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TANKLESS WATER HEATERS

Post in 'The Green Room' started by Todd, Jul 28, 2006.

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  1. Mo Heat

    Mo Heat Mod Emeritus

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    Is that because the tankless is Natural Gas and your old tank DHW heater was electric? If so, did your NG bill increase at all?

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  2. jabush

    jabush Feeling the Heat

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    Now that is very interesting. I never would have thought of the big picture implications.
    Good find.
  3. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Kinda the same thing as the local sewer utilities at halftime during the Superbowl.
  4. cbrodsky

    cbrodsky Member

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    Interesting article.

    I think the real solution to the hot water question is well insulated storage linked to a network controlled pricing system. As utility companies gear up to supply data over the same utility lines, and wireless communication technology becomes pervasive, it's not ridiculous to think your hot water tank could get a signal to load up at 3AM when your local grid has no demand for the power on it - the power company can give you a great discount while making more money selling power that is otherwise worthless.

    Same reason timer-controlled plug-in hybrids could be so useful to take advantage of off-peak production.

    -Colin
  5. brian_in_idaho

    brian_in_idaho New Member

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    Another item to consider with tankless designs, since they heat on demand, they need to put ALOT of watts into water in a short period of time, hence you have high peak demands and therefore lots of current draw. I haven't looked that much, but I suspect that the power draw is far in excess of the 30 amps typical for a fixed system, that can tolerate a more gradual buildup in heat. The reason I bring this up is that your wiring will probably need to be considerably larger wire (priced copper lately?), and if you have 2 of these, you may put quite a drain on a 200 amp service, especially with other loads.

    check out the ones here:
    http://www.gotankless.com/downloads/twh_brochure.pdf

    The 3 gpm unit requires a 120!!!! amp breaker.

    Bri
  6. got wood?

    got wood? New Member

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    My cousin is a plumber (30 years) in Oregon and he said one of the more efficient ways to reduce your hot water demand started with the pipes and not the heat source. His opinion is that if you have a circulating loop of hot water throughout the house (assuming reasonably well insulated pipes), you get hot water instantly and don't run several gallons first for trivial things like washing your hands, etc. He claimed that water leaving the hot water source at 120 or so degrees returns a few degrees cooler from its loop-trip to be re-heated up by just a bit of fresh hot water. He likened it to spinning a basketball on your finger and using your other hand to hit the ball in order to keep it spinning. He said a circulator pump runs at 20-40 watts...Interesting idea for new construction, but converting an otherwise star-pipe topology to a loop might be a bit much for us non-plumber-types.
  7. brian_in_idaho

    brian_in_idaho New Member

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    I'm doing that at my place, the plumbing loop is in place, I still have to install the water heater and pump (new construction). I installed a parallel loop to the main "trunk line" I have feeding the house, there is maybe a 10 ft run max from the trunk to each fixture. My only concern with the circ pump is that you are constantly pumping hot water through some 100 or so feet of pipe, loosing heat all the while. I need to research effective pipe insulations and get it in place.

    For DIY plumbing PEX pipe is your friend, I have soldered some amount of copper, PEX is much faster to install.
  8. cbrodsky

    cbrodsky Member

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    Interesting idea, but fundamentally flawed - all that constant circulation of hot water = more heat loss, plus you're paying for electricity to drive that process. If you didn't lose heat by circulating that hot water, then you would always have on-demand hot water - it would stay hot in the pipes.

    In fact, by constantly refreshing hot water in those pipes, you actually accelerate any heat loss you normally have after shutting off the hot water because you maintain a larger thermal differential all the time.

    It will certainly accomplish your goal of instant hot water, but it'll be pricey. That being said, there are systems like this that bridge your hot/cold water lines to do something similar, and when I have excess free solar HW in the summer, I could see using one...

    -Colin
  9. begreen

    begreen Mooderator Staff Member

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    Flawed assumption. The system should only circulate hot water when the loop drops below a set temp point. And a good one will have a timer on it so that it doesn't run when not needed. I installed a tiny Grundfos Comfort Pump and love it. The hot water lines are all insulated and I would guess that it runs for about a minute every 30 minutes on average. The loop temp is set low, comes on at about 105 degrees and off at about 110.

