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Heat exchanger for a layman

  1. Apr 11, 2012 #1
    Hello, my first post. I hope this is the right forum...

    I'm considering a heat exchanger to solve a problem.

    I've purchased a tankless water heater for home usage - look at it as a black box, cold water flows in, hot water flows out.
    Output temperature is quite constant due to a digital servo.

    It just happens the model I've purchased minimum output temperature is a few deg C above the temperature I feel comfortable with during summer.
    I cannot mix cold water with the hot water.

    If I use a heat exchanger consisting of two copper tubes soldered together side by side, input cold water would flow trough one, output hot water trough the other one, how many deg C of cooling per meter could I expect from this setup?
    (The digital servo will compensate for the increase in cold water temp so output temp will be the same).

    Copper tube diameter 20mm, wall 1mm, water flux 10 liters/min. Input water temp 20 deg C, output water temp 37 deg C.

    Thanks for your help!
  2. jcsd
  3. Apr 11, 2012 #2
    Re: Heath exchanger for a layman

    Welcome to the forum!

    This is more of a educational forum you might have better luck on an Engineering forum.

    If the heat exchange is home made it's very hard to predict the outcome, the bottom line is that you will have to test your setup.

    Good luck !

  4. Apr 12, 2012 #3
    Re: Heath exchanger for a layman

    Thanks, Roman
  5. Apr 12, 2012 #4


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    Re: Heath exchanger for a layman

    This is a fairly complex problem to solve, because to calculate heat transfer in the tube you need to calculate the Reynold's number of the fluid and use that to estimate the convection coefficient inside the tube.

    Before we walk you through the equations, may I ask what you're trying to accomplish by heating water with a water heater, only to cool it with another tube of water? What's the goal of the system?
  6. Apr 12, 2012 #5
    Re: Heath exchanger for a layman

    The heather is used for bath shower, taps, etc. Minimum servo temp is 37 deg C and to me this is hotter than I like to shower during summer.
    During winter I would rise servo temp setting if needed (plenty of room here).
  7. Apr 12, 2012 #6
    Re: Heath exchanger for a layman

    What do you think about installing a water tap with two lines (two in - one out): hot and cold? It would dilute the hot water with cold water to a comfortable temp.


  8. Apr 12, 2012 #7
    Re: Heath exchanger for a layman

    Any chance that the "black box" can be reprogrammed for a lower water temperature?

    It might be a lot easier to add a cold water inlet into the hot water plumbing after the water heater. Use a valve on the cold supply to adjust your temperature.

    I chimed in a little late.
    Last edited: Apr 12, 2012
  9. Apr 12, 2012 #8


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    Re: Heath exchanger for a layman

    I would suggest this as well. Otherwise the cold water is just wasted, where if you mix the two you get a better solution.
  10. Apr 12, 2012 #9
    Not so simple here. The tankless heater is associated with a small pump so water pressure from it is about 7m water column, while the cold water comes from a tank about 1m above the shower.
    So, if one tries to mix cold water to the hot one, the pressure will pump hot water into the cold water piping.

    And the cold water is not wasted - the cold water, after being slightly heated in the heat exchange process goes into the tankless heater.

    I'm talking to the manufacturer to see if is there a way for the servo to be set to a lower temp.
    Just as an info, it's made by Rinnai

  11. Apr 12, 2012 #10
    OK, I'm familiar with that pressure problem. A very simple solution (use to have that in my military base) is when you have two different pressures in the hot and cold taps (but above atmosphere): you get two pipes (hot and cold) with each having it's own separate controllable valve and connect them after the valves to a third pipe where they mix and where the water is coming from (atmosphere pressure). makes sense?

  12. Apr 13, 2012 #11

    I've already tried you suggestion. There's the hot water pipe/tap, the cold water pipe/tap, and they join to a single shower pipe.

    I've tried opening the cold water tap, the hot water one and then I started closing slowly the hot water tap hoping they would mix.
    But then the hot water flow becomes too small and the tankless heater turns off... (the tankless heater requires the pump to operate).

    That's why I'm looking to the weird heat exchanger solution!
  13. Apr 13, 2012 #12
    JorgeO, this is your shower you're talking about? How are you mixing the cold and hot currently? Are you just turning on the hot water only? Is it possible to just feed the cold side of your shower valve (assuming you have one) with pressurized cold water from the tankless heater inlet pipe? That would solve your pressure problem and allow for control. Your suggestion would work, as you stated it, but requires considerable surface area to get a substantial drop.
  14. Apr 13, 2012 #13
    This is kind of an interesting problem, but would agree that Highspeed's suggestion would be best if it's possible. The heater (according to their website) has a min. temp. setting of 98F (37C). The basic Value model I looked at (RV53e) has a flow range of .6 to 5.3 GPM, which is pretty wide. So, assuming the pump is far enough upstream of the heater and not built-in it might well be possible to tap into the pressurized cold water (downstream of the pump/upstream of the heater inlet) and divert that to the cold water supply of the shower valve, and still have enough flow/pressure drop across the heater to turn it on. In essence that's what any house does, except the 'pump' might be located miles away at the water company well.

