, did you try that piss test, on your hand last night??
    you sould be able to measure the Delta T while it was passing over your hand.
    with a thermometer.....

    I think that this data could be revelant......leave no stone un-turned.

    Here is a digital thermometer that you could use to test your flow.... just be sure to clean it off before sticking it in the Thanks Giving turkey


    [TD="bgcolor: transparent"]


    Last edited: Nov 7, 2013
  11. DonL

    DonL Jack of all trades Master of one

    Houston, TX

    That looks painful not digital.

  12. Dana

    Dana In the trades

    Mark- in-re DIY drainwater heat exchangers using soft-copper:

    Key to getting the heat transfer efficiency is contact area between the potable and the drain. The very original prototype designs in the early 1980s did round 1/2" soft-copper wrap and dipped them in solder but those were expensive to make, and not as efficient as 2nd & 3rd generation units that used flattened potable wraps tempered for some springiness, pre-formed to suitable flatness, then and heat-expande to be able to slip it over the copper drain. It has to be a sufficiently controlled process that a gas-tight contact was made between the flat-formed potable and the copper drain to minimize degradation of the thermal transfer efficiency over time from oxidation crystals pushing the potable away from the drain pipe as slightly insulative oxidation layer grows.

    Up for self-engineering all that?


    Of the newer designs Renewability does multiple rectangular cross section parallel potable wraps brazed on to manifolds at the connections to the distribution plumbing. All others use either square-ish cross section potable wraps or D-section wrap with the flat side up against the copper drain. Both work pretty well at single shower flow rates, and 4' or shorter units. With higher flow rates & longer units Renewability units have a slight edge from having less of a pressure drop and less thermal mass and higher turbulence in the potable wrap from the multi-path ribbon-pipe.

    These things are dumber than a box o' rocks (even the Canuckistanis can manage 'em! ;-) ), and in no way comparable to the short-cycling smoke with mirrors represented by overwrought tankless HW heater efficiency claims. They're zero maintenance, and really DO work as-advertised in the real world if you mount them reasonably plumb. (At least within the measuring accuracy I get on the limited number of field-unit's I've measured with IR thermometers. I'm on year 5 with the the example in my basement.)

    The inability to measure the return from a tepid toilet flush doesn't mean they considered it a joke. They are subsidizing it to the amount of net lowered cost achieved by the other ratepayers/taxpayers all over the country to where it's revenue neutral, based on the econometric models of the lifecycle reduction in demand these things achieve, much as other efficiency subsidies are done. If they had been able to measure tepid-water heat recovery that would have gone into the subsidy analysis, but since they couldn't, they stuck to the performance that they can actually measure. And since 2.5 gpm is actually a bit higher than the average real-world shower flows, the real world efficiency is slightly ahead of the NRCAN test numbers.

    I don't put any stock in vendor hype about payback & performance, but I DO have some faith in the certified test numbers listed on the NRCAN site. How you use the thing and your actual hot water heating efficiency & fuel cost determines the true payback, but the NRCAN numbers are good enough to design systems for minimum burner size & storage volumes required meet showering loads.
  13. my mistake....

    DonL.... Now that does look painflul when you think about it....

    I dont think I would attempt to get that good a reading
    on my flow, . it would take all the fun out of the experiment

  14. I don't put any stock in vendor hype about payback & performance, but I DO have some faith in the certified test numbers listed on the NRCAN site. How you use the thing and your actual hot water heating efficiency & fuel cost determines the true payback, but the NRCAN numbers are good enough to design systems for minimum burner size & storage volumes required meet showering loads.

    it looks like a good idea, just too expensive for me to swallow....

    of course there is something else that is not factored into the whole
    thing.... there is some heat comming out of the sewer line going out the roof
    too... wether you be on a sewer or septic it would be nominal but I suppose it would
    keep the tubes at a minimum of 55degrees all the time

    if you really have one in your basement, it would be interesting to see
    what or how much difference it would be to install one of those things on the out-going flu pipe

    did you try that turkey thermometer on yourself yet??
    Last edited: Nov 8, 2013
  15. Dana

    Dana In the trades

    The acidity of natural gas exhaust isn't materially compatible copper drain, and the laminar flows on the flue gas side won't make for very effective heat exchange, as previously mentioned.