    I thought they might go up, but our electric bills went down as soon as we stopped running water in the upstairs bathroom just to get it hot. Difference - before about 90 sec. to get hot water. Now, about 5 sec. With 4 people taking showers that can add up quickly. We pay for our water and it's expensive, so it was a double savings.

    http://www.plumbingsupply.com/grundfoscomfort.html
  10. cbrodsky

    cbrodsky Member

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    Yes, you don't wait for hot water, but that's because it runs to fill the line up all the time before you get a chance. Without such a device, you would be charging your lines up with hot water only when you use hot water. But by keeping the pipe hot, you are regularly using up hot water every 30 minutes keeping that line charged up to temperature. You can reduce this behavior with a timer, but not eliminate it.

    Comfort device, yes I can buy, and will probably put one in our house. Possibly even cost saving if you're paying big $$$ for water/sewer expense on that first bit of water you throw down the drain waiting for the hot water to come or doing rainwater collection with your backup being trucked in water. But not energy saving - doesn't come for free.

    Perhaps Grundfos' website says it best in their FAQ:

    "Does a hot water recirculating system use up a lot of energy? Both options use energy. But not as much as most people think."

    That, I can agree with :)

    -Colin
  11. Homefire

    Homefire New Member

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    These circ systems for the home should be on a timer that you set to run when you
    normaly use water, say for the morning shower or after 5
    when everyone is hom. Then set to turn off at bed time and while your at work..
    Not perfect but it will save a bucket load of electricty to heat that little bit of water to wash your hands.
  12. cbrodsky

    cbrodsky Member

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    Folks, believe it or not, these do not save you heat.

    Without such a device, you flush out the cold water in the pipe, and when hot water gets there, you wash. When you're done, only the hot water left in the pipe cools down, and it cools down once. That heat is lost.

    With such a device, for a period of time you are keeping that pipe hot even when not using water, losing heat before you even decide to use hot water. And when you're done washing, the pipe will still have hot water. But instead of letting it cool down, you continue to put heat into it to keep it up to temperature until the timer turns off. And you lose more heat by holding it hot that long.

    Heck, even the manufacturer makes that point clear on their website... it does cost energy. Maybe not a lot, but it's most certainly not saving energy.

    -Colin
  13. Homefire

    Homefire New Member

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    But you don't stand there forever waiting on the water to get hot as it is already hot and instead of the luke warn water going down the drain it returns to the tank as opposed to cold water from the well or street. There is most certainly an electrical savings. IMO
  14. begreen

    begreen Mooderator Staff Member

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    Colin. These units have a thermostat. -*- They don't run all the time. -*- Ours runs about 24 minutes per day total.

    It's also an error to assume that the hot water heater is going to kick on just because the pump as running. It doesn't always and won't unless the tank temp drops below the set point. What I think the pump manufacturer is trying to cover is for installations where the pipes are not insulated. But I can say from direct experience, insulating them makes a big difference in the frequency of cycling of the pump and the length of time it runs.
  15. cbrodsky

    cbrodsky Member

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    I don't think you're understanding the point - you only lose heat out of the hot water pipe if you keep it hot. So, your thermostat is designed to do just that - keep the entire hot water pipe run hot - all the time. Whether you're using it or not. And by keeping something hotter than the surroundings, you will lose heat. No different than your house - keep it hot all day, lose more heat. How it does that is somewhat immaterial - energy is energy. Keep it hot for a longer period of time, you lose more heat to the outside. Nobody is saying it always runs your water heater, but it is regularly taking hot water out of your heater and refilling it with lukewarm water even when you don't use your hot water. That is wasted energy plain and simple, and it will cause more total usage in the long haul.