    Another question is--does the pump start only when the heater fires up, or does it run all the time to supply cold water pressure to other outlets? If it's the latter, then there is likely already a line coming off the pump discharge, flowing to a pressure regulator and then back to the pump inlet somewhere. This is necessary to keep the pump from 'dead heading' when there's no external flow, assuming it's an impeller type pump and not reciprocating. That would be the line to tap, between the pump discharge and regulator inlet.
    Last edited: Apr 13, 2012
  15. Apr 13, 2012 #14

    I'm a retired EE with a second degree in telecoms. Before going to college I had worked as a service technician.

    Mixing hot and cold water would require masonry work, and it would be much cheaper to purchase a heater from another supplier (minimum temp 30 deg C), no way to return the Rinnai.

    I've tried all I could before I started to think about ways to cool the hot water a few deg C - say 3~4 deg.
    (It makes a difference - there are two showers in my home, one close to the heater at the other side of a wall, the other one some 6 meters away. The farther one is 1 deg cooler, and I like it better, but still prefers a bit colder).
  16. Apr 13, 2012 #15
    Last edited: Apr 13, 2012
  17. Apr 14, 2012 #16
    Thanks, NeuronsAtWork

    Your link was most helpful!

    Now, could we go back to the original question (BTW, regarding thermodynamics my knowledge is very close to zero...)
  18. Apr 14, 2012 #17
    I've been a facility engineer for 35 years, and have *some* experience with heat exchangers, be it air to air, water to air, and water to water which is what you are attempting. It is beyond my expertise to actually do the math required to give you a definitive answer on what you suggested in your OP, using the specific numbers given. I can tell you that placing two tubes of similar diameter side-by-side and soldering them together is going to have a fairly negligible effect on the water temp of either pipe, given the minimal amount of contact versus the percentage of pipe exposed to the ambient air around them. Although copper is a very good conductor of heat, it comes down to the amount of surface area of each pipe being exposed to the other, if that makes sense. Every water to water heat exchanger I've ever worked on uses a pipe within a pipe (or multiple pipes within a pressure vessel), similar in many ways to the one on the site I sent you. This is because the entire surface area of the inside pipe is exposed to the flow in the outside pipe. So, I cannot give you any other answer than to try, room permitting, some version of that. If the average ambient air temperature in the area of the heater is below 37C, then I would flow the hot water through the outside tube, and cold through the center. That way, at least some heat loss will be generated to the outside air. If the surrounding air temp is fairly warm, then I'd probably run the hot through the center pipe, cold through the outer, and insulate the whole length. As I said, I can't back this up with the math, but I have a fair amount of general experience and have at one time or another done pretty much everything related to plumbing, electrical, etc. in the course of my job. Finally, the length would certainly matter, and might have to be determined experimentally. The design on that site seems sound, easy to construct, and inexpensive. And I am certain it would work better than a side-by-side design.
    These are my two cents--I hope it helps you think this through, and perhaps someone else can chime in if I'm totally off base or if there is a more elegant solution.
    Please let us know how you ultimately solve this--I'd be curious to know. Good luck!

    One more thing I just remembered--if you should decide to do this design, the flow in each pipe should be in opposite directions to maximize heat transfer...
    Last edited: Apr 14, 2012
  19. Apr 15, 2012 #18
    Thanks again!

    One last question - suggested internal tube and external tube diameters?

    As I've posted before, I'm in contact with Rinnai to see if is there a way to set the servo to a lower temp.

    Otherwise, I will try this design with a length of about 1 meter in the heat exchange part. I hope it lowers the 3~4 degrees C I would like.

    And if I build it I will post photos and let you know the results.

    Take care!
  20. Apr 15, 2012 #19
    You know, it's a hard call, but if I were going to do it I'd probably use your current supply line diameter for the inside pipe, and ~13mm (1/2") larger for the outside pipe. I am most familiar with US imperial measurement, so for me I'd say 3/4" nominal (which for us is actually 7/8" OD) for the inside, and 1 1/4" (which is I think 1 3/8" OD) for the outside pipe. Depends on cost, availability of the right fittings, etc. You want the velocity of flow to be relatively slow to ensure decent heat transfer. If you like to be 'blasted' in the shower and aren't using a low-flow shower head, you could consider increasing the length, or you could even build two and put them in parallel.
    I'd love to see pics, but all I ask is that if this *doesn't* work, PLEASE don't hold it against me!! I enjoy experimenting and trying new things, but I have not tried this in this sort of application and my personal experience tells me that stuff does not always work out as planned. Hopefully you'll be able to find an easy route and just lower the outlet temp of the heater. Either way, good luck!
    Last edited: Apr 15, 2012
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