    There's very little prospect of dropping the drain stack temp below 55F with one of these things, even with extended draws with 35F incoming water temps. I you're really curious, I'll take IR thermometer temp readings at the incoming water pipe at the base of the heat exchanger, and the section of drain a few inches below it, which would give some indication of where the mid-winter minimum would lie. (If it was actually possible to create a low stack-temp problem it would probably have shown up first in Canada, eh?) The average temp in the drain stack would probably be the temperature in the basement & stack chase, which runs primarily through conditioned space. Any cooling by the heat exchanger would be temporary in any case.
  16. I honestly would not waste the time to do it...
    you have to remember that your room temp wil lbe higher
    than the temps inside the pipe......so its basically a total circle....

    I would be interested in seeing what one would do on the flu pipe...
    go ahead and wrap the chimmney with aluminum foil to protect the copper
    from the flu pipe if you must..... that would be a better experiment because of the
    higher heats....
    Last edited by a moderator: Nov 9, 2013
  17. Dana

    Dana In the trades

    That's not necessarily going to be the case. With only a 50% heat exchange with 100F water going down the drain and 40F water coming in, the air in drain near the bottom will be the same temp as the ~70F water clinging to the inside of the drain at that point. (The air in the drain nearest the top of the HX would be about 100F.) In a 65F basement that's a higher temp than room temp, not lower. If the HX is performing at 70% (a really tall one), you might bring the stack temp below room temp with the heat exchanger, but maybe not. I'm really not sweating this one.

    Higher temperature of flues doesn't make up for the much lower thermal conductivity and thermal mass of gases vs. liquid water- the heat transfer rate is still pretty lousy. If it were that easy to extract heat from flue gases, condensing boilers & water heaters would be dirt-cheap, and you wouldn't need complex forms on cast-iron boilers to get the combustion efficiency into the 80s. Knock yerself out, take good measurements, but the exhaust gas heat extraction road is a well-trodden path in the industrial boiler applications, and they're neither simple nor cheap. (Were they simple and scalable it would have long since found it's way in to domestic hot water applications.)
    Last edited by a moderator: Nov 9, 2013

  18. what I do remember from my solar days would probably hold true with
    what you are saying.... when the heater came on there would probably
    have to be a sensor installed on the pipe which would kick on a low flow pump to keep
    the water in the fins touching the flu pipe from overheating.....

    something like on the solar panels but with much less return

    its basically all just a huge waste of time....
  19. Dana

    Dana In the trades

    Yep- a stack economizer on a 40 KBTU/hr hot water heater burner is only sipping from the 8 KBTU/hr streaming up the flue. It would reap at best 3/4 of that (in your dreams!) for a net 6 KBTU/hr, worth maybe 10% of the heat going down the drain a shower flow. How much time are you going to spend to save that intermittent 6 KBTU/hr, which has no consequential effect on apparent capacity?

    The drainwater heat recovery for showers delivers a much higher heat rate, applied immediately, extending showering capacity. In some instances the extra capacity and not having to upsize the burner or the tank will pay for most/all of the drainwater heat recovery unit in reduced hot water heater sizing. In other situations it won't. If the water heating fuels are expensive, there is still a pretty good IRR on drainwater heat recovery on fuel savings for homes where the shower gets a half-hour of use per day. Where the fuels are cheap or the shower only runs 10 minutes/day it gets harder to make any financial case.

    The one in my house was installed for capacity reasons, but it'll be more than a decade before the fuel savings will add up to the installed cost, when the fuel savings are looked at in isolation. But it was cheaper (and takes less space) than adding an indirect HW heater as a separate zone, which would have been necessary to hit similar HW performance without having to raise the heating system water temps. As currently configured, the water in the radiation heating system never breaks 130F, the return water to the boiler never breaks 125F, yet I never have to wait between showers for the buffer to recover, and it all just works. Without the drainwater heat recovery it wouldn't. Clearly YMMV.
  20. Skeptical_thinker

    Skeptical_thinker New Member

    Massachusetts Metrowest
    Newbie late to the party here but this laundry is near where I grew up.


    It gets wicked cold there.

    Take from that what you will
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Water Heater Forum, Tanks Water Heater prices and installation Jan 14, 2011

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