    All of this can be reduced with pipe insulation, timers, etc... but none are as energy efficient as just letting the cold plug of water in the pipe go down the drain, and using the hot water when it arrives. (obviously if you flush it way beyond that point, then yes, you are wasting a lot more)

    Don't get me wrong - I see why people like them, I'm sure it's not a huge loss of energy, and I'll probably put on in our house - with solar HW I won't even worry about the loss. However, you guys want to believe you've somehow beaten basic thermodynamics and heat transfer. More power to you, but even the manufacturer admits that there is lost energy when you install such a system. Right on their website. Why in the world would they admit that if it wasn't true?

    -Colin
  16. velvetfoot

    velvetfoot Minister of Fire

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    The heat is not really wasted during the heating season since it warms the house, no?
  17. cbrodsky

    cbrodsky Member

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    Only if they are within a sealed heated envelope of the house. (which they should be... but often aren't...) And of course if you're using electric to heat that water, pretty expensive way to throw heat into your home. Not to mention much of the year, it will work the other way, and dump heat into your house which now you have to get out of the house with A/C so you still come out behind there :)

    I'm sure the actual loss is relatively minor, but point is that it's not zero. Simple common sense tells us that, and even the manufacturer makes that point clear. And the further your pipes run, the greater the volume of water your constantly refreshing with new hot water, and thus the greater the losses.

    -Colin
  18. got wood?

    got wood? New Member

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    I think what you're saying NY Soapstone is that it uses energy, but how much is the question. Let's take a simple example: compare a `star' or `home-run' to the hot water source example (what most of us have) to a loop setup. Let's say it's a kitchen sink that is 25' away from the hot water supply with 1" properly insulated copper piping.

    pi*r^2*h will give you the cylinder volume in cubic inches of the pipe, divide that by 231 to get gallons.

    3.1415*1^2*(25*12)=942.45 cu/in or 4.08 gallons of cold water in the pipe for a non-loop system. You want 1 gallon of hot water in the kitchen, that would require 5.08 gallons of hot water to purge the line and deliver the hot water (not counting heat loss of that 1st gallon of water in cold pipe).

    In a loops system: 3.1415*1^2*(50*12)=1884.9 cu/in of hot water in the pipe loop or 8.16 gallons. You want 1 gallon of hot water in the same kitchen, that would require 1 gallon of water from the hot pipe.

    What I've ignored here is that it took 8.16 gallons to heat that pipe in the loop system, so for that 1 gallon, it's actually 9.16 gallons. What about 30 minutes later you want another gallon of hot water. Standard non-loop system would require 10.16 gallons to deliver that 2nd gallon of hot water, the loop system would require 10.16 gallons. For the 3rd gallon an hour later, we're looking at 15.24 gallons in the non-loop configuration and 11.16 gallons in the loop configuration.

    This example ignores the fact that additional hot water would been needed to keep that loop hot...how much is something I do not know off the top of my head. Would it require 2 gallons of hot water added over 2 hours to maintain a desired temp? I don't know. How much energy does a circulator pump draw if it comes on every 20 miunutes for a minute or so? Maybe 50 watts, maybe more but not much.

    This was a simple example of course. If you have multiple bathrooms, kitchen sink, dishwasher, clothes washer on this loop vs. a star topology to each hot water fixture, I think the savings might become more apparent.

    my $0.02
  19. cbrodsky

    cbrodsky Member

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    I agree, and I think the basic key here is just what you state - you can't ignore that loss, but it's also very hard to estimate not knowing specifics. All we can say is that the wasted heat here is > 0. The circulator manufacturer admits this issue and encourages you to minimize this component as much as possible using extra pipe insulation and timers, but it will never go to zero. They pitch it on raw water volume savings and convenience - not energy savings.

    If not using a circulator, the pipe would have cooled to a certain point requiring you to draw in new hot water to fill the pipe and meet demand. In the simplest case, suppose that entire 8 gallons is at room temperature - now you refill it. But if you had a circulator, you still have to draw in enough BTUs with fresh hot water to maintain the same 8 gallons at the hot water demand temperature. Additionally, you have to do this both when using it and when not using it. Even more, the rate of heat loss will be greater because you constantly maintain a larger temperature differential to ambient conditions at all times. Finally, the icing on the cake is you pay some small amount of electricity to drive this entire process with a circulator.

    So, when you sum all these issues up, there is some real loss. The amount will vary tremendously from one situation to another.

    -Colin
  20. brian_in_idaho

    brian_in_idaho New Member

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    I think homefire is hitting on one of the key points. With a non-circulating system, it's true that you loose more heat out of the pipe, delta T is higher all the time, driving more heat loss. However, the difference is that with this system you are still returning 100 or so degree water to the tank. With a "standard" system, when you flush out the cool water in the line to your fixture, you are replacing that water in the tank with 50 degree or so "cold" water, in effect loosing the additional 50 degrees of temperature difference, and however many watts/btus that were in the line. If I get the chance to play with some scenarios and typical heat loss calculations I will.

    Bri
  21. cbrodsky

    cbrodsky Member

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    That is true that there is potential to recover some of the heat, just like using timers to only run part of the day, and other tricks to minimize the impact. But at the end of the day, heat is being lost... there is no escaping that fact with these systems. The hot water cools down a bit in the hot water pipe, is then sent back into the cold water line, where it cools some more, and then it eventually goes back into the water heater to heat back up again. This cycle continues as long as you are running a circulator. That takes heat that you don't have when not using a circulator. Think of your water pipes as relatively ineffective baseboard raditors - you are just constantly churning hot water through them and they radiate it out, albeit slowly.

    -Colin
  22. begreen

    begreen Mooderator Staff Member

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    Yes there will be a minor heat loss, just as there is from the hot water heater itself. But there will be much more heat lost with incoming 45 degree water replacing depleted hot water in the tank. If this incoming water is run just to get the tap or shower hot, it is much more wasteful and takes much more energy to recover from. Have 4 people doing this with every shower and face washing in the sink every day and it really adds up. If you are paying for your water it adds up more than doubley quick.

    Also, the loop in our system does not feed back into the cold water. We have a 3/4 HW feed going upstairs. Just before the 2nd floor shower I have a tee where a 1/2" recirc line returns to the pump which circulates back to the bottom of the HW tank. It doesn't touch the cold water line.
  23. cbrodsky

    cbrodsky Member

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    I think the basic point is still lost on most in this thread - the circulator is still costing more than the alternative of no circulator. Remember, the only reason the circulator runs is because you are losing heat.

    I'll try to explain another way...

    Case 1 - you use hot water, and then you go away. You do not come back for hot water until the pipes have completely cooled to ambient, thus your heat requirement to prepare for next use is to bring groundwater up to hot water temperature for whatever volume of piping you have.

    Case 2 - you have a circulator. In the time it took the pipe to cool in case 1, you have had many repeated cycles of injecting hot water to sustain a higher setpoint temperature in the pipe and discharged some lukewarm water back to the hot water tank. This is assured because in case 1, you cool from say 120 to 70. On the other hand, every single time you drop below say 110 in case 2, you refill that hot water line with 120 degree water. So you do this again and again compared to case 1. Over the same time period as case 1, you have worked to maintain a higher differential temperature between your hot water line and ambient. This drives increased heat loss from your hot water line. Heat loss is proportional to the difference in temperature of your heat source and surroundings. This loss assuredly exceeds any recovered heat you gain out of sending the partially cooled water back to the hot water tank. That step of course helps, but you are still behind. Why? This just boils down to fundamental thermodynamics and heat transfer principles. You cannot run this cycle without losing heat in the process compared to doing nothing, letting it cool, and warming the pipe up again only at the time of use.

    Surely there are some other chemical/mechanical engineers on the thread that could take a stab at explaining this point?

    It has also been pointed out that the manufacturer makes it clear that there is some moderate energy penalty. Additionally, it has been pointed out that these devices come with timers. Why? If you gain energy savings by running it, why not run it all the time? The answer is because it in fact increases heat use and the timer is key to help reduce those losses. Entropy always wins.

    -Colin
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